Hull Speed Calculator

What is hull speed?

Hull speed is the natural speed limit of a displacement hull — the point where the boat's bow and stern waves combine into a single wave as long as the hull itself. Below this speed the boat slips along efficiently; pushing past it means trying to climb its own bow wave, which takes a steep, often impractical increase in power. For most sailing yachts, trawlers and other displacement boats, hull speed is the realistic maximum cruising speed under engine or sail.

Hull speed formula

The classic formula relates hull speed to waterline length (LWL):

Hull speed (knots) = 1.34 × √(LWL in feet)

In metric, that is roughly 2.43 × √(LWL in metres). This calculator uses the metric waterline length you enter and returns the theoretical hull speed in knots. Only the waterline length matters — overhangs at the bow and stern don't count, which is why a long-keeled cruiser can be faster than a longer boat with short waterline length.

Worked example

A yacht with a 9 m (≈29.5 ft) waterline: 1.34 × √29.5 ≈ 7.3 knots. Adding a bigger engine won't make it cruise much faster — it will simply burn more fuel for a fraction of a knot. To go faster you need a longer waterline or a hull designed to plane.

Typical hull speeds by waterline length

Can a boat go faster than hull speed?

Yes — but it depends on the hull. Light, powerful planing boats (most runabouts, RIBs and ski boats) climb over the bow wave and ride on top of the water, leaving hull speed far behind. Semi-displacement hulls can push 10–30% past it. Heavy displacement hulls effectively can't, so hull speed is their practical ceiling. To estimate the speed a planing boat will actually reach, use our horsepower-to-speed calculator.

Note: this is a theoretical figure. Real maximum speed depends on hull design, power available, loading and sea conditions.