Hull Speed Calculator
Calculate a displacement hull's theoretical hull speed in knots, mph and km/h from the waterline length.
How it works
Hull speed (knots) = 1.34 × √(waterline length in ft)
A displacement hull pushes a wave along its side as it moves, and its own length sets the length of that wave. As the boat speeds up, the wave lengthens until, near "hull speed", the boat sits in a trough between its bow and stern waves and needs sharply more power to go faster. The rule of thumb estimates this speed as 1.34 times the square root of the waterline length in feet, giving knots. It is an approximation — hull shape, weight and power all matter, and light, narrow or planing hulls can rise onto their bow wave and comfortably exceed it — so treat it as a guideline rather than a hard limit.
Worked example
A sailboat with a 25 ft waterline has a hull speed of about 1.34 × √25 = 1.34 × 5 = 6.70 knots — roughly 7.71 mph or 12.41 km/h. Doubling the waterline length only raises hull speed by about 41%, since it scales with the square root of length.
Frequently asked questions
What is the difference between waterline length and overall length?
Waterline length (LWL) is how much of the hull actually touches the water; overall length includes overhangs at the bow and stern. Hull speed depends on the waterline length, which is usually shorter than the overall length.
Can a boat go faster than its hull speed?
Yes. Light, narrow or planing hulls can climb over their own bow wave and plane, exceeding the figure here. For heavy displacement hulls, though, pushing past hull speed takes a large jump in power for little extra speed.
Why do longer boats sail faster?
A longer waterline supports a longer bow wave, allowing a higher speed before the boat gets trapped in its own trough. Because the formula uses the square root of length, gains taper off — you need four times the length to double the hull speed.
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Educational estimate. Real-world results vary with conditions, tuning and measurement.