Before you repower a boat or just want to know what a spec sheet’s horsepower figure actually translates to on the water, Crouch’s formula gives a quick, widely-used ballpark for top speed based on power and weight alone. This calculator applies it across five boat categories, from average runabouts to racing catamarans.
How to use this boat speed calculator
- Select your Boat Type from cruisers/runabouts, high-speed cruisers, racing boats, hydroplanes, or racing catamarans/sea sleds.
- Enter your engine’s Shaft Horsepower (or kW).
- Enter your Boat Displacement in pounds or kilograms.
- Choose your preferred speed unit and read the estimated top speed off the gauge.
What this boat speed calculator does
Crouch’s formula estimates a boat’s top speed from just two inputs, shaft horsepower and displacement, using a constant that varies by hull/boat type to account for how efficiently different hull designs convert power into speed.
S = √(P ÷ D) × C
Where S is speed in mph, P is shaft horsepower, D is displacement in pounds, and C is the constant for your boat type: 150 for cruisers/average runabouts/passenger vessels, 190 for light high-speed cruisers/high-speed runabouts, 210 for racing boats, 220 for hydroplanes, and 230 for racing catamarans/sea sleds.
Why the constant changes by boat type
A hydroplane and a passenger cruiser can have identical power-to-weight ratios and still reach very different top speeds, because hull shape, planing efficiency, and wetted surface area all affect how much of that power actually converts into forward speed rather than drag. The C constant is Crouch’s way of encoding that hull-efficiency difference into a simple formula, higher-performance hull designs get a higher constant and therefore a higher estimated speed for the same power-to-weight ratio.
What this formula doesn’t account for
Crouch’s formula is a classic naval architecture rule-of-thumb, not a precise hydrodynamic model. It doesn’t account for hull length, specific hull shape efficiency beyond the broad category constant, propeller efficiency, trim, or sea state, so treat the result as a ballpark top-speed estimate rather than a guaranteed figure. Actual top speed on the water will vary with load, trim, propeller selection, and water conditions.
Frequently asked questions
What is Crouch’s formula?
It’s a classic naval architecture rule-of-thumb, S = √(P/D) × C, for estimating a boat’s top speed from shaft horsepower (P), displacement (D), and a constant (C) that depends on the boat/hull type.
Why do different boat types use different constants?
The constant reflects each hull category’s typical planing efficiency, higher-performance designs like hydroplanes and racing catamarans convert power into speed more efficiently than a passenger cruiser hull, so they get a higher constant for the same power-to-weight ratio.
How accurate is this estimate?
It’s a ballpark estimate only. Crouch’s formula doesn’t account for hull length, specific hull shape beyond the broad type constant, trim, or sea state, so actual top speed will vary with load, propeller selection, and water conditions.
