Boat Top Speed Calculator (Crouch's Formula — Horsepower, Weight & Hull Constant)
Predict a planing powerboat's top speed from horsepower and loaded weight with Crouch's formula, pick the hull constant for your boat type, see the horsepower needed for a target speed, the pounds per horsepower, and how extra weight or a bigger motor changes the result.
How it works
Crouch: speed (mph) = C ÷ √(displacement ÷ hp) · hp for a speed = displacement ÷ (C ÷ speed)² · knots = mph × 0.869 · C ≈ 120 heavy cruisers, 150 average runabouts, 190 light high-speed hulls, 210 race boats, 220 hydroplanes
George Crouch's 1930s formula is still the standard first estimate for planing boats: speed depends only on how many pounds each horsepower has to push, scaled by a constant that captures how slippery the hull is. Because speed goes with the square root of power, doubling the horsepower adds only about 40% to the top speed, while shedding weight is comparatively cheap. Use it to sanity-check a repower, decide whether a boat will plane with the family aboard, or see what a heavy load will cost you.
Worked example
A 20 ft centre-console loaded to 3,200 lb with a 150 hp outboard carries 21.3 lb/hp, so with C = 150 it should run about 32.5 mph (28 knots). Reaching 45 mph at that weight would need about 288 hp; adding 600 lb of people and fuel drops the 150 hp boat to about 29.8 mph.
Frequently asked questions
Which constant should I use?
Start with 150 for an ordinary runabout, bass boat, centre-console or small cruiser. If the boat is unusually light and efficient use 170–190; heavy, beamy or semi-displacement hulls 120–130. Racing hulls and hydroplanes are 210–230. Calibrate: compute C from a known GPS speed and your real weight, then use that C for what-ifs.
Does the formula work for displacement or pontoon boats?
Not well. Displacement hulls are limited by hull speed (1.34 × √LWL knots) regardless of power, and pontoons have their own drag behaviour. Crouch is for hulls that plane.
Is rated horsepower the same as horsepower at the prop?
For outboards and sterndrives the rating is already at the propeller shaft. For inboards with a gearbox and shaft, deduct 5–10%; old or poorly maintained engines make less than the badge says, and high altitude costs ≈ 3% per 1,000 ft.
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Boating figures are planning estimates based on published rules of thumb and standards (USCG, ABYC, manufacturer tables). Your boat's capacity plate, builder documentation and local regulations govern; conditions at sea vary.