Understanding Hull Speed

A displacement sailboat pushes through the water rather than planing on top of it. When such a vessel moves, its hull creates a system of waves. As the boat goes faster, the spacing between these waves increases. Eventually the wavelength becomes comparable to the boat’s waterline length, and the hull sits in a trough between the bow wave and the stern wave. The energy required to climb its own bow wave rises sharply, so further acceleration demands vastly more power. This limitation gives rise to the famous “hull speed” rule of thumb.

The classic formula states that hull speed in knots equals 1.34 times the square root of the waterline length in feet. The factor 1.34 stems from empirical observations and stems from the physics of wave propagation at the water’s surface. In equation form you will often see:

where LWL is the length of the boat’s hull at the waterline measured in feet, and V_h is the theoretical maximum efficient speed in knots. Though the constant 1.34 works well for many hull shapes, some designers adjust it slightly to account for hull fineness or modern shapes that can exceed the simple limit.

Below is a table listing typical hull speed values for a range of waterline lengths. Notice how speed increases with the square root of length, meaning that doubling the waterline produces only about a 41 % improvement in hull speed. This relationship highlights why long racing yachts can cruise fast while smaller boats reach a modest ceiling.

LWL (ft)	Hull Speed (kt)
15	5.2
20	6.0
25	6.7
30	7.3
35	7.9
40	8.5
45	9.0

While sailing on a modern cruiser, you will often see that motoring faster than hull speed yields diminishing returns. The bow climbs ever steeper as the stern squats down, producing a large wake. In heavier seas the boat might even become uncomfortable or unstable. Designers may therefore consider waterline length when planning a vessel’s cruising range because each foot of LWL yields a slight speed advantage.

Some boaters wonder whether hull speed is an absolute barrier. In short, it is not a brick wall but rather a speed at which the boat becomes inefficient to push any faster. A light planing hull or multihull can climb its own bow wave and exceed the simple limit, but a typical displacement monohull rarely manages that without large power or surf waves pushing from behind. Even a heavy keelboat can sometimes surf down a wave to speeds well above hull speed, but sustained operation at that pace requires extraordinary power.

Calculating hull speed is useful when comparing designs or estimating passage times. Keep in mind, however, that actual cruising speed may be 10–20% lower depending on sea conditions and how heavily laden the vessel is. The rule also does not apply to planing boats or slender racing hulls with long overhangs that only touch the water at speed.

The formula stems from a deeper relationship between wave period and gravitational acceleration. One can derive that a wave traveling in deep water has a speed given by , where g is gravitational acceleration and L is the wavelength. If we set L equal to the waterline length, convert units appropriately, and note that 1 knot equals 1 nautical mile per hour, we recover the empirical factor near 1.34. In practice the constant can vary from about 1.3 to 1.5.

Even though hull speed offers just an approximation, it has served sailors for generations. When selecting a cruising boat, one often considers not only comfort and safety but also how efficiently a vessel can make passages. A few extra feet of waterline length may shave hours off a trip. Nevertheless, the joys of sailing are not measured solely in knots. Many sailors deliberately slow down to appreciate the sea and sky. But if you want to maximize your boat’s potential, computing hull speed is a solid first step.

To use this calculator, simply enter the length at the waterline in feet. The script will immediately display the theoretical hull speed in knots. You can then copy that number and compare it with actual speeds observed while sailing. If you wish to experiment, change the length to see how a larger or smaller design might perform.