Hull Speed Calculator
Widely quoted as a limit. It is not one.
Work out Hull Speed. Widely quoted as a limit. It is not one. Shows the working, not just the answer.
Hull speed
7.68 kt
14.21 km/h from a 10 m waterline (32.8 ft)
Hull speed is 1.34 × √(waterline length in feet) — the speed at which the bow and stern waves are one hull length apart, so the boat sits in the trough of its own wave. Climbing out of that trough costs enormous power. It is widely quoted as a limit and is not one. Light displacement hulls exceed it readily; heavy ones spend fuel disproportionately trying, which is why the same length of boat can be a 7 kt cruiser or a 20 kt planing hull. Waterline length is what matters, not overall length. Long overhangs contribute nothing until the boat heels and immerses them — which is exactly why classic yachts sail faster heeled than upright.
How the Hull Speed Calculator works
Hull speed from waterline length, with the speed-length ratio, Froude number and which regime the boat is actually in — displacement, semi-displacement or planing.
Also known as: hull speed from waterline length · how fast can a displacement hull go · speed length ratio boat · is hull speed a real limit
Frequently asked questions
What is hull speed?
1.34 × √(waterline length in feet). It is the speed at which the bow and stern waves are one hull length apart, so the boat sits in the trough of its own wave.
Is hull speed a hard limit?
No, though it is usually described as one. Light displacement hulls exceed it readily; heavy ones spend fuel disproportionately trying, because climbing out of the wave trough costs enormous power.
Does overall length matter?
Only through the waterline. Overhangs contribute nothing until the boat heels and immerses them — which is exactly why classic yachts sail faster heeled than upright.
Why is a longer boat faster?
Hull speed goes with the square root of waterline length, so four times the length is only twice the speed. It is why length is expensive and why racing rules have always regulated it.
What is the Froude number?
Speed divided by √(g × length) — a dimensionless version of the same relationship, used because it makes model tank tests scale to full-size hulls.
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