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Rounded hulls are displacement hulls where the V of the bow is not very pronounced. These are typical of trawlers or slow powerboats with a long range. This hull shape provides a large forward volume and good stability when at anchor.
They are generally prone to rolling in motion because powerboats lack keels.
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This hull shape is mainly used in small pleasure boats, such as dinghies. In fact, before the advent of inflatables, the classic tender in recreational boating was the flat-bottomed boat!
The flat bottom, combined with moderate speed, creates a wide wake.
This characteristic is utilized by ski and wakeboard boats. These vessels feature a V-shaped bow that broadens toward the stern, eventually becoming flat.
In professional maritime fields, ferries and short-distance passenger ships often have flat-bottomed hulls.
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The V-shaped hull is very common on motorboats. The angle of the V (expressed in degrees of "deadrise" in English) is the angle formed by the boat’s bottom in relation to the horizontal. Flat-bottomed hulls have little to no V, while boats with more pronounced V-shapes have deadrise angles ranging from 21 to 27 degrees.
The V-shape cuts through waves where rounded hulls tend to pound. This shape also provides better directional stability and, by deflecting waves, helps keep the boat dry. However, V-shaped hulls have deeper drafts than flat or rounded hulls, and because they have less forward volume, they tend to roll more at anchor.
These are the hulls of fast boats.
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In some cases, the V can even invert and become concave, maximizing lift generated by the bow waves, as seen on the SRD (Schulz Reverse Deadrise) of Shannon boats.
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A chined hull is characterized by more or less sharp angles where the bottom and the sides meet.
These angles, called "chines," mark the junction between the different panels that form the hull’s shape.
Chines offer two main advantages: they sometimes simplify hull construction, such as with plywood hulls (seen in RM sailboats) or certain amateur-built steel sailboats of the past.
The design of a powerboat hull doesn’t stop at choosing the hull shape characteristics. Powerboat hulls, which generally aim for speed or at least planing, benefit from certain "secrets" that boatbuilders are reluctant to disclose in detail.
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Once on plane, this Jeanneau Cap Camarat 10.5 WA has only a fraction of its hull in contact with the water.
This reduction in wetted surface limits drag and enhances either top speed or fuel efficiency at cruising speed.
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Spray Rails
The term "spray-rails" in English is more descriptive than its French counterpart. These are longitudinal ridges designed to deflect bow waves, keeping the boat’s occupants dry.
They can often be seen below the strakes on boats like the Cap Camarat.
Strakes
A strake is a relatively horizontal surface that provides lift to a V-shaped hull. It’s the reverse chine seen in the illustration below.
V-shaped hulls lack forward lift, and strakes help maintain lift in this area.
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The strakes are clearly visible on each side of this Jeanneau Merry Fisher 795.
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If one subject has stirred debate in the world of motorboat hulls, it’s the use of steps.
A step is a transverse drop in the longitudinal line of the hull, designed to trap air or a mix of air and water, providing additional lift to the boat.
The hull then rests on this step, creating a sort of air cushion that improves sliding and comfort.
On a two-step hull, as illustrated above, the wetted surface is greatly reduced by the "air cushion" generated by the step. As a side effect, the step allows the lift point to move forward compared to a hull of the same wetted surface, improving maneuverability and stability.
Beneteau makes extensive use of steps with its Airstep 1 and now Airstep 2 models. The original Airstep 1 featured external air intakes that directed air to the hull.
Airstep 2 has air scoops directly under the hull, eliminating the need for tubes.
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Petestep is a step system where pronounced steps and spray rails are implemented along three-quarters of the hull’s length.
Since spray accounts for 30% of a hull’s friction, Petestep systems harness part of this spray energy as lift.
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A young British company is pushing the step concept even further with Pascal Technologies’ Airhull. This system relies on an air cushion (Pascal = Pressure, air cushion, vertical thrust) forced through ducts by an electric fan.
The Airhull lifts the boat about 20 cm, significantly reducing drag, by as much as 50%.
Foils are typically mobile devices, somewhat like flaps, designed to provide localized lift, allowing it to be transferred from the hull to the foils.
Though still rare on pleasure boats, foils have garnered attention with electric models like the Candela C-8.
Most of the time, mobile foils are only useful in calm seas.
The boat lifts onto its foils, but in rough waters, it could stop abruptly if it encounters a wave.
More recently, SeaAir introduced a semi-rigid boat with mobile foils, the SeaAir Flying Tender, which rises 20 cm on its foils and is said to consume 50% less fuel than a standard RIB. However, these systems are very complex and inevitably priced out of reach for small-scale recreational boating. The Princess R35 (35' open) was as expensive as a 50' cruiser without foils from the same brand!
Fixed foils are also starting to appear on large luxury yachts, offering lift without increasing draft or complicating systems.
