sailboat hull design theory

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Prof. Jerome Milgram

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Mechanical Engineering

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Mechanical Design
Ocean Engineering

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Sailing yacht design (13.734), course description.

This subject teaches students, having an initial interest in sailing design, how to design good yachts. Topics covered include hydrostatics, transverse stability, and the incorporation of the design spiral into one’s working methods. Computer aided design (CAD) is used to design the shapes of hulls, appendages and decks, and is an important part of this course. The capstone project in this course is the Final Design Project in which each student designs a sailing yacht, complete in all major respects.

The central material for this subject is the content of the book Principals of Yacht Design by Larssson and Eliasson (see further description in the syllabus ). All the class lectures are based on the material in this book. The figures in the book which are shown in class (but not reproduced on this site), contain the essential material and their meaning is explained in detail during the lecture sessions. Mastery of the material in the book and completing a design project provides the desired and needed education.

This course was originally offered in Course 13 (Department of Ocean Engineering) as 13.734. In 2005, ocean engineering subjects became part of Course 2 (Department of Mechanical Engineering), and this course was renumbered 2.996.

Sailing yacht design created using a Computer Aided Design (CAD) program.

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INTRODUCTION TO SAILBOAT DESIGN: A TECHNICAL EXPLORATION

Sailboat design is a complex and fascinating field that blends engineering, hydrodynamics, and aesthetics to create vessels that harness the power of the wind for propulsion. In this highly technical article, we will delve into the key aspects of sailboat design, from methodology to evaluation.

1) Design Methodology

Designing a sailboat is a meticulous process that begins with defining the vessel’s purpose and performance goals. It involves understanding the intended use, whether it’s racing, cruising, or a combination of both. Sailboat designers must also consider regulatory requirements and safety standards.

Once the design objectives are established, naval architects employ various computational tools and simulations to create a preliminary design. These tools help in predicting the boat’s performance characteristics and optimizing its geometry.

Design methodology also encompasses market research to understand current trends and customer preferences. This information is critical for creating a sailboat that appeals to potential buyers.

2) Hull Design

The hull is the heart of any sailboat. Its shape determines how the boat interacts with the water. Hull design encompasses the choice of hull form, its dimensions, and the material used. The hull’s shape affects its hydrodynamic performance, stability, and overall handling.

For example, a narrow hull design with a deep V-shape is ideal for speed, while a wider, flatter hull provides stability for cruising. The choice of materials, such as fiberglass or aluminum, impacts the boat’s weight and durability.

The hull design is a balance between achieving efficient hydrodynamics and providing interior space for accommodations. As a designer, finding this equilibrium is a constant challenge.

3) Keel & Rudder Design

The keel and rudder are critical components of a sailboat’s underwater structure. The keel provides stability by preventing the boat from tipping over, while the rudder controls its direction. Keel design involves selecting the keel type (fin, bulb, or wing) and optimizing its shape for maximum hydrodynamic efficiency.

Rudder’s design focuses on ensuring precise control and maneuverability. Both components must be carefully integrated into the hull’s design to maintain balance and performance.

Keel and rudder design can be particularly challenging because they influence the boat’s behavior in different ways. A well-designed keel adds stability but also increases draft, limiting where the boat can sail. Rudder design must account for both responsiveness and the risk of stalling at high speeds.

4) Sail & Rig Design

Sail and rig design play a pivotal role in harnessing wind power. Sail choice, size, and shape are tailored to the boat’s intended use and performance goals. Modern sail materials like carbon fiber offer lightweight and durable options.

The rig design involves selecting the type of mast (single or multiple), rigging configuration, and mast height. These choices influence the sailboat’s stability, maneuverability, and ability to handle varying wind conditions.

Balancing the sails and rig for optimal performance is a meticulous task. The sail plan should be designed to efficiently convert wind energy into forward motion while allowing for easy adjustments to adapt to changing conditions.

5) Balance

Balancing a sailboat is crucial for its performance and safety. Achieving the right balance involves a delicate interplay between the hull, keel, rudder, and sail plan. Proper balance ensures the boat remains stable and responds predictably to helm inputs, even in changing wind conditions.

Balance is not a static concept but something that evolves as the boat sails in different wind and sea conditions. Designers must anticipate how changes in load, wind angle, and sail trim will affect the boat’s balance.

Achieving balance is both an art and a science, and it often requires iterative adjustments during the design and testing phases to achieve optimal results.

6) Propulsion

While sailboats primarily rely on wind propulsion, auxiliary propulsion systems like engines are essential for maneuvering in harbors or during calm conditions. Integrating propulsion systems seamlessly into the boat’s design requires careful consideration of engine placement, fuel storage, and exhaust systems.

The choice of propulsion system, whether it’s a traditional diesel engine or a more eco-friendly electric motor, also impacts the boat’s weight distribution and overall performance.

7) Scantling

Scantling refers to the selection of structural components and their dimensions to ensure the boat’s strength and integrity. It involves determining the appropriate thickness of the hull, deck, and other structural elements to withstand the stresses encountered at sea.

Scantling is a critical aspect of sailboat design, as it directly relates to safety. A well-designed boat must be able to withstand the forces exerted on it by waves, wind, and other environmental factors.

8) Stability

Stability is a critical safety factor in sailboat design. Both upright hydrostatics and large-angle stability must be carefully assessed and optimized. This involves evaluating the boat’s center of gravity, ballast, and hull shape.

Achieving the right balance between initial stability, which provides comfort to passengers, and ultimate stability, which ensures safety in adverse conditions, is a delicate task. Designers often use stability curves and computer simulations to fine-tune these characteristics.

9) Layout

The layout of a sailboat’s interior and deck spaces is a blend of functionality and comfort. Designers must consider the ergonomics of living and working aboard the vessel, including cabin layout, galley design, and storage solutions. The deck layout influences crew movements and sail handling.

Layout design also extends to considerations like ventilation, lighting, and noise control. Sailboats are unique in that they must provide both comfortable living spaces and efficient workspaces for handling sails and navigation.

10) Design Evaluation

The final phase of sailboat design involves rigorous evaluation and testing. Computational fluid dynamics (CFD) simulations, tank testing, and real-world sea trials help validate the design’s performance predictions. Any necessary adjustments are made to fine-tune the vessel’s behavior on the water.

The evaluation phase is where the theoretical aspects of design meet the practical realities of the sea. It’s a crucial step in ensuring that the sailboat not only meets but exceeds its performance and safety expectations.

Conclusion:

In conclusion, sailboat design is a highly technical field that requires a deep understanding of hydrodynamics, engineering principles, and materials science. Naval architects and yacht designers meticulously navigate through the intricacies of hull design, keel and rudder configuration, sail and rig design, balance, propulsion, scantling, stability, layout, and design evaluation to create vessels that excel in both form and function. The harmonious integration of these elements results in sailboats that are not just seaworthy but also a joy to sail, and this process is a testament to the art and science of sailboat design.

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Basics of Sailboat Hull Design – EXPLAINED For Owners

There are a lot of different sailboats in the world. In fact, they’ve been making sailboats for thousands of years. And over that time, mankind and naval architects (okay, mostly the naval architects!) have learned a thing or two.

If you’re wondering what makes one sailboat different from another, consider this article a primer. It certainly doesn’t contain everything you’d need to know to build a sailboat, but it gives the novice boater some ideas of what goes on behind the curtain. It will also provide some tips to help you compare different boats on the water, and hopefully, it will guide you towards the sort of boat you could call home one day.

Displacement hulls, semi displacement hulls, planing hulls, history of sailboat hull design, greater waterline length, distinctive hull shape and fin keel designs, ratios in hull design, the hull truth and nothing but the truth, sail boat hull design faqs.

white yacht in body of water during daytime

Basics of Hull Design

When you think about a sailboat hull and how it is built, you might start thinking about the shape of a keel. This has certainly spurred a lot of different designs over the years, but the hull of a sailboat today is designed almost independently of the keel.

In fact, if you look at a particular make and model of sailboat, you’ll notice that the makers often offer it with a variety of keel options. For example, this new Jeanneau Sun Odyssey comes with either a full fin bulb keel, shallow draft bulb fin, or very shallow draft swing keel. Where older long keel designs had the keel included in the hull mold, today’s bolt-on fin keel designs allow the manufacturers more leeway in customizing a yacht to your specifications.

What you’re left with is a hull, and boat hulls take three basic forms.

Displacement hull
Semi-displacement hulls
Planing hulls

Most times, the hull of a sailboat will be a displacement hull. To float, a boat must displace a volume of water equal in weight to that of the yacht. This is Archimedes Principle , and it’s how displacement hulled boats get their name.

The displacement hull sailboat has dominated the Maritimes for thousands of years. It has only been in the last century that other designs have caught on, thanks to advances in engine technologies. In short, sailboats and sail-powered ships are nearly always displacement cruisers because they lack the power to do anything else.

A displacement hull rides low in the water and continuously displaces its weight in water. That means that all of that water must be pushed out of the vessel’s way, and this creates some operating limitations. As it pushes the water, water is built up ahead of the boat in a bow wave. This wave creates a trough along the side of the boat, and the wave goes up again at the stern. The distance between the two waves is a limiting factor because the wave trough between them creates a suction.

This suction pulls the boat down and creates drag as the vessel moves through the water. So in effect, no matter how much power is applied to a displacement hulled vessel, it cannot go faster than a certain speed. That speed is referred to as the hull speed, and it’s a factor of a boat’s length and width.

For an average 38 foot sailboat, the hull speed is around 8.3 knots. This is why shipping companies competed to have the fastest ship for many years by building larger and larger ships.

While they might sound old-school and boring, displacement hulls are very efficient because they require very little power—and therefore very little fuel—to get them up to hull speed. This is one reason enormous container ships operate so efficiently.

Of course, living in the 21st century, you undoubtedly have seen boats go faster than their hull speed. Going faster is simply a matter of defeating the bow wave in one way or another.

One way is to build the boat so that it can step up onto and ride the bow wave like a surfer. This is basically what a semi-displacement hull does. With enough power, this type of boat can surf its bow wave, break the suction it creates and beat its displacement hull speed.

With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat’s bottom to channel water away and sit on the surface. Once it is out of the water, any speed is achievable with enough power.

But it takes enormous amounts of power to get a boat on plane, so planing hulls are hardly efficient. But they are fast. Speedboats are planing hulls, so if you require speed, go ahead and research the cost of a speedboat .

The most stable and forgiving planing hull designs have a deep v hull. A very shallow draft, flat bottomed boat can plane too, but it provides an unforgiving and rough ride in any sort of chop.

white and gray boat on sea during daytime

If you compare the shapes of the sailboats of today with the cruising boat designs of the 1960s and 70s, you’ll notice that quite a lot has changed in the last 50-plus years. Of course, the old designs are still popular among sailors, but it’s not easy to find a boat like that being built today.

Today’s boats are sleeker. They have wide transoms and flat bottoms. They’re more likely to support fin keels and spade rudders. Rigs have also changed, with the fractional sloop being the preferred setup for most modern production boats.

Why have boats changed so much? And why did boats look so different back then?

One reason was the racing standards of the day. Boats in the 1960s were built to the IOR (International Offshore Rule). Since many owners raced their boats, the IOR handicaps standardized things to make fair play between different makes and models on the racecourse.

The IOR rule book was dense and complicated. But as manufacturers started building yachts, or as they looked at the competition and tried to do better, they all took a basic form. The IOR rule wasn’t the only one around . There were also the Universal Rule, International Rule, Yacht Racing Association Rul, Bermuda Rule, and a slew of others.

Part of this similarity was the rule, and part of it was simply the collective knowledge and tradition of yacht building. But at that time, there was much less distance between the yachts you could buy from the manufacturers and those setting off on long-distance races.

Today, those wishing to compete in serious racing a building boat’s purpose-built for the task. As a result, one-design racing is now more popular. And similarly, pleasure boats designed for leisurely coastal and offshore hops are likewise built for the task at hand. No longer are the lines blurred between the two, and no longer are one set of sailors “making do” with the requirements set by the other set.

Modern Features of Sailboat Hull Design

So, what exactly sets today’s cruising and liveaboard boats apart from those built-in decades past?

Today’s designs usually feature plumb bows and the maximum beam carried to the aft end. The broad transom allows for a walk-through swim platform and sometimes even storage for the dinghy in a “garage.”

The other significant advantage of this layout is that it maximizes waterline length, which makes a faster boat. Unfortunately, while the boats of yesteryear might have had lovely graceful overhangs, their waterline lengths are generally no match for newer boats.

The wide beam carried aft also provides an enormous amount of living space. The surface area of modern cockpits is nothing short of astounding when it comes to living and entertaining.

If you look at the hull lines or can catch a glimpse of these boats out of the water, you’ll notice their underwater profiles are radically different too. It’s hard to find a full keel design boat today. Instead, fin keels dominate, along with high aspect ratio spade rudders.

The flat bottom boats of today mean a more stable boat that rides flatter. These boats can really move without heeling over like past designs. Additionally, their designs make it possible in some cases for these boats to surf their bow waves, meaning that with enough power, they can easily achieve and sometimes exceed—at least for short bursts—their hull speeds. Many of these features have been found on race boats for decades.

There are downsides to these designs, of course. The flat bottom boats often tend to pound when sailing upwind , but most sailors like the extra speed when heading downwind.

How Do You Make a Stable Hull

Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel.

There are two types of stability that a design addresses . The first is the initial stability, which is how resistant to heeling the design is. For example, compare a classic, narrow-beamed monohull and a wide catamaran for a moment. The monohull has very little initial stability because it heels over in even light winds. That doesn’t mean it tips over, but it is relatively easy to make heel.

A catamaran, on the other hand, has very high initial stability. It resists the heel and remains level. Designers call this type of stability form stability.

There is also secondary stability, or ultimate stability. This is how resistant the boat is to a total capsize. Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability but very high ultimate stability. A sailor would likely describe this boat as “tender,” but they would never doubt its ability to right itself after a knock-down or capsize.

On the other hand, the catamaran has extremely high form stability, but once the boat heels, it has little ultimate stability. In other words, beyond a certain point, there is nothing to prevent it from capsizing.

Both catamarans and modern monohulls’ hull shapes use their beams to reduce the amount of ballast and weight . A lighter boat can sail fast, but to make it more stable, naval architects increase the beam to increase the form stability.

If you’d like to know more about how stable a hull is, you’ll want to learn about the Gz Curve , which is the mathematical calculation you can make based on a hull’s form and ultimate stabilities.

How does a lowly sailor make heads or tails out of this? You don’t have to be a naval architect when comparing different designs to understand the basics. Two ratios can help you predict how stable a design will be .

The first is the displacement to length ratio . The formula to calculate it is D / (0.01L)^3 , where D is displacement in tons and L is waterline length in feet. But most sailboat specifications, like those found on sailboatdata.com , list the D/L Ratio.

This ratio helps understand how heavy a boat is for its length. Heavier boats must move more water to make way, so a heavy boat is more likely to be slower. But, for the ocean-going cruiser, a heavy boat means a stable boat that requires much force to jostle or toss about. A light displacement boat might pound in a seaway, and a heavy one is likely to provide a softer ride.

The second ratio of interest is the sail area to displacement ratio. To calculate, take SA / (D)^0.67 , where SA is the sail area in square feet and D is displacement in cubic feet. Again, many online sites provide the ratio calculated for specific makes and models.

This ratio tells you how much power a boat has. A lower ratio means that the boat doesn’t have much power to move its weight, while a bigger number means it has more “get up and go.” Of course, if you really want to sail fast, you’d want the boat to have a low displacement/length and a high sail area/displacement.

Multihull Sailboat Hulls

Multihull sailboats are more popular than ever before. While many people quote catamaran speed as their primary interest, the fact is that multihulls have a lot to offer cruising and traveling boaters. These vessels are not limited to coastal cruising, as was once believed. Most sizable cats and trimarans are ocean certified.

Both catamarans and trimaran hull designs allow for fast sailing. Their wide beam allows them to sail flat while having extreme form stability.

white sailboat on sea near green mountain under blue sky during daytime

Catamarans have two hulls connected by a large bridge deck. The best part for cruisers is that their big surface area is full of living space. The bridge deck usually features large, open cockpits with connecting salons. Wrap around windows let in tons of light and fresh air.

Trimarans are basically monohulls with an outrigger hull on each side. Their designs are generally less spacious than catamarans, but they sail even faster. In addition, the outer hulls eliminate the need for heavy ballast, significantly reducing the wetted area of the hulls.

Boaters and cruising sailors don’t need to be experts in yacht design, but having a rough understanding of the basics can help you pick the right boat. Boat design is a series of compromises, and knowing the ones that designers and builders take will help you understand what the boat is for and how it should be used.

What is the most efficient boat hull design?

The most efficient hull design is the displacement hull. This type of boat sits low in the water and pushes the water out of its way. It is limited to its designed hull speed, a factor of its length. But cruising at hull speed or less requires very little energy and can be done very efficiently.

By way of example, most sailboats have very small engines. A typical 40-foot sailboat has a 50 horsepower motor that burns around one gallon of diesel every hour. In contrast, a 40-foot planing speedboat may have 1,000 horsepower (or more). Its multiple motors would likely be consuming more than 100 gallons per hour (or more). Using these rough numbers, the sailboat achieves about 8 miles per gallon, while the speedboat gets around 2 mpg.

What are sail boat hulls made of?

Nearly all modern sailboats are made of fiberglass.

Traditionally, boats were made of wood, and many traditional vessels still are today. There are also metal boats made of steel or aluminum, but these designs are less common. Metal boats are more common in expedition yachts or those used in high-latitude sailing.

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

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4.2 Conceptual design in sailing boat hulls

Hull shapes for sailing boats typically fall into the three types shown in Figure 22 . The 'fin and skeg' design is typical of modern mass-produced yachts and small dinghies. The long-keel design is typical of working boats designed to be handled by a small crew. These two forms represent extremes of stability and performance. The dish shaped hull of the fin-and-skeg will skate on the sea's surface and turn quickly, whereas the long-keel yacht will be stable in a rough sea and manoeuvre slowly enough for a small crew to have time to handle sails and ropes. The compromise design is typical of a 1950s yacht. It has better turning performance than the long-keel design and more stability than the fin-and-skeg design.

Image showing three forms of sailing boat hull

Many large, modern sailing boats, designed and made for both cruising and racing, have the same overall form as small dinghies. These dinghy shapes are cheap to manufacture, lively, quick, and none too comfortable. You might imagine that they are advertised for speed rather than ease and cheapness of manufacture.

The sketch of three hull forms shows no superstructure (cabins etc.) on the hulls. Clearly somewhere must be provided for the crew to eat and sleep, for storage of sails and equipment etc. Depending on the design of the hull of the yacht, there will be different available solution spaces.

There is a clear compromise between sailing performance and interior space that a designer must consider. Anyone sailing on the boat will need to have headroom. This can be achieved either by building up from the deck level of the keel – which introduces surfaces that are exposed to the wind and affects the sailing properties – or by situating the living space inside the keel itself. If a fin-and-skeg design is chosen the only way to provide headroom is to build upwards, whereas a long-keel design can provide headroom inside the hull. Whether the headroom that is required is for standing or sitting extends the problem. A large, high cabin to maximise the interior space and carry people in well-lit comfort is a common design solution for modern pleasure yachts that are unlikely to be used in difficult conditions.

Consider now, how space might be subdivided in the two extremes of fin-and-skeg design and long-keel design. A decision tree can be created to show the different design solutions, Figure 23 . (Note that for simplicity the fin-and-skeg design is shown simply as a finned keel.)

The design decisions concerning how space is apportioned as the design progresses are shown as a hierarchy. Clearly, the choice between a shallow hull with a fin keel and a deep hull with a long keel is a high-level decision that imposes significant constraints on the decisions that follow. The next choice, between a low and a high cabin, on the fin–keel branch of the tree, will lead to very different designs of yacht. The long cockpit, for easy sail handling, naturally goes with a low cabin, for reduced wind resistance, on a racing design. Going for the high-cabin option creates a design where interior accommodation is more important than performance, and a shorter cockpit gives more length in the cabin to split up the space between cooking facilities, lavatories and bunks. It is unlikely that the buyer of a high-cabin boat will want a large cockpit. The low cabin design will give interior space for a sitting person, but not for a standing or a stooping person.

Figure showing a decision tree of yacht designs

With a long-keel design (top left of Figure 23 ), the extra depth in the hull means that a high superstructure is not required, so the first decision shown is a choice between a high (and therefore wide) floor, giving sitting room, and a low floor, giving standing room. The compromise here is to accept sitting room as a price worth paying for a large floor area that gives more freedom to arrange tables, beds, sinks, lockers and chairs in different ways.

A variable more significant in the high-floor design is the width of the cabin superstructure. A wide cabin gives narrow side decks that inhibit working above deck but might give more headroom inside the yacht. A design where a person can sit underneath a side deck gives a very different use of space than a design where sitting is only possible under the cabin roof.

A further branching of the tree is shown on the basis of a decision about the length of the cockpit and how the room underneath the cockpit floor is used. Shown are two design solutions that show sitting and lying room underneath. Clearly another solution is to have no interior space under the cockpit floor, but to use it all for lockers accessible from the outside. These might be used to store sails and rope.

From this decision tree it can be seen that the design choices about geometry are connected and hierarchical. Also the choices can be made from their implications on the performance of the yacht, without having to model the detail of the aerodynamic and hydrodynamic principles which govern the yacht's performance.

The route taken through this maze of interlinked decisions can lead to yachts of very different types and functionalities.

How does the initial choice of fin-and-skeg or long-keel design for the hull influence the provision of a superstructure for accommodation?

The long-keel design has more available space within the keel, so it is not necessary to build as large a superstructure on the deck. For equal available space, a fin-and-skeg design requires more superstructure to be built above the level of the deck.

In the next section, two different design solution spaces are going to be evaluated with the help of some simple formulae.

Types of Sailboat Hulls

Last Updated by

Daniel Wade

June 15, 2022

Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls.

There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats, and dense full-keel displacement cruisers. Multihull designs include catamarans and trimarans.

In this article, we'll cover the most common types of sailboat hulls along with their best uses. We'll explain the difference between monohulls and multihulls, along with how keel shape influences sailboat performance.

We sourced the information for this article from sailing experts, hull shape guides, and the written wisdom of famous sailboat designers. Additionally, we researched sailboat sales figures to determine the most popular vessel configurations available today.

Table of contents

‍ Importance of Sailboat Hull Design

A sailboat is defined by its rig and hull shape. Sailboat hull shape is one of the deciding factors on how it will handle. Additionally, the shape (and displacement) of a sailboat hull can be used to determine its strengths and weaknesses. Learning about sailboat hull shape can help you understand what kind of boat you need and what your vessel is capable of.

You can easily categorize sailboats based on their hull shape. For example, a heavy deep-draft displacement hull is likely a slow, steady, and comfortable cruiser. In contrast, a sleek flat-bottomed sailboat or catamaran is likely built for speed and could easily outpace even the most nimble displacement cruisers.

The most common kind of sailboat is the monohull. When you think of a sailboat, probably think of a monohull. The term simply means that the vessel has one single hull and nothing more. This is in contrast to multihulls such as catamarans, which are easy to spot and differentiate from traditional designs.

Monohulls are popular because they work. They're easy to build and narrow enough to fit in most marina dock spaces. Monohulls are also generally easy to handle in a variety of conditions, both fair and foul.

One drawback of monohull designs is that they are not quite as stable as most multihulls, though monohulls can recover more easily from a serious roll or capsize. They also cost a lot less, as the vast majority of production sailboats ever constructed were of the same basic single-hull configuration.

Centerboards and Swing Keels

The windward performance of sailboats is greatly improved by the use of a long keel or centerboard. The centerboard is the most simple type of stabilizing device used on sailboats. Usually, the centerboard is simply a long fin that protrudes from the bottom of the hull.

The centerboard keeps the boat on track when the wind is not moving in the boat's direction of travel. This is why sailboats can sail at different angles to the wind without being pushed to the side. A key characteristic of centerboards is that they can be raised and lowered, which is convenient on small boats that need to be trailered or beached.

Swing keels are similar to centerboards in that they can be raised and lowered, though they pivot on a hinge instead of sliding up and down in a truck. Swing keels are either recessed into the hull or held in a housing just below it, which usually also contains much of the boat's ballast. Swing keel designs free up cabin space that would normally be occupied by a bulky centerboard trunk.

Centerboards and most swing keels are an alternative to a permanently affixed keel. They're generally not considered to be as seaworthy as other hull designs, so their use is confined primarily to inland and coastal cruising.

Monohull Sailboat Hull Shapes

When in the water, it's difficult to distinguish between the different types of monohull shapes. In most cases, you have to pull the boat out of the water to figure out what hull shape you're dealing with. Next, we'll go over the most common monohull sailboat shapes and their uses.

Flat-Bottom Sailboats

Flat bottom sailboats are the easiest to build and often the fastest. These vessels have a very shallow draft and are often lightweight, so they slide easily and quickly across the water. Flat bottom sailboats make excellent racing boats and 'gunkholers,' which are primarily used for camping and hopping between shallow Islands.

Flat bottom sailboats usually have centerboards or swing keels, which makes them great for shallow water, beaching, and towing on a trailer. The use of flat bottom sailboats is confined primarily to inland and coastal waters, as a flat bottom does not handle well in swells and rough weather. Flat bottom sailboats pound hard on chop, and they lack the low center of gravity that's necessary for good stability.

Fin Keel Sailboat Hulls

The fin keel is a popular alternative to centerboards, and vessels utilizing this low-profile hull shape have proven to be quite seaworthy. Fin keels are popular on fast racing boats and lightweight cruisers. A fin keel resembles a centerboard, but it usually extends much further from the base of the hull.

The majority of a sailboat's draft comes from the fin keel, as the hulls of these sailboats tend to be rounded and shallow. They resemble flat-bottom designs, but slight rounding significantly increases comfort. Fin keel sailboats are ideal for racing and coastal cruising, and some models can be used for extended offshore passages.

Bulb Keel Sailboat Hulls

A bulb keel sailboat hull usually resembles most fin keel varieties. The hulls of these vessels tend to be shallow and rounded, with a long and thin fin extending from the base of the hull. A bulb keel is essentially just a thin blade with a bulb on the bottom.

Bulb keels are different from fin keels as they usually contain additional ballast weight for stability. The hydrodynamic properties of bulb keels are proven to be efficient. As a result, these boats can also be quite fast. In a direct comparison, a vessel with a bulb keel will likely be more seaworthy than the same sailboat with only a fin keel or a centerboard.

Bilge Keel Sailboat Hulls

The hull shape of a bilge keel sailboat usually resembles that of a bulb or fin keel sailboat, with one major distinction. Instead of one long and thin keel descending from the center of the hull, a bilge keel sailboat has two lengthier fins offset on the port and starboard side.

The idea behind the bilge keel design is that when the vessel heels to one side, one of the two keels will be straightened out. This, in theory, provides better tracking and improves stability. It also distributes ballast evenly on both sides. Bilge keels can also improve motion comfort, and they can reduce the vessel's draft by a small margin.

Bilge keel sailboats offer a balance between seaworthiness and speed. These vessels can be used as bluewater cruisers and coastal cruisers. They can also hold their own in any yacht club regatta.

While a bilge keel sailboat may not be ideal for cruising the North Atlantic during the winter, it can certainly make a safe and comfortable passage maker that can gain a knot or two of speed above its heavier counterparts.

Semi-Displacement Sailboat Hulls

Now, we'll look at some true bluewater cruising designs. The semi-displacement hull features a long and deep keel that runs from about the center of the hull all the way back to the rudder. Semi-displacement hulls get deeper the further back you go, reaching their longest point at the very aft end of the boat.

The offshore benefits of a long and deep keel are numerous, as this hull shape provides an enormous amount of stability and a very low center of gravity. The design itself it's quite old, and it's featured on many classic cruising sailboats and workboats.

Though less common in the modern era than more contemporary fin keel designs, a traditional semi-displacement sailboat offers easy handling and enhanced motion comfort. Semi-displacement hulls tend to have a deep draft and therefore are not ideal for shallow water. They handle confidently in all conditions, though they usually aren't as fast as newer designs.

Displacement Sailboat Hulls

Displacement hulls, also known as full keel hulls, are the bulldozers of the sailboat world. These traditional vessels are deep, heavy, relatively slow, and capable of plowing through the roughest weather conditions.

Displacement hulls have a long keel that begins at the bow and extends all the way after the rudder. Like semi-displacement hulls, full keel sailboats offer excellent motion comfort and confident handling.

Displacement hulls have the best directional stability and downwind maneuvering abilities. Their handling is more forgiving, and they're less jumpy at the helm. Many of these boats heel gently and give the crew more time to respond to changing conditions.

The primary downside to displacement hulls is their high cost and sheer mass. Displacement boats are large and take up a lot of space. They're usually too tall and heavy for trailering, so they tend to remain in the water most of the time.

Displacement hulls aren't made to just sit at the dock or jump around the lake; they're designed for real-deal offshore sailing. They also have the roomiest cabins, as the hull extends further down and longer than any other hull shape.

Now, let's examine multihull sailboat designs and why you may want to consider one. Some of the earliest seagoing vessels had multiple hulls, usually featuring one long hull (occupied by the crew) and a small stabilizing hull off to one side.

Multihulls have only recently become popular, and they make up a decent portion of the modern production boat market. This is because of their numerous design benefits and spacious cabins. Multihulls are almost guaranteed to be more expensive than monohulls (both new and used), and the used market is still saturated with expensive luxury cruising sailboats.

Modern multihull sailboats feature a large pilothouse in the center and plenty of cabin space in each full-size hull. They offer excellent motion comfort and achieve very high speeds. Due to their wide beam, they provide spacious living spaces and excellent stability. Here are the two main types of multihull sailboats.

From above, a catamaran looks like two thin monohull sailboats lashed together and spaced apart. Fundamentally, that's exactly what they are. Except catamarans have a very shallow draft and the capability to reach very high speeds.

Catamarans have two hulls instead of one, and each hull is typically a mirror of the other. They achieve their space using width rather than length, so a 30-foot catamaran has significantly more interior room than a 30-foot monohull.

Their primary drawback is that, due to their width, catamarans usually require two standard dock spaces instead of one. But at sea, they don't heel over dramatically like monohulls, which makes them much more comfortable to eat, sleep, and cook inside of.

Trimarans follow the same basic design principles as catamarans, except they have a third hull in the center. From above, a trimaran looks like a monohull with two smaller hulls lashed to the sides. Unlike a catamaran, the primary living space of a trimaran is in the large center hull. Trimarans are essentially just monohulls with stabilizers on the side, resembling ancient sailing canoes.

Trimarans have the same spatial and stability benefits as catamarans, though they can achieve higher speeds and better sea keeping. This is because of the additional stability provided by the center hall. Trimarans tend to be costlier than catamarans, though many sailors believe that the benefits outweigh the cost.

Best Sailboat Hull Shape for Speed

If we take wave height and weather conditions out of the equation, the fastest sailboats are usually the longest. Sailboats are limited by hull speed and sail plan size regardless of their hull shape. That said, the fastest sailboats tend to be flat bottom monohulls, fin keel monohulls, and trimarans.

Best Sailboat Hull Shape for Motion Comfort

The best sailboat for motion comfort is the catamaran. These wide and seaworthy vessels 'stance up' and minimize rolling. They also come close to completely eliminating heeling.

Wide and stable multihulls are popular because they alleviate some of the most common complaints of sailors. Trimarans are also an excellent choice for comfort, as their stabilizers minimize the effect of rolling in heavy seas.

Most Seaworthy Sailboat Hull Shape

Today, many people consider multihulls to be the most seaworthy design on the market. However, seaworthiness is more than just average stability in rough weather. Many Sailors argue that traditional displacement sailboat hull designs are the most seaworthy.

Displacement hulls have a low center of gravity which improves their knockdown survivability. In other words, in the (rare) event of a displacement boat knockdown, the weight of the keel is more likely to swing the boat back up and out of trouble. Multihulls cannot recover from a knockdown, as they like the pendulum-like recoil ability.

Most Spacious Sailboat Hull Type

The most spacious hull sailboat type is the catamaran. Catamarans have two nearly full-size hulls (one on each side) plus a large central pilothouse that resembles the main cabin of a large powerboat.

Many typical catamarans fit an entire kitchen into the Pilot House along with four private births and two full-sized heads in its hulls. Some mid-size catamarans even come with a bathtub, which is essentially unheard of on equivalent monohulls.

Spaciousness varies on small monohulls. Larger cabins are usually found on bulb and bilge keel designs, as swing keel and centerboard boats need somewhere to hide their skegs. Centerboard boats are the least spacious, as the centerboard trunk must occupy the middle of the cabin space.

I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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The Illustrated Guide To Boat Hull Types (11 Examples)

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

What are the different boat hull types? There are three boat hull categories: displacement hulls, which displace water when moving; planing hulls, which create lift at high speeds; and semi-displacement hulls, which displace water and generate lift at low speeds. The most common hull types are round-bottomed, flat-bottomed, multi, V-shaped, and pontoon hulls.

But that's all pretty abstract if you ask me, so below I'll give a simple overview of what it all means. After that, I'll give a list with pictures of all the different designs.

A Simple Overview of Boat Hull Types

Your boat hull will be the biggest factor in how your boat handles or sails, how wet it is, how bumpy - absolutely everything is determined by the hull shape. So it's important to understand what different hulls will do for you, and what each hull is best for. First, let's slice it up into rough categories.

Roughly, you can divide boat hulls into three categories:

Displacement hulls - Lie inside the water and push it away when they move
Planing hulls - Lie on top of the water and don't push it away
Semi-displacement hulls - Lie inside the water and push it away, but can generate lift

Everything I'll be mentioning below is one of those three, or something in between.

There are five common boat hull types:

Round-bottomed hulls - handle well in rough water: sailboats
Flat-bottomed hulls - very stable for calm inland waters: fishing boats
Multihulls - very stable and buoyant: catamarans
V-Shaped Hulls - fast and comfortable in chop: powerboats
Pontoon hulls - fast and stable: pontoon boats

And then there's everything in-between.

Here's a quick and handy overview of the different hull types

In each category, we find different designs and styles that have different characteristics. There isn't a real clear distinction between categories and styles: there are semi-displacement hulls and so on. So I thought the best way to learn you the different hull types is by simply creating a list with lots of pictures, instead of getting all theoretical about it.

So below I've listed all the different hull styles I could possibly think of, mention what category and type it is, the pros and cons of each one, and give you examples and illustrations for each one.

Displacement hulls displace water when moving. These hulls lie in the water, instead of on top of it. The amount of water they displace is equal to the boat's weight. Displacement hulls handle way better in rough waters than flat-bottom hulls. That's why most cruisers have some sort of displacement hulls. There are actually all kinds, shapes, and forms of the displacement hull design, which we'll go over later.

The most important thing to understand about the displacement hull, is that it operates on buoyancy. This means that most of the boat's weight is supported by its capacity to float . Planing hulls, on the other hand, operate on lift instead, but we'll dive into that later.

Sailboats typically have displacement hulls, but also fishing boats, trawlers and crabbers. All in all, it's used for each boat that needs to handle well in rough conditions.

Learn everything there is to know about displacement hulls in this article . It lists all the pros and cons and really goes into detail on the nitty-gritty about how displacement hulls actually work .

But they are also slower than flat and planing hulls because the boat creates more resistance when moving. It has to push the water aside. In fact, this type of hull has a built-in upper-speed limit.

This upper-speed limit is called maximum hull speed . It means that the length of a displacement hull directly determines the maximum speed. It can't go faster, because the water-resistance increases with the boat's speed. To learn everything about calculating maximum hull speed , please check out my previous article here.

A round-bottomed hull is a type of displacement hull - it lies in the water and has to power through it. But since it's rounded, it creates little resistance and is effortless to move through the water. It's a very smooth ride and typical for any sailboat that sort of glides through the waves. In contrast, powerboats really have to eat their way through the water.

Examples: Canoes, sailboats

They are also one of the least stable. Since the bottom is rounded, your boat or canoe will rock plenty when boarding or moving around. They are also easy to capsize. That's why pro canoers learn to do a 360 in their canoes. I've never did a roll myself but came close enough a couple of times.

Almost all sailboats use a round bilge as well. This provides it its buoyancy and makes sure it handles well in waves. But since a rounded bilge is easy to capsize, a lot of sailboats have some sort of keel, which stabilizes the roll.

Nearly all ocean-going vessels use some sort of displacement hull, and the round bottom is the most common one. But our next guest is very popular as well.

The catamaran is similar to the pontoon hull (read on to learn more on that one), but it is a displacement multihull instead of a planing one. So it has two hulls, that lie inside the water and displace it. Like the pontoon, you will have to try really hard to capsize this design (and it won't work).

Examples: well, catamaran sailboats. But also this cool catamaran trawler:

Catamarans are extremely popular ocean cruisers. Their biggest pro is their extreme stability and buoyancy. And they have a very shallow draft for a displacement hull, making them very popular for sailing reefs and shallow waters, like the Caribbean.

Some cons for the catamaran are less agile than monohulls. They have a large turning radius, making them less maneuverable. Also, expect to pay high marina fees with this one.

Speaking of marina fees, our next one can go either way.

I think trimarans are incredibly cool, and especially the second type.

There are two types of trimarans:

a catamaran with three hulls instead of two,
or a displacement monohull with two floaters.

The first has the same characteristics as the catamaran: it's a displacement multihull, but now with three hulls:

The second can be a regular displacement monohull, with two pontoon-type floaters that provide extra buoyancy, making the total thing a hybrid between pontoon and displacement:

This last one has all the pros of a catamaran in terms of stability, but: you can simply wheel in those floaters whenever you head for port. That saves you a lot of money. And you can trailer her! Imagine that, a towing a trimaran home.

So those were the most common displacement hulls, aka what lives in the water. Let's move on to the planing hulls, aka what lives on the water.

Planing hulls are a hybrid between the flat-bottom and displacement hulls. Planing hulls displace water at low speeds , but create lift at higher speeds . The shape of their hull + speed lifts them out of the water, making them glide on top of the water. Most powerboats look like flat-bottom boats but use a shallow V-shape that helps the boat to handle better at higher speeds.

Examples: Water sports boat, powerboats

The most important thing to understand about planing hulls is that they operate mainly on lift instead of buoyancy. This means the weight of the boat is mainly supported by dynamic forces 1 . With the right amount of power, this design generates lift, which results in less resistance. This is why they are a lot faster than boats with displacement hulls, but also a lot rougher, even with mild chop.

A lot of powerboats use some sort of planing hull. Again, there are many designs and variations on the planing hull, and I'll try to mention as many as I can below.

Because the wedge of the hull runs into the water, it is much easier to handle at high speeds. At lower speeds, it is able to keep its course, even with a bit of wind. However, whenever the boat starts planing, it is prone to wind gusts, since the wedge shape no longer stabilizes the boat.

The flatter the hull, the faster it will go, but also the more poorly it will handle. Other powerboats use deep V-hulls, which I'll discuss below. But first, let's take a look at the flattest hulls you'll ever see.

A flat-bottom hull lies on top of the water and doesn't displace water (okay, very little) as it moves. Since there is no displacement, there is also little to no friction when moving. This makes it potentially fast, but it handles pretty poorly. It is one of the most stable hull design.

Examples: rowboats, (old) high-performance powerboats, small skiffs, small fishing boats, tug boats

They aren't just incredibly stable, they're also very practical. Because the bottom is practically flat, they maximize boat surface. But they are also extremely choppy in rough weather and waves. They will handle very poorly with stiff winds, as the wind can simply catch them and blow them across the water surface. That's why this design is almost exclusively used for calm, small, inland waters.

This type of hull operates mainly on buoyancy , like the displacement hull, but it doesn't require the same amount of power to propel, which is why it's faster.

Because of the uncomfortable ride, not a lot of boats use a perfectly flat bottom. Most boats nowadays use some sort of v-hull or hybrid design, like a semi-displacement hull; especially larger boats. So not a lot of boats have a real flat bottom. However, we do call a lot of boats flat-bottomed. How come?

There are two types of hulls we call flat-bottoms:

Of course boats with an actual flat bottom
Boats with almost no deadrise

What is the hull's deadrise? The deadrise is the angle of the front of the hull to the horizontal waterline.

As you can see, the green sailing dinghy in the picture above has a deadrise that's barely noticeable.

Let's move on to other variations of the planing hull. One of the most popular hull design for modern-day powerboats is the Deep Vee hull. And that's as cool as it sounds.

This is a type of planing hull that combines the best of both worlds.

These types of hulls are very popular on modern-day powerboats, and no wonder. With a V-shape that runs from bow to stern, deep into the water, you can handle this boat even in offshore conditions. It handles a lot better than flat-bottomed hulls, while it's at the same time extremely fast.

Examples: Most modern powerboats.

The Deep V-shape acts as a tiny keel of sorts, stabilizing the boat and making it more reliable and maneuverable. The rest of the hull acts as a planing hull, giving the boat its fast edge. Even at high speeds, the Deep V will cut into the water, making it more handleable.

The deep-V design is just one of many variants on the V-hull. Below we'll talk over another, the modified V hull.

The modified V hull is the ultimate crossover of all planing hull types. It's a mix of the flat-bottom and Deep V hull. It is one of the most popular hull designs for small motorboats. It's flat in the back and then runs into a narrow V-shape to the front. The flat back makes it more stable, and adds a little speed, while the V-shape front ensures good handling.

It is, in short, kind of the compromise-family-sedan of boat hulls. It's the fastest design that's also stable, that's also safe, and that also handles well. But it's not the best in any of those things.

Most powerboats you've seen will have some sort of Vee or Modified-V hull.

Stepped hulls are used on high-performance powerboats. It's a type of planing hull that reduces the hull surface by adding steps, or indents in the hull below the waterline. It looks something like this:

It is said to work extremely well at high speed (60 knots and up) and adds up to 10 knots to your top speed.

On to our next design. There are also planing multihulls, and they might even look like catamarans to you. Meet the pontoon hull.

Pontoon hulls float on top of the water using pontoons or floaters that create lift. It's a type of planing multihull that doesn't lie in the water, so it doesn't displace a lot of water. They don't really handle well. As with any multihull, they aren't agile - they're not great at maneuvering. They also have a very large turning radius. But they are extremely stable: there's no chance you'll capsize this.

Examples: Cruisers, modern trawlers, motor yachts, Maine lobster boats

Semi-displacement hulls are smack bang in the center of planning and displacement hulls. They are a bit better for speed than displacement hulls are. They are a bit better for handling rough waters than planing hulls are. This makes them very versatile.

You can see these a bit like being 'half-planing' hulls. These hulls are designed to plane at lower speeds than normal planing hulls - somewhere in the range of 15 - 20 knots, depending on the length of the boat. It also requires less power. When the hull lifts, it reduces drag (water resistance), making it faster and more efficient.

Semi-displacement hulls are perfect for boats that need to be steady and seaworthy but fast at the same time.

For more information about semi-displacement hulls, please check out my in-depth guide to semi-displacement hulls here . It has a diagram and lists all the pros and cons.

So those were my 11 examples, and my step by step explanation of the different types of boat hulls and functions. You now have a solid basic understanding of boat hulls, and can recognize the most common ones. I hope it was helpful, and if you want more good sailing information, be sure to check out my other articles below.

https://www.soundingsonline.com/boats/how-different-hull-types-react-in-rough-water . ↩

I was wondering what your opinion would be on the ship uss Texas as far as hull type and bow type. I think it has a plumb bow and it looks to have a displacement or flat bottom hull. Im doing some research and a better trained eye would be of great help. I used images “bb-35 dry dock” to help see the hull shape. Thank you

Shawn Buckles

Hi Kirk, I don’t know about trained but here we go. I’ve checked the picture, it’s definitely a displacement hull I’d also say it’s a plumb bow.

Hahahahaa imagine liking boats hehehehehe Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water

You may also like, a complete guide to displacement hulls (illustrated).

The displacement hull is the classic go-to hull design for sailboats and one of the most recognizable ones out there. In this guide, I explain all there is to know …

Semi-Displacement Hulls Explained (Illustrated Guide)

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Professional BoatBuilder Magazine

Notes on sailing-yacht hullforms, part 2.

By Jay Paris , Mar 22, 2022

In “Notes on Sailing-Yacht Hullforms” ( Professional BoatBuilder No. 196, page 32) naval architect Jay Paris, citing drawings and photographs of his own designs as well as the lines of other designers, detailed some characteristics, attributes, and design methods used to develop monohull sailboat hullforms. Because of space constraints in the print magazine, we are publishing Paris’s additional notes and reference material on Proboat.com. In addition to his sections on fin-keel variants and parent hullforms, Paris includes a reference list of hullforms worthy of note for a number of reasons—aesthetics, excellence as examples of their type, historic significance, or design methodology. All but one of the hullforms in this Part 2 are round bottomed, and all the yachts cited, which were built during the past century, are wooden boat designs. The lengths for the hullforms in this article are length of hull (LOH) since length overall (LOA) includes extensions to the hull such as bowsprits and boomkins and length on deck (LOD) is often less than the hull due to bulwarks and reverse transoms. —Ed.

Twin-Keel and Retracting-Foil Variants

A variant of the fin-keel design is the twin-keel. Most common on coastal cruisers intended to take the ground when the tide is out, they reflect an empirical approach to their design. A few are serious offshore passagemakers, including the noteworthy Bluebird of Thorne .

The Bluebird's hullform is unusual for twin keel yachts

This design is the combination of Robb’s talent and tank testing along with Lord Riverdale’s experience with his previous Bluebirds when he was R.A, Balfour. Bluebird of Thorne ’s keels canted outboard at 20°, and normal to her near circular midsection, were low aspect ratio, highly swept back asymmetrical foils that Robb implied were towed inboard and certainly were more sophisticated than those of almost all other twin keelers. Robb’s strong favorable opinions about this design concept have been frequently quoted in detail.

A more current fin-keel variation is the use of canting keels, with or without lift-generating sections, employed on offshore racers to shift the ballast bulb to weather to increase their righting moments.

Parent Sailing-Yacht Hullforms and Evolution

Historically, designers have used specific earlier boats as the basis for follow-on designs, in some cases very closely but with changes in the overall size and proportions. For example, Nathanael Greene Herreshoff had noticeable success using his 26 ’ (7.92m) Alerion III NGH 718 of 1912 as the parent for designs as large as the 33.42 ’ (10.19m) Newport 29 of 1914 and the 43.25 ’ (13.18m) Fishers Island 31, designed by his son Sidney using NGH’s offsets.

The lines shown incorporate the cutaway deadwood aft with a balance portion of the rudder forward of the rudderstock as found in the Alerion copies by the Sanford Boat Company. Eliminating these features would restore the lines to the original NGH configuration.

Maynard Bray, in his book Aida , includes a photograph of Herreshoff’s offset booklet pages used in the mathematical expansion to create the 33.67 ’ (10.26m) Aida of 1926. The Buzzards Bay 25 of 1914 looks perhaps to be the least likely of being an Alerion derivative.

The Buzz Bay's hullform also packs some punch

L.F. Herreshoff said the Buzzard’s Bay 25 was his favorite of his father’s designs.

Another famous parent hullform is the Pilot S&S 539, 32.92 ’ (10.03m) wooden stock class sloop of 1945. Olin Stephens wrote in Lines 2002 that the Pilot was “drawn in the Boston office by K. Aage Nielsen, who was, in theory, working under my supervision.” The later S&S 1727 fiberglass version from the New York office was lengthened to 36.07 ’ (10.99m).

Since there are not lines available in my references for the 1962 Pilot, this rendering by Al Mason gives a good sense of her hullform. Mason drew perspectives for many S&S designs.

The beautiful Anitra S&S 1358 48.42 ‘ (14.76m) yawl of 1958, the largest of the Pilot family, was the overall winner of the 1959 Fastnet Race.

In my own practice, The Paris Design P-32 (9.75m) Petrel is the parent of the later P-37, G-37, and OCC-47 designs.

While the P-32 hullform had many atypical features, a number of them were optimized for her longitudinally framed wood-foam-wood composite construction.

The P-32’s longitudinals are shown fitted into the molds setup at her stations. The clamps and longitudinals are laminated mahogany with the latter located on her waterlines ready for the double-diagonal planking to be glued and fastened. The temporary pine longitudinals in the underbody will be protected from any glue by duct tape prior to the first layer of planks being installed. The staples into the longitudinals are pulled as the second layer of planking is glued and stapled to the first. All permanent staples are Monel. The uppermost pine longitudinals are just above the tops of the bulwarks.

Those using state-of-the-art yacht design software often morph parents to the extent that their design “DNA” is no longer apparent.

Hullform evolution is similar but less slavishly derivative than the use of parent hullforms. The Viking ships of a thousand years ago are candidates for the most elegant hullforms of all time. The Scandinavian preference for double-enders has followed this fashion in some degree to modern times. Colin Archer’s Redningsskoite (rescue boats), which evolved from them, have inspired designers to this day. William Atkin’s 38 ’ (11.58m) ketch Ingrid , his highly refined granddaughter of this type, is considered to be one of his best designs.

Ingrid ’s fine lines with a wineglass midsection and 6 tons of outside and internal ballast contribute to the design’s good performance.

Knud Reimers’s Cohoe is a 1932, 32 ’ enlargement of his famous and beautiful 27 ’ (8.23m) Tumlare of 1927. These designs introduced lighter displacement and finer lines than earlier Scandinavian double-enders. Many consider the Tumlares to have the most beautiful hullforms of all time.

Knud Reimers, well-known for his Square-Meter boats, applied the same design philosophy to this double-ended cruiser. Reimers and I discussed his diagonals and contracted fairing approach during a week of sailing in the early 1970s.

A small Danish fishing boat type with very full sterns typical of the sound north of Copenhagen was the parent of the Spidsgatter classes of sailing yachts. Aage Nielsen, a Dane, was familiar with the type. Some of his smaller designs from 15 ’ to 26 ’ (4.57m to 7.92m) were referred to as Nielsenized Spidsgatters. Their full sterns were models for that of Holger Danske , his 42.5’ (12.95m) cruising ketch that won the 1980 Newport-to-Bermuda Race by a large margin.

One of Aage Nielsen’s favorite designs, Holger Danske , had a slippery hull, a low rating, and sailed like fury on a reach.

Over time, many working craft have served as models for sailing yachts. The danger, at times, has been designers ignoring the original function, loading, and the wind and sea conditions for which working craft were developed.

Design evolution can also be seen amongst ocean racers. Olin Stephens’s Stormy Weather came seven years after the famous Dorade of 1929, which had introduced the forms of his earlier inshore Six-Meter designs to the offshore world. These two sisters established a trend for blended-body ocean racers for some four decades.

Stormy Weather , slightly longer than Dorade , of similar displacement but with 22% more beam, was a favorite of her designer.

Sources From My Library

Starting before high school with the original five Uffa Fox books of 1934/1938, my office library has grown to almost a thousand books on sailing and sailing yachts with most containing technical material as well as numerous technical reports. I also have some 100 volumes addressing hydrodynamics, aerodynamics, and aerospace design containing material applicable to sailing yachts not addressed in yachting publications.

Lines drawings from my library that I consider worthy of note and reference but not embedded in the text are shown below with captions and sources. Note: Lines in the early five Uffa Fox books were redrawn by Fox.

Starling Burgess. Nina , 59’ Staysail Schooner, 1928.

Just one of Starling Burgess’s many beautiful sets of lines from the multi-talented aviation pioneer and yacht designer. Nina ’s blended body hullform was a very early example of the long waterline with short overhangs concept currently (2022) in vogue.

Howard I. Chapelle. Clipper-Bowed Centerboarder, 1936

Howard I. Chapelle was an eminent yacht designer and a historian who chronicled the designs of American working sailing craft. He chose a traditional clipper-bowed centerboarder to illustrate the steps in developing a lines drawing in his classic text on design of 1936.

Frederick A. Fenger. Diablesse , 38’ Wishbone Ketch, 1935

Frederick Fenger was an advocate of the dhow hullform with its unusual maximum draft near the bow. Diablesse ’s maximum draft was 23% of the waterline abaft the bow. Her forebody had V-shaped sections, hollow waterlines, and easy diagonals.

William Garden. Oceanus , 60’ Narrow Doubled-Ended Sloop, 1954

An example of a designer’s atypical boat for himself. An elegant, narrow sixty-footer (18.29m) intended for a crew of two, Garden and his wife. Interestingly, to quote an article in the January 1961 issue of Motor Boating : “ Oceanus was launched bottom up; there were considerable advantages in building her hull that way, and she was run into the water, parbuckled, and pumped out at insignificant cost and no risk compared with the tricky job of turning a hull of that size on shore.”

N.G. Herreshoff. Aida , 33.67’ Keel/Centerboard Yawl, 1926

Lines drawn by Doug Hylan from Original HMC 1002 Offsets. For the subtitle of his book, Maynard Bray wrote, “N.G. Herreshoff’s finest shallow-draft yawl.” Adding to the praise is the quote in the Preface that L. Francis Herreshoff called her “probably the finest boat of her size in the world.” In his book, Bray documents the evolution of Aida from Alerion through Pleasure NGH 907, the second of his shallow keel/centerboarders with low-aspect-ratio triangular foils.

L. Francis Herreshoff. Rozinante , 24’ Canoe Yawl (Ketch Rig), 1956

LFH’s graceful and leaner interpretation of the English canoe yawl was the only example of its type to be widely seen on American waters. Roger Taylor quoted LFH: “A canoe yawl can be about the safest that can be had since her design is based on those most seaworthy open boats ever known—whaleboats.”

George Holmes. Trent , 25’ Canoe Yawl, 1910

George Holmes, a designer and artist, was in the forefront of the English canoe yawl world. The term canoe yawl deserves explanation. It has nothing to do with the rig but rather with size, it being the same size or smaller than a ship’s yawl boat. The term canoe relates to it being either a day boat or coastal cruiser.

Robert Perry. Amati , 40’ High-Aspect-Ratio Fin-Keel, Spade-Rudder, Wood-Foam-Wood Composite, Fast Cruising Sloop, 1999

A significant departure from Perry’s early double-enders, this is one of his light-displacement sleds, for a couple with the emphasis on off-the-wind speed. His early designs were often double enders including the iconic Valiant with its full-ended cruiser stern. The term cruiser stern refers to the sterns on the cruiser class of warships prior to World War II.

Knud Riemers. Moose , 43.0’ 30-Square-Meter Sloop, 1934

Moose was the 1935 US 30-Square-Meter champion. Uffa Fox stated in 1936 that she was “probably the finest example of a 30-Square-Meter in the world.”

Sparkman & Stephens. Babe , 30.5’ Fractional Sloop, 1935

Babe , a favorite of Olin Stephens, was one of the 49 designs he illustrated and commented on in his book Lines. The modern S&S 30, purported to be an update of Babe and assigned a “1935” based design number 97-c2, has nothing in common with the original’s hullform.

Sparkman & Stephens. 1834 Intrepid , 64.25’ 12-Meter Sloop, 1967

Although conventional thinking states that deep draft is required for optimal windward performance, the heavily ballasted 12-Meters with minimal salient keel prove the exception. While articles about S&S designs often stated new from the board of OJS, it was not so during my time at the Madison Ave. office. Olin’s board did not have any room for drawings, so he would come into the main portion of the office to review a designer’s work and suggest changes, for example, to Mario Tarabocchia’s 12-Meter lines. I had the privilege of being involved in this process with the preliminary design of S&S 1770 Ta Aroa a New Zealand A Class racer. Olin had me start with a 5.5 meter’s lines, doubling their size, and proceeding under his guidance.

E.G. van de Stadt. Zeevalk , 41’ V-Bottom Plywood, Fin-Keel, Spade-Rudder, Fractional Sloop, 1949

This second design of E.G. van de Stadt’s and the first for Bruynzeel established his reputation as an innovative designer with a preference for fin-keel, spade-rudder configurations. This perspective or isometric illustrates Zeevalk ’s V-bottom hullform and the short-chord trim tab on the trailing edge of her fin keel.

Joel White. Dragonera , JW Design 49 74’ Fin-Keel Ketch, 1993

Joel White’s designs varied from traditional hullforms and construction in the fashion of Herreshoff and Nielsen to cold-molded fin keel types.

Jay Paris Designs:

No. 10 Aeromarine 50, 50 ’ Tall Mizzen Ketch 1968

No. 24 Lone Star, 54 ’ LOH Clipper Bow Ketch 1976

No. 27 Freedom 33, 33 ’ Cat Ketch for Gary Hoyt 1977

No. 33 P-32, 32 ’ Keel/Centerboard Sloop 1982

No. 40 P-45/Y-45, 45 ’ Ketch for YW/CW Contests 1988

No. 43 OCC/Sail, 43 ’ Sloop for Sail Article 1992 (An Ocean Cruising Club member survey design)

No. 48 P-37, Yawl for IBEX and METS Lectures 2007

No. 51 G-37, Keel/CB Yawl for a CCA Lecture 2014

No. 53 OCC-47, Keel/Centerboard Ketch 2018 (A rethink of the 1992 OCC member survey requirements)

Lines Drawings Reading List

In the following books, design examples illustrate the step-by-step creation of the lines reflecting the hullforms in fashion when the texts were written:

Chapelle, Howard I. Yacht Designing and Planning , 1936.

Clipper Bow 33.33 ’ DWL Shoal-Draft Centerboarder.

Kinney, Francis. Skene’s Elements of Yacht Design , 8th Ed., 1973.

Pipe Dream 25.42 ’ DWL Cruising Yacht.

Phillips-Birt, Douglas. Sailing Yacht Design , 3rd Ed., 1966 and 1976.

Full Keel 30 ’ DWL Fast Cruiser Including design using Diagonals.

Larsson, Lars and Rolf Eliasson. Principals of Yacht Design . 3rd/4th Eds., 2007 and 2014.

YD-40 39.5 ’ 2007 and YD-41 41 ’ 2014 Fin Keel Fast Cruising Yachts.

These books discuss various technical design details and parameters used by yacht designers:

Brewer, Ted. Ted Brewer Explains Sailboat Design , 1985.

Killing, Steve. Yacht Design Explained , 1998.

Perry, Robert H. Yacht Design According to Perry , 2008.

For a more theoretical explanation of the factors influencing sailing yacht design, I recommend:

Marchaj, C.A. Sailing Theory and Practice , 2nd Ed., 1982.

Marchaj, C.A. Seaworthiness: The Forgotten Factor , 1996.

Fossati, Fabio. Aero-Hydrodynamics and the Performance of Sailing Yachts , 2007.

Slooff, J.W. The Aero- and Hydrodynamics of Keel Yachts , 2015.

Bruce, Peter. Heavy Weather Sailing , 7th Ed., 2016.

About the Author: While attending Webb Institute Jay Paris worked two winters and one summer at Sparkman & Stephens. His graduate studies at MIT focused mostly on oceanography and research-vessel design. He also managed the MIT towing tank and operations of the yacht-testing program and participated in the international investigations into the 1979 Fastnet Race disaster. After MIT he was involved in the design of winches for the America’s Cup and high-end racing yachts, the design and construction of various sailboats, and the writing and editing of articles on sailing yachts for a number of different journals.

Saildrone Launches 65′ Aluminum USV

Richard Jenkins, an Englishman trained in mechanical engineering, stepped into the limelight on March 26, 2009, in California’s Mojave Desert by setting a 126.1 mph (203 km/h) land sailing speed… Read more »

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Boat Hulls 101: Complete Guide to Boat Hull Types, Shapes, and Designs

Table of Contents

Last Updated on August 17, 2023 by Boatsetter Team

If you’re new to boating, then you may not have even considered a boat’s hull , its importance, and the way that it affects your time on the water. With the hull being the part of the boat in the water, it is perhaps the most important part as it gives your boat the ability to float. Not only that, but it affects every single characteristic of your boat and the smoothness of your ride. This article on boat hulls will equip you with the technical knowledge and expertise necessary to understand hulls and the way they work.

What is a Boat Hull?

First of all, we’ll go into a bit of detail on what a boat hull is. The hull is the body of the boat. It is sealed to prevent water from transmitting its way through and keeping your boat afloat. A hull can be open where you sit in it, such as a small dinghy, or a deck may cover it as you would find on a yacht.

When there is a deck placed on top of a hull, it opens up many more options for utilizing the space on your boat more appropriately as it is raised to the top of the hull, where more space is apparent. For example, on a deck, you can place a cabin -like you would find on a center console or even a mast and sail rigs to create a sailboat.

When the hull is open, options to use your space effectively are reduced as you sit at the bottom of the bowl shape. In addition to having less space, you also feel the rock of the water in a more pronounced manner as it is just the keel of the boat (the bottom) separating you from the water. Therefore, every wave and lurch in the water that rocks the boat is felt, which may cause you discomfort if you haven’t quite found your sea legs.

Why Are Hulls Important?

The knowledge of how a boat floats is fundamental if you are looking to get into boating. Without actually knowing, you put yourself at risk of compromising your boating activities and creating a danger that you cause your boat to sink. The key line to this knowledge is that the air encapsulating your boat must be denser than the water it sits upon. This not only includes the air but the items on your boat as they contribute towards the pressure that your boat’s hull puts upon the water.

The greater the amount of weight your boat holds, the further it pushes itself into the water, lowering or raising the level that your hull sits in the water. This force displaces the water to a level that is equal to the boat. If the average density of the boat is greater than the water, then the boat shall sink. You can see this in action if you have a small dinghy; the more people you place on it, you’ll notice that your boat edges itself ever so slightly more into the water as the boat’s weight is rising.

Different types of Boat Hulls

We’ll now walk you through the different types of boat hulls that you come across. The design of the boat’s hull changes the type of boat that you have. If you are browsing through our boat rentals, you’ll notice the various types of boats. Each of these boats has a different type of hull design. For example, a pontoon boat rental is designed for calm waters, whereas a giant yacht is designed for taking on the rough seas, meaning that their hulls vary greatly.

There are two main types of hull: displacement and planing. We’ll give you the rundown of both of these types and the other sub-varieties within them.

Displacement Hulls

The first variety of hulls that we shall examine are displacement hulls. These hulls are typically found on boats that need to carry a heavy load, such as a large fishing boat and big yachts. The hull sits deeper into the water, and the boat is supported by buoyancy, as opposed to its thrust.

Due to the boat sitting deeper in the water, it might be slower, but it will ride steadier. These larger boats are particularly good for the sea as they can handle stronger waves and currents as the boat can stabilize themselves better. This is why you’ll see container ships and other varieties that need to bear a heavy load using these types of hulls.

When it comes to boat rentals, you are most likely to find a sailing boat with a displacement boat hull. The hull is rounded at the bottom, allowing the sailboat to lurch strongly to one side while turning without any danger of capsizing. Thus, we can see the impact that the hull has on your boat rental as it gives your sailboat the extra capacity to lurch around sharp turns and enjoy some exhilarating fun.

Planing Hulls

The other main type of hull is the planing hull. This hull’s design allows the boat to accelerate to higher speeds due to less hull being placed in the water. When a boat with a planing hull is cruising at lower speeds, it operates similarly to a boat with a displacement. When it starts to hit around 15 knots, things start to change depending on the weight of the boat’s load. The flatter surface of the planing hull allows the boat to propel itself upwards to skiff itself across the water. This is what causes boats with planing hulls to obtain higher speeds. In addition, because the bulk of the hull is not placed below the water’s surface, there is less tension from the water holding the boat back, meaning that it can move through the water faster and using less power to obtain a speed that a boat with a displacement hull can.

There is not only one type of planing hull but instead many different varieties. We’ll look through these varieties to examine how it affects your boating experience so that you can make a more informed decision when choosing your next boat rental.

Flat Bottomed Hulls

As the name suggests, these hulls do not have the traditional curved hull that reaches a point at the bottom but has a flat surface instead. These tend to be small skiffs or fishing boats where you cast out from. Due to them having a flat hull, they are excellent for getting into shallow water where some of your favorite catches may lie. These boats don’t need much power for the planing power to come into action and reach quick speeds in no time. They also tend to handle well not just on the flats but also on the sea, with choppy water not being a big issue. So, if you’re looking for some gentle fishing on the flats or maybe out in some nearshore waters, check out our range of small flat bottomed hull boats to truly enjoy some great fishing experiences.

Pontoons are one of the great boat rentals for cruising around and enjoying time with friends because the design of their hulls allows for more space to be created. Pontoons have two-cylinder hulls that sit parallel to each other on the surface of the water. The deck is placed atop these two cylinders, and because they are placed on cylinders, the deck can expand beyond the cylinders, creating more space. This allows for a comfortable seating/social area to be created on the boat, allowing you to use it for parties and some relaxed exploring with the wider family. These boats are best used on inland and flat waters. This is due to waves rocking them a lot more, and a storm at sea can even put them at risk of capsizing. For some fun on a lake, however, pontoons are hard to beat.

In recent years many tritoons have started to crop up on the boat rental market. These are similar to pontoons, but they have a third cylinder that gives them some extra stability. It also means that they can handle a more powerful engine that can bring them up to higher speeds than a pontoon boat. If a pontoon has an engine that is too powerful, then its planing hull can lift it too far above the water’s surface, causing great instability. However, when it comes to tritoons, the greater speeds that you can reach allow you to expand upon other activities and add in some wakeboarding or tubing action onto your party on the water!

V Bottom Hull

The shape of a v bottom hull has a sharper decline that accumulates in having a meeting point at the bottom, creating a v shape, as stated in the name. Because of the honed hull, one of these boats can cut through the water at decent speeds and are particularly good when out on seawater. However, they require a powerful engine for the boat to go into a planing mode. One of the most common types of v bottom hull boat rentals is center consoles. These are great vessels for going for some nearshore or offshore fishing or some general saltwater exploring. Their v bottom hull allows them to cut through the waves so that you can rush to the best fishing grounds in no time at all.

The tri-hull design is a variation of the v bottom hull. It has a v-shaped hull in the center and two parallel smaller hulls on either side of the main central one. This gives the tri-hull boat some extra stability when going forward . Additionally, this also allows the boat to have more deck space as the hull covers a wider range. One of the big drawbacks of the tri-hull – also known as a cathedral hull – is that the bat rocks more when it is in choppier water because the hull is wider. Nevertheless, tri-hulls make for a great option for fishing or exploring on lakes or calm coves.

Catamaran: A Multi-Hulled Boat

Perhaps the most popular multi-hulled boat is the catamaran. This type of boat has two separate hulls that run parallel to each other. These hulls sit on either side of the boat and the deck connects them. This type of design allows forecast amounts of space onboard . Many catamarans are luxury boats that can have the space to hold swimming pools and even helipads. Because they have dual hulls, catamarans can get themselves in shallow waters and lagoons where other luxury boats cannot. This makes them the perfect boat rental if you plan to visit a location where there are multiple small islands such as Hawaii or The Bahamas. The multi-hull system also provides a lot more stability and comfort, so they are perfect boat rentals if you are prone to suffering from seasickness . Catamarans are not only luxury liners as smaller versions with a trampoline-designed deck can also be found that make for great day adventures.

As we hope you have been able to discover in this blog post, the type of hull that your boat has affects everything about your boat. By having a little bit of knowledge on how the design of a boat’s hull has an impact on your boating experience, you can begin to make more informed decisions on which boat rental is best for you. To reinforce this information a little bit further, check out this video !

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How hull shape affects comfort at sea

julianwolfram
September 15, 2021

Understanding how your hull shape responds to waves will keep you and your crew safe and comfortable in a blow, says Julian Wolfram

Understanding how your hull shape responds to waves will keep you and your crew safe and comfortable. Credit: Richard Langdon

There are many desirable attributes sailors want in their cruising yacht.

Julian Woolfram is a physicist, naval architect, former professor of ocean engineering at Heriot-Watt in Edinburgh and a Yachtmaster Offshore who has cruised and raced for 45 years

Julian Wolfram is a physicist, naval architect, former professor of ocean engineering at Heriot-Watt in Edinburgh and a Yachtmaster Offshore who has cruised and raced for 45 years

Stability is obviously a crucial factor, but there are other important factors that should be considered when judging a sailing yacht and the balance of these will depend on the type of cruising envisaged and whether there may be some club racing involved.

Comfort, or seakindliness , is high on many people’s list and a boat that bounces around or slams in a choppy sea is generally shunned by all except hardened racers who will put up with any discomfort in the pursuit of speed.

However, most sailors likes to sail and a boat that can’t maintain a reasonable speed in light winds and frequently requires the use of the engine is also undesirable.

When the wind picks up a bit, a yacht that responds with a good turn of speed is a delight and can make the difference between arriving in daylight and tying up in the dark after a long passage.

A traditional long-keel working boat style cutter – Heard 35. Credit: Graham Snook

Finally, there is course-keeping and manoeuvrability .

A yacht where the helm can be left unattended for a minute or two can be a godsend to a short-handed crew , but the same crew will welcome good manoeuvrability when going into a crowded harbour or marina.

A yacht regarded as being ‘well-found’ is usually a boat that responds slowly and gently to the waves and doesn’t slam when sailing or motor-sailing up wind in a bit of sea, but she may not be the fastest boat out of the blocks.

Comfort depends on low accelerations, and research has shown that the travelling public feel ‘general discomfort’ when accelerations exceed 0.2g (more than 20% of the acceleration due to gravity).

A medium displacement fin and skeg sloop – Hallberg Rassy 352. Credit: Graham Snook/Yachting Monthly

To put this in context, the acceleration at the top of a big, very steep wave can reach 0.5g.

There are international standards on comfort, based on acceleration figures, that have to be met by operators of public transport systems and these are used in the design of manned and unmanned rail and road vehicles.

Whilst we can maintain the smoothness of rails and roads you can’t do the same for the sea, so ferries have ‘operability criteria’ that limit the sea conditions in which they can operate.

Of course bigger vessels are able to operate in more severe conditions than smaller ones. There are also Motion Sickness Indices based on accelerations that predict the number of passengers likely to be ill.

When it comes to small yachts it’s up to the skipper and crew to decide upon the conditions in which they will stay in port.

Light displacement, modern wide hull – Pogo 10.50

This decision should depend, at least in part, on how comfortable the yacht is in a seaway.

Now the comfort of yachts of the same size can be remarkably different. Take yachts of around 35ft or 10.6m long – a common yacht length.

Three examples show how much the displacement can vary for this size of vessel. The Heard 35, a traditional long keel working boat type design, weighs 12.7 tonnes; the Hallberg Rassy 352 , with quite a long keel but separate skeg-hung rudder, weighs 6.7 tonnes; and the Pogo 10.50, which has a very modern wedge-shaped hull with a narrow but deep lifting keel and spade rudder, weighs 3.6 tonnes.

Hull shape: Rolling

Consider these three yachts moored in a line when a fat motor boat goes past with a big wake.

The heave (lifting) force produced by the waves will depend on the yachts’ waterplane area and these will all be roughly the same for all three.

So, initially, they will all experience the same heave force. Now remember Newton’s famous law: force = mass x acceleration. This can be rewritten for boats as:-

Acceleration = Force/Mass (displacement + AV M)

Of course Newton’s Law is usually applied to objects in air, and when a boat moves in water it moves the water around it as well, and this must be accounted for.

This is known as the added virtual mass (AVM) and it will be broadly similar for all these yachts when heaving, and has the beneficial effect of reducing the acceleration.

When it comes to rolling, weight is also an advantage, given careful design and a metacentric height (GM – is the distance from the centre of gravity to the point where the vertical line through the centre of buoyancy, when heeled through a small angle, intersects the centreline. It is a measure of the initial stability or stiffness of a yacht (for small angles, GM multiplied by the angle of heel gives the righting lever) that is not unnecessarily large.

Does your boat slam or hobby-horse over waves? Knowing the difference helps you decide when you’re happy to put to sea and when you’d rather wait. Credit: Richard Langdon

The sail-carrying capacity of a yacht depends on the product of its displacement and GM, so if the displacement is large the GM can be smaller.

A lower GM will lead to lower angular accelerations and a more comfortable rolling motion.

Let’s assume that all the yachts will heel to the same angle when setting full sail in a light breeze on a close reach.

The Heard sets 92 sq. m, with its topsail up, the Hallberg Rassy 64 sq. m and the Pogo 71 sq. m which means the Heard can have a lower GM than the Hallberg Rassy and both can be lower than the Pogo.

Continues below…

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The result is that the Pogo 10 will accelerate nearly twice as quickly as the Heard 35 and about 30% more quickly than the HR352.

So the Pogo 10 motion will be lively whereas the Heard 35 will respond slowly and the HR352 will be somewhere in the middle.

A single wave breaking against the side of the hull will be more noticeable jerk in the Pogo 10 than in HR352 and less noticeable again in the Heard 35.

Interestingly the late Ed Burnett, in discussing the design for his long keeled yacht Nomad , decided against a carbon fibre mast and in favour of a wooden one so as to keep GM smaller and the rolling motion more comfortable.

Credit: Maxine Heath

The reasoning behind this decision is reflected in the diagram above, which shows the estimated roll angles and corresponding angular accelerations for our three yachts whilst on their moorings with waves on the beam.

In a harbour with a 6-knot speed limit the waves generated by passing craft will be between 2m and 6m long, enough to get resonant rolling of the Pogo 10.

But it needs a passing vessel to go at 9 knots to get the Heard 35 to roll.

More significantly, although the maximum roll angles are reasonably similar, the acceleration is much higher for the light Pogo 10.

No wonder racing crews tend to sleep ashore whenever possible. (It should be mentioned that these curves are based on simple linear theory and it is assumed the damping is the same in each case, however these approximations do not affect the broad thrust of the results.)

So in general, for a given length of boat, a greater displacement will result in a more comfortable motion in the sort of waves you might encounter in an anchorage.

Of course the bigger the boat the greater the comfort, and large vessels don’t move much when the fishing boats tear out at 0400!

Hull shape: Pitching

When sailing, of course, the sails will damp out the rolling motion but not the heaving and pitching.

The diagram below shows our yachts going into head seas at 5 knots. Here the wave height is 30cm with a wavelength of 5m.

A digram showing the impact of pitching on different boat hull shapes

This goes up to 1.3m when the wavelength is 35m, which is roughly what may happen when the sea rises as the wind increase to Force 6 (25 knots).

The natural period of pitching is larger for a heavy yacht and resonant pitching will occur when driving into longer and generally higher waves than for a lighter yacht.

This is the one occasion when heavy displacement can be a negative in terms of comfort.

The more symmetrical hulls normally associated with heavy displacement have less damping and tend to ‘hobby horse’ in these conditions.

However, they can drive through a short chop comfortably with small accelerations when the light boat is having a more torrid time.

Of all the yacht motions, slamming is often the one that produces the most discomfort and accelerations of over 3g have been measured by Southampton University on racing yachts going upwind.

Slamming was first studied analytically by Theodore Von Karman in connection to seaplane floats in 1929.

Von Karman studied, among other things, the vortex shedding behind slender cylinders, now known as a Karman Street, that gives rise to the strumming vibration of rigging and halyards in high winds.

He showed how important it was to have a ‘V’ shape to the bottom of a seaplane float to reduce the slamming impact on landing. Naval architects call the angle the hull makes with the horizontal at the centreline the ‘dead rise’ .

So a flat bottom has 0° dead rise.

Von Karman showed if you doubled the deadrise angle the impact force should half.

Impact pressure = 1/Tangent of deodorise angle

So, approximately if you increase the deadrise angle from 5o to 10° you halve the impact pressure and significantly reduce the slamming. (This formula doesn’t work when there is no deadrise, i.e. a flat bottom, and Von Karman produced a more complex analysis for this case).

Unfortunately, a trend was started in the 1970s to give sailing yachts flat-bottom bows following the introduction of the International Offshore Rating rule.

The rule wanted to get an estimate of displacement for each yacht without actually weighing it. That was considered too difficult at the time.

So they introduced a complex series of measurements to various points around the hull beneath the waterline including some at the bow.

However, yacht designers soon started to distort the hull shape at the measurement points to kid the rule that a light hull was actually heavier, which gave the yacht a better handicap than would otherwise be the case.

The diagram above shows what was typically done in the bow region to get a lighter boat with lower wetted surface area.

My first personal experience of this trend came in the 1980s when I changed my Trapper 500 for a Westerly Fulmar.

The Fulmar was quicker and had much better accommodation, but it was much more prone to slamming.

A Trapper 500 with V-shaped bow sections

I remember motor-sailing, to catch a tide, down the Irish coast into a short chop when it seemed to slam on every other wave.

The Trapper, on the other hand, rarely slammed even when my kids decided to take a shortcut through overfalls at the top of Skye while I was having an off-watch nap below – it was the pitching that woke me, not any slamming.

The photos show clearly the difference in bow shape.

A ‘V’ or well-rounded bow shape is the best to avoid slamming and if there is any flat area near the bow it should be small to ensure the deceleration on a slamming impact is minimised.

The Westerly Fulmar has a flattened section aft of the bow

If you look at lifeboats and RIBs you will see they have deadrise angles of at least 20o, even at the stern.

Going at high speed in a flat-bottom boat through a steep chop can be an ankle-shattering experience!

Draw a chalk line perpendicular to the centre line and view from ahead to get a sense of your boat’s boat sections. Credit: Julian Woolfram

Drawing a chalk line around the hull mid way between the bow and the keel from the waterline on one side to the waterline on the other side and viewing from the bow will give a good impression of how seakindly, or otherwise, the bow is.

If you have the body plan for the hull look at the shape of the sections near the bow.

Hull shape: Wave slap

Now look at the stern, if it is wide and rises at a shallow angle through the waterline to accommodate a spacious aft cabin, it will probably suffer from ‘wave slap’.

Ideally the angle should be about 17o.

Much steeper, especially on a wide stern, and water flow separation will increase drag; much shallower and there will be wave slap (see below).

I watched a fat little fishing boat going out at a fair lick from an inner berth in Le Crouesty marina last year and nearly half the sailing yacht sterns it passed were slapped on their undersides by its wake.

Not a great way to wake up (excuse the pun) in your sumptuous aft cabin at 0530.

Above and below decks

The other factor to consider in how your crew will experience the seakindliness of any boat is where they are on the boat.

We all know that sleeping in the bow cabin at sea can be like a fairground ride compared to the saloon.

A good sea boat will have this factored into its cockpit and accommodation arrangement, with everything that will be used at sea in the middle third of the boat’s length – heads, chart table, galley and sea berths all need to be here, close to the point of minimum motion just aft of amidships and close to the waterline (see below).

A diagram showing layout downbelow which impacts hull shape

The same is true on deck.

Ideally the helmsman and crew should be able to operate the boat from the forward part of the cockpit and not have to go forward of the mast, or to be perched right at the stern.

While many larger boats, and some boats as small as 30ft, now have twin wheels shoved out to the aft quarters to create more cockpit space and a better view forward, this isn’t the most comfortable place to be.

Tiller steering on small boats is no bad thing as it allows the helm to be much further forward, where there is less motion and more shelter.

Seaberths, galley heads and chart table in the middle third of the boat provide more comfort at sea. Credit: Graham Snook

However, another person’s ideal sea boat may be very different to mine.

It’s just important to understand the difference that their design makes to the conditions you are happy to set out in, understanding when it’s going to be uncomfortable, and how to handle your boat when the wind does get up.

How a Sail Works: Basic Aerodynamics

The more you learn about how a sail works, the more you start to really appreciate the fundamental structure and design used for all sailboats.

It can be truly fascinating that many years ago, adventurers sailed the oceans and seas with what we consider now to be basic aerodynamic and hydrodynamic theory.

When I first heard the words “aerodynamic and hydrodynamic theory” when being introduced to how a sail works in its most fundamental form, I was a bit intimidated.

“Do I need to take a physics 101 course?” However, it turns out it can be explained in very intuitive ways that anyone with a touch of curiosity can learn.

Wherever possible, I’ll include not only intuitive descriptions of the basic aerodynamics of how a sail works, but I’ll also include images to illustrate these points.

There are a lot of fascinating facts to learn, so let’s get to it!

Basic Aerodynamic Theory and Sailing

Combining the world of aerodynamics and sailing is a natural move thanks to the combination of wind and sail.

We all know that sailboats get their forward motion from wind energy, so it’s no wonder a little bit of understanding of aerodynamics is in order. Aerodynamics is a field of study focused on the motion of air when it interacts with a solid object.

The most common image that comes to mind is wind on an airplane or a car in a wind tunnel. As a matter of fact, the sail on a sailboat acts a bit like a wing under specific points of sail as does the keel underneath a sailboat.

People have been using the fundamentals of aerodynamics to sail around the globe for thousands of years.

The ancient Greeks are known to have had at least an intuitive understanding of it an extremely long time ago. However, it wasn’t truly laid out as science until Sir Isaac Newton came along in 1726 with his theory of air resistance.

Fundamental Forces

One of the most important facets to understand when learning about how a sail works under the magnifying glass of aerodynamics is understanding the forces at play.

There are four fundamental forces involved in the combination of aerodynamics and a sailboat and those include the lift, drag, thrust, and weight.

From the image above, you can see these forces at play on an airfoil, which is just like a wing on an airplane or similar to the many types of sails on a sailboat. They all have an important role to play in how a sail works when out on the water with a bit of wind about, but the two main aerodynamic forces are lift and drag.

Before we jump into how lift and drag work, let’s take a quick look at thrust and weight since understanding these will give us a better view of the aerodynamics of a sailboat.

As you can imagine, weight is a pretty straight forward force since it’s simply how heavy an object is.

The weight of a sailboat makes a huge difference in how it’s able to accelerate when a more powerful wind kicks in as well as when changing directions while tacking or jibing.

It’s also the opposing force to lift, which is where the keel comes in mighty handy. More on that later.

The thrust force is a reactionary force as it’s the main result of the combination of all the other forces. This is the force that helps propel a sailboat forward while in the water, which is essentially the acceleration of a sailboat cutting through the water.

Combine this forward acceleration with the weight of sailboat and you get Newton’s famous second law of motion F=ma.

Drag and Lift

Now for the more interesting aerodynamic forces at play when looking at how a sail works. As I mentioned before, lift and drag are the two main aerodynamic forces involved in this scientific dance between wind and sail.

Just like the image shows, they are perpendicular forces that play crucial roles in getting a sailboat moving along.

If you were to combine the lift and drag force together, you would end up with a force that’s directly trying to tip your sailboat.

What the sail is essentially doing is breaking up the force of the wind into two components that serve different purposes. This decomposition of forces is what makes a sailboat a sailboat.

The drag force is the force parallel to the sail, which is essentially the force that’s altering the direction of the wind and pushing the sailboat sideways.

The reason drag is occurring in the first place is based on the positioning of the sail to the wind. Since we want our sail to catch the wind, it’s only natural this force will be produced.

The lift force is the force perpendicular to the sail and provides the energy that’s pointed fore the sailboat. Since the lift force is pointing forward, we want to ensure our sailboat is able to use as much of that force to produce forward propulsion.

This is exactly the energy our sailboat needs to get moving, so figuring out how to eliminate any other force that impedes it is essential.

Combining the lift and drag forces produces a very strong force that’s exactly perpendicular to the hull of a sailboat.

As you might have already experienced while out on a sailing adventure, the sailboat heels (tips) when the wind starts moving, which is exactly this strong perpendicular force produced by the lift and drag.

Now, you may be wondering “Why doesn’t the sailboat get pushed in this new direction due to this new force?” Well, if we only had the hull and sail to work with while out on the water, we’d definitely be out of luck.

There’s no question we’d just be pushed to the side and never move forward. However, sailboats have a special trick up their sleeves that help transform that energy to a force pointing forward.

Hydrodynamics: The Role of the Keel

An essential part of any monohull sailboat is a keel, which is the long, heavy object that protrudes from the hull and down to the seabed. Keels can come in many types , but they all serve the same purpose regardless of their shape and size.

Hydrodynamics, or fluid dynamics, is similar to aerodynamics in the sense that it describes the flow of fluids and is often used as a way to model how liquids in motion interact with solid objects.

As a matter of fact, one of the most famous math problems that have yet to be solved is exactly addressing this interaction, which is called the Navier-Stokes equations. If you can solve this math problem, the Clay Mathematics Institute will award you with $1 million!

There are a couple of reasons why a sailboat has a keel . A keel converts sideways force on the sailboat by the wind into forward motion and it provides ballast (i.e., keeps the sailboat from tipping).

By canceling out the perpendicular force on the sailboat originally caused by the wind hitting the sail, the only significant leftover force produces forward motion.

We talked about how the sideways force makes the sailboat tip to the side. Well, the keep is made out to be a wing-like object that can not only effectively cut through the water below, but also provide enough surface area to resist being moved.

For example, if you stick your hand in water and keep it stiff while moving it back and forth in the direction of your palm, your hand is producing a lot of resistance to the water.

This resisting force by the keel contributes to eliminating that perpendicular force that’s trying to tip the sailboat as hard as it can.

The wind hitting the sail and thus producing that sideways force is being pushed back by this big, heavy object in the water. Since that big, heavy object isn’t easy to push around, a lot of that energy gets canceled out.

When the energy perpendicular to the sailboat is effectively canceled out, the only remaining force is the remnants of the lift force. And since the lift force was pointing parallel to the sailboat as well as the hull, there’s only one way to go: forward!

Once the forward motion starts to occur, the keel starts to act like a wing and helps to stabilize the sailboat as the speed increases.

This is when the keel is able to resist the perpendicular force even more, resulting in the sailboat evening out.

This is exactly why once you pick up a bit of speed after experiencing a gust, your sailboat will tend to flatten instead of stay tipped over so heavily.

Heeling Over

When you’re on a sailboat and you experience the feeling of the sailboat tipping to either the port or starboard side, that’s called heeling .

As your sailboat catches the wind in its sail and works with the keel to produce forward motion, that heeling over will be reduced due to the wing-like nature of the keel.

The combination of the perpendicular force of the wind on the sail and the opposing force by the keel results in these forces canceling out.

However, the keel isn’t able to overpower the force by the wind absolutely which results in the sailboat traveling forward with a little tilt, or heel, to it.

Ideally, you want your sailboat to heel as little as possible because this allows your sailboat to cut through the water easier and to transfer more energy forward.

This is why you see sailboat racing crews leaning on the side of their sailboat that’s heeled over the most. They’re trying to help the keel by adding even more force against the perpendicular wind force.

By leveling out the sailboat, you’ll be able to move through the water far more efficiently. This means that any work in correcting the heeling of your sailboat beyond the work of the keel needs to be done by you and your crew.

Apart from the racing crews that lean intensely on one side of the sailboat, there are other ways to do this as well.

One way to prevent your sailboat from heeling over is to simply move your crew from one side of the sailboat to the other. Just like racing sailors, you’re helping out the keel resist the perpendicular force without having to do any intense harness gymnastics.

A great way to properly keep your sailboat from heeling over is to adjust the sails on your sailboat. Sure, it’s fun to sail around with a little heel because it adds a bit of action to the day, but if you need to contain that action a bit all you need to do is ease out the sails.

By easing out the sails, you’re reducing the surface area of the sail acting on the wind and thus reducing the perpendicular wind force. Be sure to ease it out carefully though so as to avoid luffing.

Another great way to reduce heeling on your sailboat is to reef your sails. By reefing your sails, you’re again reducing the surface area of the sails acting on the wind.

However, in this case the reduction of surface area doesn’t require altering your current point of sail and instead simply remove surface area altogether.

When the winds are high and mighty, and they don’t appear to be letting up, reefing your sails is always a smart move.

How an Airplane Wing Works

We talked a lot about how a sail is a wing-like object, but I always find it important to be able to understand one concept in a number of different ways.

Probably the most common example’s of how aerodynamics works is with wings on an airplane. If you can understand how a sail works as well as a wing on an airplane, you’ll be in a small minority of people who truly understand the basic aerodynamic theory.

As I mentioned before, sails on a sailboat are similar to wings on an airplane. When wind streams across a wing, some air travels above the wing and some below.

The air that travels above the wing travels a longer distance, which means it has to travel at a higher velocity than the air below resulting in a lower pressure environment.

On the other hand, the air that passes below the wing doesn’t have to travel as far as the air on top of the wing, so the air can travel at a lower velocity than the air above resulting in a higher pressure environment.

Now, it’s a fact that high-pressure systems always move toward low-pressure systems since this is a transfer of energy from a higher potential to a lower potential.

Think of what happens when you open the bathroom door after taking a hot shower. All that hot air escapes into a cooler environment as fast as possible.

Due to the shape of a wing on an airplane, a pressure differential is created and results in the high pressure wanting to move to the lower pressure.

This resulting pressure dynamic forces the wing to move upward causing whatever else is attached to it to rise up as well. This is how airplanes are able to produce lift and raise themselves off the ground.

Now if you look at this in the eyes of a sailboat, the sail is acting in a similar way. Wind is streaming across the sail head on resulting in some air going on the port side and the starboard side of the sail.

Whichever side of the sail is puffed out will require the air to travel a bit farther than the interior part of the sail.

This is actually where there’s a slight difference between a wing and a sail since both sides of the sail are equal in length.

However, all of the air on the interior doesn’t have to travel the same distance as all of the air on the exterior, which results in the pressure differential we see with wings.

Final Thoughts

We got pretty technical here today, but I hope it was helpful in deepening your understanding of how a sail works as well as how a keel works when it comes to basic aerodynamic and hydrodynamic theory.

Having this knowledge is helpful when adjusting your sails and being conscious of the power of the wind on your sailboat.

With a better fundamental background in how a sailboat operates and how their interconnected parts work together in terms of basic aerodynamics and hydrodynamics, you’re definitely better fit for cruising out on the water.

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8000 Engineering Business
8200 Fluids
8210 Fluid Dynamics
8220 Resistance
8230 Propulsion
appendage resistance
power yacht
righting moment
sail side force
sailing yacht
skene element of yacht design
viscous resistance
wave making
wave resistance

Figure 2-1: Example of Sailboat Surfing [2]

The TRUTH of Hull Speed: How to Break the Sailing Speed Limit

Hull speed is bogus, but not because the math is wrong. This formula supposedly predicts the maximum speed of a yacht based solely on length. It gets discussed frequently in sailing yacht communities. Surprisingly, hull speed is partially correct, with a very simple and strong theoretical basis. But don’t sell yourself short; there are ways around hull speed limits. Today we discuss the basis for hull speed and fill in the rest of the story.

1.0 Theory Behind Hull Speed

The hull speed theory predicts a maximum speed for sailing yachts from a simple formula.


Boat length in feet = L	Boat length in meters = L
Speed limit in knots = V	Speed limit in knots = V
Hull Speed:	Hull Speed:

Ever wonder where those coefficients came from? They result from two interesting quirks of physics. First, any waves generated by the hull have to move at the same speed as the hull. Our boat generates two major sets of waves: one at the bow, and one at the stern. Once they spread away from the hull, they get to slow down. Until then, they must do whatever is necessary to keep up with the boat.

And just how do they manage to keep up with the boat? By changing their wavelength. This isn’t the wave height. It’s the distance from one wave crest to the next. Our second quirk of physics: that wavelength gets tied to the speed limit for the wave. If the wave wants to go faster, it must stretch out longer, following this simple formula:

Where g is the acceleration due to gravity. Plug in the constants, work out the unit conversions, and you get the formulas in Table 1.

But that formula only describes the speed limit for a wave, not a boat. This is where we get to put the two ideas together. As the boat goes faster, the waves at the bow and stern get longer. When you reach hull speed, the bow wave lines up with the stern wave. The two waves double up on you. Your boat appears to sink down into one big wave trough. (Figure 1‑1) The hull speed theory states your sailboat will not go any faster once this happens.

2.0 The Rest of the Story

If you leave hull speed at this simple theory, you miss all the good parts of the story. So far, hull speed is this impenetrable limit, like the speed of light. But physicists already imagined theoretical ways to get around lightspeed. And this story has a few good parts that let us get around the hull speed limit.

It doesn’t break any rules to go faster than hull speed. If you push beyond the speed limit, the wavelength gets longer than your boat length. No law against that. At this point, most boats start to surf on their own bow wave; nothing wrong with that. (Figure 2‑1) No limiting formulas here. Sure, hull speed is a difficult hump to get over, but the shape of your hull determines the resistance from those waves. Not some magical formula.

Hull speed is bogus. It predicts when you face a big hump, true enough. But we reach too far when we assume this creates an impenetrable speed limit. It only tells you a speed when the bow and stern waves get bigger. So what? Your boat sees big waves on a stormy day, and you go through those just fine. The ultimate speed of your boat depends on only two things: resistance and power.

3.0 What Really Makes Resistance

Resistance depends completely on the shape of your hull. The best hulls are long and skinny; they minimize the size of your bow wave and stern wave. To minimize wave resistance, we want a hull that cuts straight through the wave instead of bouncing over it.

With an efficient hull, hull speed becomes just another hump in the resistance graph. A typical resistance graph for a ship looks like Figure 3‑1. The humps happen when waves line up and add to increase your resistance. You get multiple humps, and hull speed is just one of those humps. The ship powers over the hump and continues to go faster, if you have the right hull shape. Of course, getting the best hull shape becomes tricky due to the other major component: viscous resistance.

Viscous resistance is basically surface friction, plus some extra pieces. When the water runs along your hull, it generates friction, which slows you down. Especially when you travel in the range of 0 – 3 knots, almost all your resistance comes from friction. This is why many sailing hulls are shaped to reduce their underwater surface area as they heel over in light winds. All part of the strategy to minimize surface friction. Even at higher speeds, friction can form 20-40% of your total resistance. The catch is that not all hull surfaces are equal.

Appendages are the worst source of viscous resistance, in my opinion. Your bow thruster, rudder, propeller, and keel all form the extra pieces of viscous resistance, and they concentrate a lot of penalty into a small area. Consider a bow thruster. As the water runs over that, it ducks into the thruster tunnel, swirls around, and pops out all confused and turbulent. (Figure 3‑2) All that motion takes energy. Energy that slows down your ship. Similar stories for the other appendages. Sometimes, these little bits and pieces add up to more than the skin friction of your entire hull. And don’t forget about wave resistance. Put all the resistance sources together, and you see that going faster becomes increasingly difficult.

4.0 Limits on Power

To go faster, we need more power. But as a sailing ship, all the power comes from those sails. You might think to add larger sails. Unfortunately, sails do not limit your maximum power. The sail plan gets limited by the righting moment of your hull. [5] [6] As the wind heels the boat over, the hull pushes back with a righting moment. Try to push more power into the sails, and the boat just heels farther, dumping that power. (Figure 4‑1) In extreme cases, this is how the wind capsizes a boat. If you want more power in your sail plan, the first step is not larger sails. The trick is more righting moment in the hull.

But how to get more righting moment from the hull? Unfortunately, you need to increase the beam of your hull. Except that increasing the beam also increases your wave resistance. This is one of the many conflicts that yacht designs have to compromise on. That is also why catamarans have a reputation for speed. They get to combine long and skinny hulls (low resistance) with a huge righting moment (high power). Though even they run into other limits. The maximum speed of your hull is an integrated part of the vessel design. It becomes a performance target, strategically decided by the naval architect. Far more complicated than a simple formula.

5.0 Conclusion

With all the added evidence, why does the myth of hull speed persist. Simply put, designers are not stupid. We designers all recognize that any given boat has a point of diminishing return, where extra power barely adds any more speed. That point of diminishing return often happens near hull speed. This coincidence reinforces the belief that hull speed predicts a reliable limit. Boats often top out near hull speed, but not exactly at hull speed. Ship design has many tricks available to push faster than hull speed. Don’t limit your expectations to a simple formula.

6.0 References

[1]	A. Lloyd, Seakeeping: Ship Behaviour in Rough Weather, Gosport, Hampshire, UK: ARJM Lloyd, 1998.
[2]	G. Day, “Choosing a Safe Sailboat,” Boats.com, 4 February 2002. . Available: http://www.boats.com/how-to/choosing-a-safe-boat-5934/#.WtgAoC4bNpg. .
[3]	R. C. Brancho, “Basics of Ship Resistance,” LinkedIn Learning, 23 June 2011. . Available: https://www.slideshare.net/adsokant/basics-ofshipresistance. .
[4]	Wikipedia Authors, “File: Pourquoi_pas_bow_thrusters.jpg,” Wikimedia Commons, 24 Oct 2006. . Available: https://commons.wikimedia.org/wiki/File:Pourquoi_pas_bow_thrusters.jpg. .
[5]	N. L. Skene, Elements of Yacht Design, Dobbs Ferry, NY: Sheridan House, 2001.
[6]	Claughton, Wellicome and Shenoi, Sailing Yacht Design: Theory, Southampton, Hampshire, U.K.: University of Southampton, 2006.
[7]	Don Adzigian, “501 Heeling Dynamics,” Anything and Everything Catalina 22, . Available: http://www.catalina22experiment.com/home/basics-for-advanced-sailors-501. .
[8]	C. Doane, “Crunching Numbers: Hull Speed & Boat Length,” Boats.com, 26 March 2010. . Available: http://www.boats.com/reviews/crunching-numbers-hull-speed-boat-length/#.WtfzIC4bNph. .
[9]	Cloughton, Wellicome and Shenoi, Sail Yacht Design: Practice, Southampton, Hampshire, U.K.: University of Southampton, 2006.

Figure 3-1: Icebreaker Healy [4]

Surviving the Arctic: Polar Class Icebreakers

Figure 2-1: Icebreaker Mackinaw in Sea Ice [3]

Ramming the Ice: Icebreaker Propulsion

Freighter in Ice

Breaking the Ice: Icebreakers

Lying with Numbers

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COMMENTS

The Basics of Hull Design Explained
A truly flat-bottomed boat has zero degrees of deadrise. Most powerboat hulls have some deadrise, giving the hull bottom its "V" shape when viewed from the bow or stern. The deep-V hull was developed in the late 1950s and proved to be optimal for high-speed offshore vessels, with transom deadrise of 18 to 24 degrees.
Sailing Yacht Design (13.734)
This subject teaches students, having an initial interest in sailing design, how to design good yachts. Topics covered include hydrostatics, transverse stability, and the incorporation of the design spiral into one's working methods. Computer aided design (CAD) is used to design the shapes of hulls, appendages and decks, and is an important part of this course. The capstone project in this ...
Introduction to Sailboat Design: a Technical Exploration
2) Hull Design. The hull is the heart of any sailboat. Its shape determines how the boat interacts with the water. Hull design encompasses the choice of hull form, its dimensions, and the material used. The hull's shape affects its hydrodynamic performance, stability, and overall handling.
Basics of Sailboat Hull Design
Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel. There are two types of stability that a design addresses. The first is the initial stability, which is how resistant to heeling the design is.
The Basics of Hull Design Explained
A boat's weight—and its distribution fore and aft—is central to hull design of any kind. We design the underwater volume of a hull to be distributed in a way that matches the optimal fore and aft center of gravity of the assembled boat plus her occupants and gear. This "center of buoyancy" is the volumetric center of the shape of the ...
PDF THE DESIGN RATIOS
with explanations of the various design ratios, publishing one a week. I admit to having an ulterior motive. Many years ago, a client of mine informed me of his invention of what he called the S Number (S#), which is a way to rate the performance of all sailboats on a scale of 1 to 10 using the Sail Area/Displacement ratio (SA/D) and the
PDF Structure design of a sailing yacht hull by rules and
tureOn the Figure 82 are shown boundary conditions and loads of yacht structure. Boundary conditions are fixed supports at the edges of the model and als. on th. top of the model where structure is connected. with the hull of the yacht.5.2.3. Total deformation of the keel by direct methodTotal deformati.
Understanding Hull Design
A shape that generates lift also produces resistance, and a wider hull that has more interior room also has more hull surface in contact with the water, and therefore experiences more resistance. However, hull design isn't just about speed and interior volume. There's also stability and tenderness, which is a boat's tendency to rock.
4.2 Conceptual design in sailing boat hulls
Hull shapes for sailing boats typically fall into the three types shown in Figure 22. The 'fin and skeg' design is typical of modern mass-produced yachts and small dinghies. The long-keel design is typical of working boats designed to be handled by a small crew. These two forms represent extremes of stability and performance.
PDF Eric W. Sponberg
(sail area) to weight (hull, rig, and crew) ratio. These are perhaps the extremes of sailboat design. Let's explore the middle ground, where most boats reside. Table 1 lists the key criteria for the design of keels for racers and cruiser/racers. Simple Planform Keels The "simple" or "standard" keels such as the one shown in Figure 1
Types of Sailboat Hulls
June 15, 2022. Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls. There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats ...
Optimization strategy for planing hull design
A recent experimental planing hull optimization study was carried out by Kim et al. (2013), in which three planing hulls were designed and adjusted based on the results of model testing to improve their resistance and seakeeping. The results obtained in such model tests are often systematic hull series to serve for the design of planing hulls.
The Illustrated Guide To Boat Hull Types (11 Examples)
There are three boat hull categories: displacement hulls, which displace water when moving; planing hulls, which create lift at high speeds; and semi-displacement hulls, which displace water and generate lift at low speeds. The most common hull types are round-bottomed, flat-bottomed, multi, V-shaped, and pontoon hulls.
Sailing Yacht Design: Theory
Books. Sailing Yacht Design: Theory. Andrew R. Claughton, J. F. Wellicome, R. A. Shenoi. Longman, 1998 - Sports & Recreation - 337 pages. This book forms part of a two-volume guide to the fundamental principles governing how and why a sailing yacht behaves in the way it does including an understanding of the physics involved and mathematical ...
Notes on Sailing-Yacht Hullforms, Part 2
By Jay Paris, Mar 22, 2022. In "Notes on Sailing-Yacht Hullforms" ( Professional BoatBuilder No. 196, page 32) naval architect Jay Paris, citing drawings and photographs of his own designs as well as the lines of other designers, detailed some characteristics, attributes, and design methods used to develop monohull sailboat hullforms.
Neville's Magic Hull Shape
The second boat has a 60-foot waterline with an 18-foot beam and a 4.5-foot draft, and displaces 75,000 lbs. The first hull has a Cb of 0.163, including the keel. The second has a Cb of 0.241. This shows that the hull form of the first boat is not nearly as full as that of the second boat.
The ABCs of Understanding Hull Design
A shape that generates lift also produces resistance, and a wider hull that has more interior room also has more hull surface in contact with the water, and therefore experiences more resistance. However, hull design isn't just about speed and interior volume. There's also stability and tenderness, which is a boat's tendency to rock.
Boat Hulls 101: Complete Guide to Boat Hull Types, Shapes ...
Tri-Hull. The tri-hull design is a variation of the v bottom hull. It has a v-shaped hull in the center and two parallel smaller hulls on either side of the main central one. This gives the tri-hull boat some extra stability when going forward. Additionally, this also allows the boat to have more deck space as the hull covers a wider range.
A design method for planing hulls, considering hydro-elasticity and
Applied rules assess the strength of the members of the hull by applying beam theory and a static design pressure, which represents the slamming load. Actually, the slamming is a violent fluid structure interaction, where dynamics, hydro-elasticity, and nonlinear structural effects may be important.
How hull shape affects comfort at sea
Light displacement, modern wide hull - Pogo 10.50. This decision should depend, at least in part, on how comfortable the yacht is in a seaway. Now the comfort of yachts of the same size can be remarkably different. Take yachts of around 35ft or 10.6m long - a common yacht length. Three examples show how much the displacement can vary for ...
How a Sail Works: Basic Aerodynamics
As your sailboat catches the wind in its sail and works with the keel to produce forward motion, that heeling over will be reduced due to the wing-like nature of the keel. The combination of the perpendicular force of the wind on the sail and the opposing force by the keel results in these forces canceling out.
The TRUTH of Hull Speed: How to Break the Sailing Speed Limit
As the boat goes faster, the waves at the bow and stern get longer. When you reach hull speed, the bow wave lines up with the stern wave. The two waves double up on you. Your boat appears to sink down into one big wave trough. (Figure 1‑1) The hull speed theory states your sailboat will not go any faster once this happens.
PDF Engineering the Sailboat Safety in Numbers
cture, in reality, will stay in one piece.Safety factors usually vary between 1.5 and 4.0 times the estimated maximum load, but on occasion can b. 10.0 or more, depending on the structure. On a racing sailboat, which will receive continuous care and where excess weight and windage in the rig are undesi. ble, the smaller safety factors are used ...