What hull speed measures
A displacement hull moving through the water makes a wave system of its own. As the boat speeds up, the waves it makes get longer, until the crest at the bow and the crest at the stern are one waterline length apart. At that point the hull sits in a trough it has dug for itself, and going any faster means climbing its own bow wave. The speed at which that happens is called hull speed. Pushing past it takes a steep increase in power, which is why most cruising monohulls rarely sail much faster, even in a strong breeze.
The formula
Hull speed depends only on waterline length (LWL), because the speed of a water wave depends only on its length.
In metric: hull speed (knots) = 2.43 × √LWL (m). In imperial: hull speed (knots) = 1.34 × √LWL (ft).
The two give the same result; the constant changes because a metre is 3.28 feet. The factor 1.34 is the
speed-length ratio, the speed in knots divided by the square root of the waterline in feet.
A boat with a 9 m waterline has a hull speed of about 7.3 knots; double the waterline to 18 m and hull speed rises only to about 10.3 knots. Length is the most effective way to go faster in a displacement boat.
How to read it
Treat hull speed as a practical cruising ceiling rather than a hard limit. A well-sailed boat in flat water reaches it on a reach in a good breeze; in light air most boats sail well below it. Waterline length grows when a boat heels and its overhangs touch the water, so older boats with long overhangs often sail a little faster than their static waterline suggests.
Limitations
- Light boats with flat aft sections do not stop at hull speed: they surf and plane, reaching speed-length ratios of 2 or more. Multihulls with slender hulls are not bound by it in the same way, so the number says little about them.
- The 1.34 factor is a convention. Naval architects use values between about 1.3 and 1.5, depending on hull shape. The result is the theoretical speed in flat water, not a prediction of passage speed.
- Published waterline lengths are measured at different loadings. A fully loaded cruising boat sits deeper and often has a slightly longer waterline than the brochure figure.
Sources
The wave-length relationship goes back to William Froude's towing-tank work in the 19th century; Dave Gerr's The Nature of Boats explains it for sailors. The 1.34 constant is the one used by sailboatdata.com and on our yacht pages, where the hull speed shown matches this calculator. See also our sources and methodology.