Boat Speed Calculator

This boat speed calculator works out the theoretical hull speed of a displacement boat from its waterline length. Hull speed is the natural speed limit of a boat that pushes through the water rather than skimming over it: as the boat moves it makes a bow wave and a stern wave, and once those waves are as far apart as the hull is long, going any faster means climbing the boat's own bow wave, which takes a sharp jump in power for very little extra speed. The classic estimate is 1.34 multiplied by the square root of the waterline length in feet, which returns a speed in knots. Enter the waterline length in metres, the length of hull that actually sits in the water rather than the overall length including any bow or stern overhangs, and the calculator converts to feet, applies the formula, and shows the result in knots, kilometres per hour and miles per hour. Yacht owners, kayakers, launch buyers and boat builders use it to gauge the efficient cruising speed of a hull and to compare designs. Remember that planing hulls, multihulls and light racing boats can exceed hull speed by rising over their bow wave, so treat this as the sensible cruising limit for a typical displacement hull, not an absolute ceiling.

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metres

Use the length of hull in the water, not the overall length including bow or stern overhangs.

7.28 knots
theoretical hull speed
Kilometres per hour13.49 km/h
Miles per hour8.38 mph
Waterline length29.5 ft

A 9 m waterline gives a hull speed of about 7.28 knots, a comfortable displacement cruising speed. Pushing much faster than this takes a steep jump in engine or sail power.

Hull speed is a theoretical guide for displacement hulls. Planing hulls, multihulls and light craft can go faster. Estimate only.

How it works

The calculator uses the traditional hull speed rule: speed in knots equals 1.34 times the square root of the waterline length in feet. Waterline length is entered in metres and converted to feet by multiplying by 3.28084 before the square root is taken. The knot result is then converted to other units: one knot is 1.852 kilometres per hour and 1.15078 miles per hour. Because the formula depends on the square root of length, doubling the waterline length only raises hull speed by about 41 percent, which is why longer boats are so much more efficient at speed than short ones.

Worked example

Take a yacht with a 9 metre waterline. That is 9 times 3.28084, or about 29.5 feet. The square root of 29.5 is roughly 5.43, and 1.34 times 5.43 gives a hull speed of about 7.28 knots. Converting, that is 7.28 times 1.852, or about 13.49 kilometres per hour, and 7.28 times 1.15078, or about 8.38 miles per hour. A boat with a 4 metre waterline, by contrast, works out at only about 4.85 knots, so the longer hull is well over a third faster.

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Frequently asked questions

What is hull speed?

Hull speed is the theoretical top speed of a displacement boat, the point where the wave the hull makes is as long as the hull itself and pushing faster takes a steep jump in power. It is estimated as 1.34 times the square root of the waterline length in feet, giving a speed in knots.

Why use waterline length and not overall length?

Hull speed depends on the length of hull actually in the water, because that sets the length of the bow and stern waves. Overhangs at the bow and stern add to the overall length but do not carry the wave, so the waterline length is the figure that matters.

Can a boat go faster than hull speed?

Yes. Planing hulls, multihulls and light racing boats can climb over their own bow wave and exceed hull speed, and even heavy cruisers surf faster down waves. Hull speed is a guide to the efficient cruising limit of a typical displacement hull, not a hard barrier.