Powerboat Engines

Below hull speed, a little power does it
It takes surprisingly little engine power to push a boat through the water at modest speeds, but the amount of power you need depends to a large extent on the design of the hull and the speed you seek.Powerboat hulls fall roughly into three classes: at either end of the spectrum are displacement hulls and planing hulls; in between are semidisplacement (or semiplaning) hulls.A displacement hull is designed to displace the water it moves through, shoving it outward and downward as it goes. In so doing, the hull creates a wave trough whose two high ends are positioned roughly opposite the ends of the boat when it approaches maximum speed. Below this speed, a displacement hull needs comparatively little power. About 2 hp per ton of displacement is sufficient to propel it at what is known as hull speed—the situation described above—in calm water. The hull speed of a displacement hull (its theoretical top speed) is roughly 1.34 times the square root of its waterline length in feet; the result is in knots. Modern powerboats have considerably more than the minimum power requirement, of course, to compensate for wind resistance and rough water.A semidisplacement hull with a lot more power can partially rise up over its own bow wave and go faster than the displacement hull, but it can never skim over the surface of the water as a true planing hull does. For example, let’s take a displacement hull of 36 feet on the waterline whose twin 120 hp diesels give it a hull speed of 8 knots. A semidisplacement hull of the same size would need almost to double the horsepower to achieve a top speed of about 12.5 knots. Even then it would not be planing cleanly, but rather simply bursting through its own bow wave.In true planing hulls, the primary factor governing speed is the power-to-weight ratio. With 1 hp delivered to the propeller for every 40 pounds of boat weight (including crew, fuel, and stores, of course), the average planing hull will do 25 knots in calm water. If there’s 1 hp for every 10 pounds, the speed increases to 50 knots, and so on.See also Bilge; Chines; Hull Shapes; Planing; Sailboat Engines.