Hull speed

For small displacement hulls, such as sailboats or rowboats, wave-making resistance is the major source of the marine vessel drag.

A simple way of considering wave-making resistance is to look at the hull in relation to bow and stern waves. If the length of a ship is half the length of the waves generated, the resulting wave will be very small due to cancellation, and if the length is the same as the wavelength, the wave will be large due to enhancement.

Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.

A heavy boat gets trapped in its own wave system. In spite of their enormous horsepower, they can't break loose from the trap of their own wave system. A light displacement boat such as a dinghy, a ULDB, or a multihull doesn't have so much water to move out of the way - so they make smaller waves. When they reach the speed that would be hull speed for a heavy boat the wave system is not big enough to trap them. They are able to exceed the "speed limit" where bow and stern waves coincide.

The phase speed c of waves is given by the following formula:

c = sqrt( (g / 2pi) * l)

Substituting, the rule of thumb is:

c (knots) = 4/3 * sqrt(length in feet)

In other words, an 11 ft paddle board will have hull speed of 4.4 knots (5.2 mph).