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Crouch Planing-Boat Speed, Power & Weight

Estimate planing speed from shaft power, loaded displacement and a hull coefficient, with unit choices and inverse calculations.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

A coefficient of 150, 100 mechanical horsepower and a loaded weight of 2,500 lb give an estimated speed of 30 mph.

Speed in mph = coefficient × √(shaft horsepower/loaded weight in lb)

Enter these known values and leave the other values blank.

Crouch’s relationship is an empirical planing-boat estimate. Its coefficient uses speed in mph, mechanical shaft horsepower and loaded weight in pounds; unit controls convert these consistently.
Preset coefficients are illustrative starting points in the retained mph convention, not measurements of a particular hull. Calibrate against a comparable boat. A coefficient from a knots-based reference cannot be substituted unchanged: multiply it by 1,852/1,609.344 to use this mph formula.
Use total loaded displacement, including occupants, engine, fuel and equipment. Shaft power is power delivered at the propeller, not necessarily engine nameplate power.
Power, weight, speed and coefficient are positive. This shortcut excludes propeller matching, sea conditions and detailed resistance; it is not a safe-speed or maximum-engine-rating determination.

At fixed weight and coefficient, twice the power gives √2 times the modeled speed. Four times the power doubles it.

Yes. Leave power unknown and enter speed, loaded weight and the coefficient. The estimate still relies on the same empirical model.

Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor.