Boat Speed Calculator
Estimate a boat's top speed with Crouch's formula from horsepower, displacement and hull type — and the power a target speed needs.
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What this tool does
Estimate a boat's top speed with Crouch's formula from horsepower, displacement and hull type — and the power a target speed needs. ESTIMATE from Crouch's published empirical formula with labelled typical hull constants. Propeller, bottom condition and sea state are not modelled.
How to use the Boat Speed Calculator
- Enter or select hull type (crouch constant).
- Enter or select shaft horsepower.
- Enter or select displacement as run (lb).
- Enter or select target speed for power check (mph).
- Read the calculated result; change any measurement to compare alternatives.
Formula
Crouch: speed (mph) = C ÷ √(lb ÷ hp); required hp for a target = lb × (target ÷ C)²
- hull
- Hull type (Crouch constant)
- hp
- Shaft horsepower
- weight
- Displacement as run (lb)
- targetSpeed
- Target speed for power check (mph)
ESTIMATE from Crouch's published empirical formula with labelled typical hull constants. Propeller, bottom condition and sea state are not modelled.
Worked example
For boat speed calculator, the following measurements illustrate the exact method: Hull type (Crouch constant): 150; Shaft horsepower: 300; Displacement as run (lb): 3500; Target speed for power check (mph): 40.
Inputs
- Hull type (Crouch constant)Average planing pleasure boat — C ≈ 150
- Shaft horsepower300
- Displacement as run (lb)3500
- Target speed for power check (mph)40
Result
- Estimated top speed (mph)43.92
- Horsepower needed for target speed248.89
- Pounds per horsepower11.67
Results explained
- Estimated top speed (mph)
- Estimated top speed (mph) from the formula above. ESTIMATE from Crouch's published empirical formula with labelled typical hull constants. Propeller, bottom condition and sea state are not modelled.
- Horsepower needed for target speed
- Horsepower needed for target speed from the formula above. ESTIMATE from Crouch's published empirical formula with labelled typical hull constants. Propeller, bottom condition and sea state are not modelled.
- Pounds per horsepower
- Pounds per horsepower from the formula above. ESTIMATE from Crouch's published empirical formula with labelled typical hull constants. Propeller, bottom condition and sea state are not modelled.
Frequently asked questions
Top speed (mph) ≈ hull constant ÷ √(displacement lb ÷ shaft horsepower), published by George Crouch for estimating planing-boat speed. For 300 hp and 3,500 lb with the planing constant 150 it predicts about 44 mph.
Published Crouch constants cluster by type: about 150 for average pleasure planing hulls, higher (170–190+) for light fast hulls, around 130 for heavier cruisers, and near 100 for semi-displacement forms. The defaults are labelled approximations — pick the closest type, not a promise.
Inverting Crouch: hp = displacement × (target speed ÷ constant)². Because of the square, small speed gains at the top end demand disproportionately more power.
The boat as run: hull, engine, fuel, crew and gear. brochure 'dry weight' understates it and flatters the prediction.
Bottom condition, prop choice and condition, altitude/air temperature for the engine, sea state and loading all subtract from the idealised estimate.