Looking at the character of different types of bottoms
Almost every hull has a bottom built to one of three basic shapes, or a combination of them: flat, V, and round bottoms.Flat-bottomed boats are the simplest and least expensive to build, but they are mostly restricted to sheltered waters because they are prone to pounding in waves, which creates considerable discomfort for passengers and strains the structure of the boat. Hulls with flat bottoms need flare in their topsides to deflect spray and provide reserve buoyancy when they heel.The sharpie hull form—a flat bottom with narrow beam and a pointed bow—makes a fast sailer in flat water, though it will pound when sailing to weather in a chop. It requires a big centerboard trunk in its narrow cabin and provides little headroom below, but sharpies can be handsome. As a powerboat, a sharpie needs a deep forefoot to reduce pounding; it should be limited to semidisplacement
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| A few representative hull sections. |
speeds for the same reason. A deep skeg aft allows it to track better in a following sea.Dories are also flat-bottomed, with narrower bottoms than a sharpie but much more flare in the topsides, giving them greater beam on deck. This hull form makes them initially tippy, but the more they tip, the more they resist further tipping as that flare comes into play. With more rocker than a sharpie, a dory is meant for slow speeds—some of the earliest powerboats were sailing-rowing dories for inshore fisheries in which small engines had been retrofitted.V-bottomed hulls (also known as deadrise hulls) are more seakindly and far less prone to pounding. Although more often associated with powerboats, they are suitable for a wide range of sailboats, from small plywood dinghies to large seagoing vessels built of steel or aluminum. Fast-planing powerboat hulls often combine a deep V at the bow (i.e., 26 degrees or so of upward slope as seen in section view) to minimize pounding with a much shallower V at the transom (say, 10 to 18 degrees) to facilitate planing. These modified V-bottoms are often called warped-plane or variable-deadrise hulls.A deep-V or constant-dead-rise powerboat carries the steep deadrise of its entry all the way back to the transom. Ever since Dick Bertram’s
Moppie whipped other offshore racing powerboats in 1961, the deep-V hull has been accepted for offshore speed. Without flat after sections, it requires more horsepower to get up on plane and is not as fast in calm water as a modified V, but it can keep flying through seas that require other boats to slow down before they knock your teeth out. Deep-V powerboats are tippy at slow speeds but acquire dynamic stability when planing. They hold course extremely well but may turn with difficulty, and they have a tendency to lay over unpredictably on a side of the bottom—a defect known as chine-walking. Longitudinal running strakes and chine flats (an outward elaboration of the chine, or corner of the bottom and topsides seen in section, into a horizontal shelf) can cure this.A hull with a V-shaped bottom will naturally have a greater draft than a flat-bottomed hull, as well as greater displacement and, therefore, carrying capacity.Round bottoms provide the largest interior space for the least area in the water and, therefore, the least hull friction at reasonably slow speeds. They, too, are used for all types of boats, from small dinghies to the biggest ocean cruisers, both power and sail. They are the least prone to pounding of all three types and are often regarded as the most pleasing to the eye. However, they are more difficult to build using some materials, such as steel, and are therefore more expensive to produce, but a round-bottomed hull can be more easily fine-tuned by a designer to the exact performance specifications required.A round-bottomed hull is ordinarily one meant to operate at displacement speeds because the flat bearing surfaces required by a planing hull are incompatible with a rounded bilge. Nevertheless, some semidisplacement powerboats have rounded hulls; the Maine lobster-boat hull type isone example. The tighter the turn of the bilge, the higher is the hull’s potential speed.See also
Bilge; Chines; Deadrise; Keels; Skegs.
