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Rounded hulls typically feature rounded bilges and relatively shallow deadrise angles, particularly towards the stern. These hulls are commonly found on trawlers and long-range displacement motorboats. This hull shape can provide generous interior volume and good initial stability, depending on the overall hull design.
These boats can be prone to rolling, particularly in beam seas, due to their rounded underwater sections and relatively limited roll damping.
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This hull shape is mainly used in small pleasure boats, such as dinghies. Before inflatable tenders became widespread, flat-bottomed dinghies were commonly used as yacht tenders.
A flat-bottomed hull can generate a significant wake, depending on its speed, displacement and trim.
Hull shape and ballast are carefully designed on watersports boats to produce the wake characteristics required for waterskiing or wakeboarding.
Many watersports boats feature a V-shaped bow that transitions into flatter aft sections.
In commercial marine applications, some ferries and short-distance passenger vessels feature relatively flat-bottomed hulls.
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The V-shaped hull is very common on motorboats. The deadrise angle is the angle between the hull bottom and a horizontal plane, measured across the hull. Flat-bottomed hulls have little or no deadrise, while deep-V hulls typically have deadrise angles of around 20 to 25 degrees or more at the transom.
A deep-V hull cuts through waves more smoothly, helping to reduce pounding in choppy conditions. This hull shape can improve handling in rough seas, while properly designed chines and spray rails help deflect spray away from the boat. Deep-V hulls generally have greater draught than comparable flat-bottomed hulls and may experience more rolling at anchor due to their narrower immersed sections.
Deep-V hulls are commonly used on high-speed powerboats.
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In some hull designs, the bottom geometry transitions into reverse-deadrise sections to improve hydrodynamic lift and efficiency, as seen in Shannon's SRD (Schulz Reverse Deadrise) hull design.
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A hard-chine hull is characterised by distinct angular transitions between the bottom and sides of the hull.
These edges, known as chines, mark the transitions between different sections of the hull.
Chine construction can simplify hull manufacturing, particularly when using flat panels of plywood, aluminium or steel, as seen on RM sailing yachts and many amateur-built boats.
Designing a powerboat hull involves more than simply selecting its basic hull shape. High-performance powerboat hulls incorporate various design features to improve hydrodynamic lift, reduce drag and enhance handling, some of which are proprietary to boatbuilders.
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Once planing, this Jeanneau Cap Camarat 10.5 WA has a significantly reduced wetted surface area.
This reduction in wetted surface area reduces frictional resistance, potentially improving top speed and fuel efficiency at cruising speeds.
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Spray Rails
The English term "spray rails" describes their function of deflecting spray away from the hull. Spray rails are longitudinal projections along the hull designed to deflect spray away from the boat, helping to keep the deck and occupants dry.
They can be seen along the lower sections of the hull on boats such as the Cap Camarat.
Strakes
A lifting strake is a longitudinal projection on the bottom of a V-shaped hull that helps generate hydrodynamic lift and deflect spray. Unlike a reverse chine, which is located along the outer edge of the hull bottom, lifting strakes are generally positioned further inboard.
Lifting strakes help increase hydrodynamic lift and improve planing performance, particularly on deep-V hulls.
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The lifting strakes are clearly visible on both sides of the hull of this Jeanneau Merry Fisher 795.
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Few hull design features have generated as much debate in the powerboating world as stepped hulls.
A step is a transverse discontinuity in the hull bottom designed to introduce air beneath the hull, reduce wetted surface area and improve planing efficiency.
The step allows air to flow beneath the hull, reducing wetted surface area and hydrodynamic drag, which can improve efficiency and ride comfort.
On a twin-step hull, as illustrated above, ventilation behind the steps reduces the wetted surface area and can significantly decrease hydrodynamic drag. Steps modify the distribution of hydrodynamic lift along the hull and can improve efficiency and handling when properly designed.
Beneteau has developed two generations of its patented Air Step hull technology: Air Step and Air Step 2. The original Air Step system used dedicated air intakes to channel air beneath the hull.
Air Step 2 features redesigned steps and increased airflow beneath the hull to improve efficiency and performance.
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Petestep is a patented hull technology that uses specially designed longitudinal spray deflectors to redirect water downwards and aft, generating additional lift and reducing drag.
Petestep is a step system where pronounced steps and spray rails are implemented along three-quarters of the hull’s length.
According to Petestep, spray generation can account for up to 30% of the hydrodynamic resistance of a conventional hull. Its deflectors redirect this spray energy to generate additional lift and forward thrust.
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Pascal Technologies has developed AirHull, an air-cushion-assisted hull system designed to significantly reduce hydrodynamic resistance. The system uses an electrically powered fan to inject air into a specially designed cavity beneath the hull, creating an air cushion that partially supports the vessel's weight.
According to Pascal Technologies, AirHull technology can significantly reduce hydrodynamic resistance and energy consumption by partially lifting the hull out of the water using a pressurised air cushion.
Hydrofoils are underwater lifting surfaces, either fixed or actively controlled, that generate hydrodynamic lift and transfer part or all of the vessel's weight from the hull to the foils.
Although hydrofoils remain relatively uncommon on recreational powerboats, they have attracted increasing attention with electric models such as the Candela C-8.
Hydrofoil performance depends on sea conditions, foil design and the vessel's active control systems.
As the boat rises onto its foils, hull contact with the waves is reduced. However, rough seas can cause ventilation, loss of lift or excessive loads, depending on the foil system and operating conditions.
SEAir developed the Flying Tender, a rigid inflatable boat (RIB) equipped with retractable hydrofoils. The manufacturer claims that its foil-assisted design can significantly reduce fuel consumption compared with conventional RIBs. However, these systems add mechanical and electronic complexity and can significantly increase purchase and maintenance costs, limiting their accessibility for smaller recreational boats. The 35-foot Princess R35 was priced at a level comparable to that of significantly larger conventional motor cruisers.
Fixed hydrofoils are also being incorporated into some larger motor yachts to provide additional hydrodynamic lift without requiring active foil-control systems, although their effect on draught depends on the installation.
