Displacement vs planing hulls

Flat-bottom hulls come in two broad varieties: displacement and planing.

• Displacement hulls tend to be long and narrow, designed for water to go around the hull. These tend to be faster in flat water.
• Planing hulls tend to be short and fat, design for water to go under the hull. They often have a rocker which makes it easier to get on top of waves. These tend to be faster in downwind conditions.

SUPs are measured by length and width. For example, a SUP can be 12’ x 24”, which means it’s 12’ long and 24” at its widest point. Most SUPs races are limited in length to 14’, so this is an effective upper bound for hull length.

The theory behind planing vs. displacement hulls is that a planing hull is slower at low speeds but once it starts to plane at higher speeds, it lifts out of the water and reduces the wetted surface, lowering friction and allowing higher top speeds.

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).

References:

• Heavy Boats, Light Boats, and Hull Speed: explained in layman terms.
• Wikipedia: Wave-making resistance, for a more physics-based explanation.