A clear and concise analysis of various types of yacht hulls.

 

The behavior of a yacht on water and its usability depend directly on the type of hull. When choosing a yacht, it’s essential to determine which hull type suits your specific needs. In this article, we’ll list the main types of hulls existing today and clearly illustrate their differences.

Two main types of hulls using the example of a knife and a box.
The development and improvement of hulls aim to reduce the hull’s resistance when moving through the water. All hulls experience wave resistance, overcome frictional force, as well as resistance from form, aerodynamics, induction, and spray. Each design copes with these forces differently. Let’s consider how different designs behave in water using the example of a knife and a box. Imagine placing them in a pool as if they were two boats. The knife easily cuts through the water, even in waves. However, it doesn’t float, and its size doesn’t allow anything to be loaded onto it. In contrast, the box behaves differently: it floats well, even with a load, but it rocks on waves, especially when stationary, and in motion, such a hull will be hit by each wave with its full width.

Of course, this is a very rough comparison, but it accurately describes the behavior of the hull in water depending on its shape and the tasks faced by yacht designers. It’s intuitive that for faster speeds, a narrow hull that easily cuts through the water and creates minimal resistance is needed, but for comfortable accommodation of a large number of people, something box-like resembling a barge would be suitable.

The key is to combine the advantages of the “knife” and the “box” in one hull.

The more the hull’s shape resembles a “knife,” the faster the yacht, and the more it resembles a “box,” the more spacious and comfortable it is. Roughly speaking, a “knife” is a planing hull, and a “box” is a displacement hull. These are the two poles in yacht design. Between them, there are transitional types of hulls, and within them, there are subtypes depending on the shape of the bottom and the lines of the sides.

Advantages and Disadvantages of Displacement Hulls
Remember the school rhyme to remember Archimedes’ law, which allows bodies not to sink in water? “A body immersed in water displaces a volume equal to the volume of the body, with a force equal to the force of the displaced water.” In the case of displacement boats, Archimedes’ force is the main force acting on the yacht. Balanced Archimedes’ force and the force of gravity suspend the hull in the water, much like a buoy. So when moving, it simply pushes through the water due to the momentum received from the propeller or sails.

To make it easier for a yacht to push through the water, the shape of the hull below the waterline is often made round, smooth – to minimize water friction. They resemble ideally polished sea pebbles through which water passes without stopping. Only the yacht’s bow is made sharp to cut through the water like a knife, without raising a large wave. Even if the hull’s construction occasionally involves sharp edges (when the contours resemble a faceted glass), they are directed so that they intersect the waterline at the bluntest angle possible or don’t intersect it at all. After all, any extra angles and recesses create unwanted whirlpools, which slow down the boat.

The biggest limitation of yachts with displacement hulls is their speed. The formula for calculating the maximum speed of a displacement hull would look like this:

Here,

Lwl represents the length of the vessel at the waterline in feet.

Vmax represents the maximum speed of the vessel in knots.

Simply put, the speed of a displacement hull is limited by its length.

 

Of course, installing an extremely powerful engine can make the yacht go faster, but most of the “extra power” will be used to overcome resistance, as the hull is not designed for high speeds and will start “plowing” through the water, pushing a waves ahead of it.

As the speed increases linearly, the required power increases cubically. For example, to double the speed of a displacement yacht, the engine power needs to be increased by eight times. And to triple the speed, the power must be increased by 27 times.

To reduce wave resistance, a bow fairing of the boat is pear-shaped form is applied. It is positioned below the waterline and forms its own wave system, which overlays the hull’s wave system and reduces its impact.

Displacement boats significantly lag behind planing boats in speed when comparing vessels of the same size.

BUT HERE LIES THE GREAT ADVANTAGE OF DISPLACEMENT YACHTS: THEY ARE MUCH MORE FUEL-EFFICIENT AND PROVIDE GREATER COMFORT FOR EMBARKING ON LONG CRUISES AND NAVIGATING LONG DISTANCES.

The low speeds of displacement hulls significantly reduce the skipper’s ability to avoid an approaching storm or escape from a thunderstorm. At low speeds, it’s more challenging to navigate through heavy seas. Additionally, you won’t be able to maneuver between waves, which could significantly enhance safety and facilitate transitioning in bad weather conditions.

Displacement hulls also face the challenge of typically having a deeper draft compared to planing hulls, which restricts their maneuverability in shallow waters.

WHILE PLANING YACHTS MIGHT THEORETICALLY EVADE ADVERSE WEATHER CONDITIONS, IN THE EVENT OF A STORM, YOU’D PREFER TO BE ABOARD A YACHT WITH A DISPLACEMENT HULL.

For longitudinal stability on waves, which rock the boat sideways, the correct ratio of hull length to width is required. For example, for boats ranging from 6 to 12 meters in length, the hull length should be 3.5–4.75 times greater than its width. A displacement yacht sails steadily on the water, without a heel to the stern (longitudinal inclination of the hull to the corresponding side), allowing travel to continue even in bad weather.

SUCH YACHTS DO NOT FEAR EITHER ROLL OR PITCH, WHERE THE BOAT LIFTS BOTH THE BOW OR THE STERN.

To maintain the boat in a normal position, it is not necessary to apply much additional effort. There can be a lot of cargo on board (including passengers), and it can be distributed relatively freely across the yacht. Although the occurrence of heel (sideways tilt) or sternward heel is undesirable, even in this case the vessel will be able to continue moving. The weight of the boat itself also plays a minor role. With planing yachts, things are different.