If you’re new to the yacht market, you may find yourself a little overwhelmed by the many variables you need to face as you choose your first vessel. One of these, and perhaps the most fundamental, is the shape and type of the hull. This is an important consideration, as it will affect everything from your initial outlay to your running costs, your yacht’s performance and handling, as well as the kinds of conditions in which you can sail.
So, to make this choice as simple as possible for you, and simply to help you understand why your boat is shaped the way it is and what that means for its function and performance, we have put together a guide to hull shapes and types.
The math and science behind hull design can be quite complicated. We are not going to go into all the finer details, but it’s worth knowing a little about it to start with. It also inspires a deeper appreciation of the art of hull design and boat-building as a whole. Let it be enough to say that, at the core of it is Archimedes’ principle of buoyancy, which states that any object placed in a liquid is acted on by an upward force which is equal in magnitude to the weight of the fluid displaced by the object. The volume of the displaced fluid is equivalent to the volume of whichever part of the object is submerged in the fluid. Finally, the buoyant force is equal in size to the weight of the object itself. What this means is that, when a boat is lowered into the water, it sinks until the weight of the water it displaces is equal to its own. As it is loaded with passengers, cargo or equipment, it sinks further and displaces more water until another level of equivalency is reached.
Now, when it comes to a boat’s motion, or how the boat moves through the water that is buoying it up, hull designers have essentially taken two approaches to Archimedes’ principle. The first actually uses the displacement of the boat to move it forward, while the other develops combinations of the hull shape and power supply to attempt to raise the boat up, lower its displacement, and thus enable speed. From these two approaches, we get the two basic hull types that we know today.
Different Yacht Hull Types
Hulls do come in several shapes. But overall, there are really only two types of hull, namely a displacement- and a planing hull. These descriptions refer to the way in which the hull moves through or over the water, and the way in which their designers treat Archimedes’ principle, whether knowingly or not.
In a nutshell, a displacement hull is designed to push through the water, thus pushing it aside or displacing it. As the name suggests, this design works with Archimedes’ principle.
A planing hull, on the other hand, is designed to rise and float across the surface of the water when there is a sufficient power supply. This design kind of uses physics against itself, using the shape of the hull and propulsion power, to give the boat a lower displacement than it would have at rest.
Boats with displacement hulls often use a relatively low powerplant and propulsion compared to planing hulls. In technical terms, the boat is propelled primarily through its own displacement, which is the volume and weight of water that it pushes aside in order to occupy any given space. A displacement hull thus moves forward by cutting out its own path through the water.
The high speed of a boat with a displacement hull is determined by its load waterline (LWL). For the mathematicians among you, the formula to calculate the top speed is 1.34 times the square root of the LWL. So a vessel with a 50 ft LWL will have a hull speed of 9.5 knots.
The main benefit of a displacement hull is that it tends to move much more smoothly than a planing one. The possible downside is that these kinds of boats are limited to lower speeds since they sit lower in the water and create a high level of active resistance against the water as they move.
This hull design is mostly used on large cruise ships and sailboats because there is a greater emphasis on the smooth motion as opposed to the speed.
Boats with planing hulls tend to skim across the surface of the water as much as possible, rather than displacing high quantities of water. They can actually operate like displacement hulls when at rest or when moving at low speed. They then rise further up out of the water as power supply and speed increase.
In fact, boats with planing hulls actually move at three different levels, depending on the speed at which they are traveling. If they are moving at low speeds, they are effectively displacement hulls. When speed is increased to a certain level, the bow rises out of the water and the boat goes forward with a plowing motion. Traveling at such a speed for a considerable period of time is not advisable, as it creates a large wake and reduces operator visibility. It’s better to increase the speed until full planing mode is achieved. At this speed, the entire boat moves across the surface of the water.
Most small, power-driven boats, as well as small sailing boats, have planing hulls. The purpose and emphasis with these designs is the attainment of high speeds, regardless of how bumpy the ride may be.
Combine the bow of a displacement hull with the stern of a planing hull, and you get a semi-displacement hull. This keeps the boat supported and stable at low speeds, while still allowing it to travel at higher speeds compared to displacement vessels of the same size. The larger space in the bow also accommodates more tankage, storage or accommodation, though this also means that a fully loaded boat will be substantially heavier than a displacement-hulled relative. These designs also tend to be heavier on fuel.
Different Hull Shapes
Within each of these categories, there are several different shapes of the hull, each of which functions to type in slightly different ways.
This is the oldest and most basic hull shape and the most obvious example of a planing hull. You are very unlikely to come across a yacht with a hull that is entirely flat at the bottom, but there are some designs that have minimal deadrise, which is about as close as you can get. This shape is intended mostly for lighter, high-performance boats that skim at high speeds across the top of the water. If you’re carrying heavy loads and heading out into unpredictable waters, this wouldn’t be the boat for you. They may be stable, but they tend to bounce and pound in a difficult current and, thus, may take on damage quite easily.
This is the wedge-shaped hull that you are probably most familiar with. Planing hulls of this type can come in quite an array of variations. They are engineered in such a way that the bottom third of the boat that is still in the water forms a wedge. This allows the hull to cut through the water with minimal displacement. They are the most adaptable to all conditions, and are a far easier ride than their flatter-bottomed relatives. There are drawbacks to V monohulls, however. They have increased draft, and they are less stable, which means that they will tend to roll in rough conditions and with sharp turns.
These combine the best of both the flat- and V-bottomed hulls. Usually, the design includes a flatter shape towards the stern and more of a V-shape towards the bow. The result is that the flatter-shaped stern provides better stability, while the V-shaped bow offers the dynamic benefits of deep-V hulls.
Most boat designers these days opt for variations on modified V-shapes, as they eliminate the drawbacks of pure deep-V hulls. In addition to the added stability, they are also more economical on fuel. A deep V design throughout the length of the boat, particularly when it tapers the overall boat space towards the stern, results in a boat that doesn’t plane as easily and has a larger surface area coming into contact with the water. What this means is that you need to increase power to get the speed you want, and that means burning more diesel or gasoline. Modified V-shapes get around this problem quite effectively.
The best examples of these shape hulls are catamarans and pontoons. These types of hulls consist of two narrow hulls bridged together. The hulls are usually of a V-shape, which means they are planing hulls with the dynamic benefits of V-shaped monohulls, while also having additional stability. The downside is that they tend to have less onboard space, in other words, the cabin and cockpit are reduced in size to varying degrees. They also require a larger area to turn.
This is the classic displacement hull, able to move smoothly and consistently through the water, even at fairly low speeds. The disadvantage of these hulls is that they can roll quite easily, although this can be offset with the addition of a deep keel in the design.
It’s All About Balance
With modern yacht designs, there is never really any question of choosing a design that sits squarely in any of the above categories or shapes. As is apparent from the discussion about modified V hulls, boat design is about establishing a perfect balance among a number of variables, such as speed, carriage space, stability, maneuverability and handling, smoothness of motion, and tenderness. The kind of hull you choose ultimately depends on which of these factors is most important to you. If you don’t need much speed and want comfort instead, you will most likely go for a displacement hull or some variation on it. A need for speed, on the other hand, will lead you to a sleeker, planing-oriented design.
This information serves to give you a basic idea of the various types of hull shapes and types. But of course, there is still a lot to consider as you search for your perfect vessel. Get in touch with us for all you need to know in the world of yachts.