Waterline Length Speed Calculator
A displacement boat makes a bow wave whose length grows with speed. When that wavelength matches the waterline length the hull effectively sits in a trough of its own making and resists going faster. The classic hull speed estimate is 1.34 times the square root of the waterline length in feet. Enter your waterline length to get the theoretical hull speed; the coefficient is editable so you can model finer or fuller hulls.
Hull speed formula
Hull speed (knots) = coefficient * sqrt(waterline length in feet)
mph = knots * 1.150779
km/h = knots * 1.852
The traditional coefficient is 1.34. Some designers use up to about 1.5 for light, fine hulls.
Worked example
- Waterline length 36 feet, coefficient 1.34.
- sqrt(36) = 6, so hull speed = 1.34 * 6 = 8.04 knots.
- That is about 9.25 mph or 14.89 km/h.
Hull speed: frequently asked questions
What is hull speed?
Hull speed is the speed at which a displacement hull's bow wave has a wavelength equal to the waterline length, making it hard to go faster without planing. The classic estimate is hull speed in knots = 1.34 times the square root of the waterline length in feet. A 36-foot waterline gives about 8.04 knots.
Where does the 1.34 come from?
It comes from the speed of a deep-water wave whose length equals the waterline. In consistent units the wave speed is 1.34 times the square root of the wavelength in feet (knots). The coefficient changes if you use different units, but 1.34 with feet and knots is the traditional rule.
Can a boat exceed its hull speed?
Yes, with enough power a hull can climb over its bow wave and plane, or a long slender hull can push past the simple limit. The 1.34 rule is a rough guide for typical displacement hulls; light, fine hulls often exceed it, which is why some designers use a higher coefficient.
Does waterline length or overall length matter?
Waterline length, the length the hull actually presents to the water, is what sets hull speed, not the overall length including overhangs. A boat that heels or is loaded so its overhangs immerse gains effective waterline and a slightly higher hull speed.
Official sources
- U.S. Navy, Naval Sea Systems Command: ship design and resistance.
- NIST: unit conversions (knot, mph, km/h).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.