Boating Calculators
Hull speed.
Enter the waterline length and get the classic displacement hull speed, then enter your actual speed to see your speed/length ratio and how close you are. Runs in your browser; nothing is sent or stored.
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Result
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Hull speed (knots) = 1.34 x the square root of the waterline length in feet. A 30 ft waterline gives 1.34 x sqrt(30) = 7.34 knots, about 8.4 mph or 13.6 km/h.
Worked example
Waterline length 30 ft: sqrt(30) = 5.477; hull speed = 1.34 x 5.477 = 7.34 knots (8.45 mph, 13.59 km/h). At an actual 6 knots you are at 82% of hull speed, a speed/length ratio of 6 / 5.477 = 1.10.
How it works
Hull speed = 1.34 x sqrt(LWL in feet) knots; in metric the coefficient is 2.43 knots per square root of a metre. The coefficient is quoted anywhere from 1.34 to 1.51 depending on the source. The idea: as a displacement hull moves it makes standing waves, and at a speed/length ratio of about 1.34 the wave as long as the hull makes drag climb steeply.
Hull speed is not a hard wall. Ship drag for a displacement hull rises smoothly as hull speed is approached and exceeded, planing hulls are designed to go faster, and modern naval architecture uses the Froude number instead of "hull speed". Treat it as a rough cruising ceiling for displacement boats such as many sailboats and trawlers.
Sources opened 2026-10-08: Hull speed - Wikipedia; Knot (unit) - Wikipedia. Planning aid only, not professional, engineering or safety advice. Your vehicle, trailer, tire, battery and boat manuals and the weight labels on your equipment come first; when in doubt, weigh it or ask a qualified professional.
FAQ
What is the hull speed formula?
Hull speed in knots is about 1.34 times the square root of the waterline length in feet, or 2.43 times the square root of the length in metres.
Is hull speed the maximum speed of a boat?
No. It is a rough speed limit for displacement hulls because drag climbs steeply. Planing and semi-displacement hulls can exceed it.
Do I use overall length or waterline length?
Waterline length (LWL), the length of the hull where it meets the water, which is often shorter than the overall length.
What is speed/length ratio?
Speed in knots divided by the square root of the waterline length in feet. A ratio of about 1.34 matches the classic hull speed.
Why do sources use different coefficients?
The constant is quoted from 1.34 to 1.51 depending on the source. This tool defaults to 1.34 and lets you change it.