This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.

The answer, verbatim from mp, is:

The formula yacht designers use is called Crouch's formula.

It takes into account the weight and horsepower at the

propeller, and assumes a 50"% to 60"% efficient prop.

Most props fall into this range. Note that it doesn't take

into account the boat length, as that doesn't matter with

planing boats.

Crouch's Formula

V = C/((DISP/HP)**.5)

Where V = boat speed in knots (1 knot=1.15 mph)

C = Constant (depends on boat type)

DISP = Displacement (pounds)

Note that boat manufacturers usually give

innacurate numbers for displacement,

typically on the low side

HP = Horsepower available at the propeller

For comparison sake, here are some average values of C:

150 Typical lightweight, planing cruiser 180 High Speed Runabout 200-230 Race boats, hydroplanes etc.

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