Boat Speed Calculator

This calculator relies on Crouch's Law, a trusted empirical formula widely used in the marine industry to estimate the top speed of planing motorboats. The core equation relates boat speed directly to engine horsepower and total vessel displacement, modulated by a characteristic constant that reflects hull efficiency and shape. The mathematical formula is expressed as: Speed = sqrt(Power / Displacement) * Constant, where Speed is measured in knots, Power is total shaft horsepower, Displacement is the loaded weight of the vessel in pounds, and the Constant varies based on hull form and design characteristics.

Imagine you are testing a modern runabout boat with a loaded displacement of 3,000 pounds, powered by an engine delivering 150 horsepower. For a typical average modern runabout, Crouch's constant is generally around 180. To find the estimated top speed, first divide the power by the displacement: 150 divided by 3,000 equals 0.05. Next, calculate the square root of that result, which is approximately 0.2236. Finally, multiply this number by the constant of 180, giving you an estimated top speed of roughly 40.2 knots.

To get the most accurate results from your boat speed calculations, ensure that your displacement figure includes the total loaded weight—accounting for fuel, gear, water, and passengers, not just the dry hull weight. Remember that Crouch's formula is specifically calibrated for planing and semi-displacement hulls; traditional heavy displacement hulls like sailboats will not yield accurate results using these constants. Always verify your engine's true shaft horsepower rather than advertised gross output to prevent overestimating your vessel's velocity.