Hull speed: why waterline length limits a boat's speed and how planing gets past it

Published on 9 October 2026 · By Ricard Prehn, industrial engineer (Ingeniero Técnico Industrial), professional recreational skipper (PPER), Spanish Yacht Captain (capitán de yate) and sailing instructor for over 27 years

What is hull speed?

It is the speed at which the wave a boat makes is as long as its waterline. From there on, the boat sits between its bow wave and its stern wave, and to go faster it would have to climb its own wave. For a displacement hull, like that of almost every sailing yacht, it is in practice its top speed.

How do you calculate hull speed?

Hull speed (knots) ≈ 1.34 × √(waterline length in feet), or 2.43 × √(waterline length in metres). A yacht with a 10-metre (33 ft) waterline: about 7.7 knots.

Two details. Use the waterline length, the part in the water, not the overall length with pulpit and pushpit. And the square root rules: to go twice as fast you would need a boat four times as long.

Table: hull speed by waterline length

That is why big racing yachts are long: every metre of waterline is tenths of a knot that no engine or sail can give them.

Why does the wave depend on speed?

Because the wave the boat makes travels with it, at the same speed, and at sea long waves run faster than short ones: a wave of length λ moves at about 2.43·√λ knots (λ in metres). If the boat speeds up, its wave has to get longer to keep up. At first several crests fit along the hull; at hull speed, only one, from bow to stern.

The "wall": why more engine does not help

Below hull speed, resistance rises gently. Once you reach it, things change: the stern crest drops behind, the stern sinks into the trough, the bow lifts and the boat is literally going uphill. The power needed shoots up: on a yacht under engine, going from 7 to 8 knots can take twice the power, and from 8 to 9, an absurd amount. All you get is a bow-up boat, a huge wake and a lot more diesel.

That is why, motoring in a sailing yacht, a sensible cruising speed sits a little below hull speed: that is where fuel goes furthest.

How can a small motorboat do 25 knots?

By planing. A planing hull is light, has a flat or shallow-V bottom aft and carries plenty of power. Past hull speed it also meets its wave (the “hump”, where it needs the most power and lifts its bow highest), but it gets over it: the water pushes it up like a water ski, the hull rises and leaves the wave behind. Length no longer limits it: a 6-metre boat with a 6-knot hull speed can do 24 or more.

Engineers measure it with the Froude number, which relates speed to length: hull speed is around a Froude number of 0.4, and a boat truly planes from about 1. In between sit semi-displacement hulls, such as many fishing boats and motor launches, which go somewhat beyond hull speed without fully planing.

Can a sailing yacht exceed its hull speed?

A little, and only now and then. In strong wind with a following sea, a cruiser can surf the waves and exceed it for a few seconds. Light sailing dinghies (a Hobie-style cat, a 470, a Laser) do plane in a breeze. And very slender hulls, such as a catamaran’s, make less wave and get past it more easily. But a loaded keelboat cruiser lives below its hull speed.

And as a sailing yacht speeds up, the wind its sails feel changes too: we explain it with another animation in apparent wind.