Crouch Planing-Speed Estimate
The planing top-speed estimate and its diminishing return, the opposite regime from the displacement hull-speed wall: Crouch's formula speed = C / sqrt(weight / hp), in MILES PER HOUR (not knots), with the hull constant C about 150 heavy cruiser, 190 runabout, 210 race. A 6,000 lb runabout with 200 hp and C = 190 makes 34.7 mph; double the power to 400 hp and it reaches only 49.1 mph -- twice the horsepower buys sqrt(2) = 41% more speed, not double, because speed scales with the square root of the power-to-weight ratio. Assumes the boat is on plane (below the planing threshold use the hull-speed tile). A planning estimate; the hull, propeller, and conditions govern.
Formula and source
speed_mph = C / sqrt(weight_lb / hp), with the hull constant C ~150 heavy cruiser / ~190 runabout / ~210 race.
Crouch's planing-speed formula (naval-architecture back-of-envelope for planing top speed), by name; the actual hull, propeller, and conditions govern.
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