George Crouch's formula has been used by naval architects for decades to estimate planing hull speed. It accounts for the two variables that actually drive top speed — power and weight — with a hull-type constant that captures the efficiency difference between a flat-bottom jon boat and a stepped fiberglass bass boat. Include everything in your loaded weight: boat, motor, full fuel tank, gear, and passengers.
Top Speed Estimator
Crouch's Formula — the standard naval architecture method for estimating planing hull speed from horsepower and loaded weight.
CROUCH · MPH
Hull & Power
Include everything in your weight: boat, motor, fuel (~6 lb/gal), gear, and people. That's the number that determines real speed.
Formula: MPH = C × √(HP ÷ lbs). Crouch published C = 172 for average runabouts. The length brackets here apply a calibrated adjustment — longer waterlines have less wave-making drag per pound at planing speeds, so a 20' aluminum genuinely outruns a 16' aluminum at the same power-to-weight ratio. Constants back-solved from real-world speed data across common hull types. Real-world speed also varies with prop pitch, trim, and bottom condition.
Estimated Top Speed
38
mph (ideal conditions)
Power-to-weight26.7 lb/HP
Planing statusPlanes well
Good power-to-weight ratio for this hull type.
USCG Max Horsepower
33 CFR 183.53 formula for boats under 20 ft without a capacity plate, plus a rough persons-capacity check.
33 CFR 183.53
Hull Inputs
This is the CFR's generic computation method — used when a boat has no existing capacity plate. If your boat already has one riveted to the transom, that number is the legal one; it overrides this estimate.
USCG Max Rating
90
max horsepower (legal ceiling)
Length × width factor104.0
Rough persons capacity6 people
Persons capacity uses the simplified length × width ÷ 15 approximation — your actual capacity plate also weighs in motor weight and gear, and the weight limit always wins if it's lower than the headcount.