Optimum Speed Calculations

Optimum Speed Calculations

Boats have an optimum speed, which is their hull speed. The displacement speed is governed by waterline length.

To calculate the maximum displacement speed of your boat, a theoretical rule of thumb calculation is as follows:

If the waterline length (WLL) is in metres:

Maximum speed in knots = 2.43 x the square root of the waterline length (WLL) of the boat in metres.

e.g. for a WLL of 15 metres the maximum speed is 2.43 x √15

= 2.43 x 3.9 = 9.5 knots (maximum hull speed for WLL of 15 metres).

 

If the waterline length (WLL) is in feet:

Maximum speed in knots = 1.37 x the square root of the waterline length (WLL) of the boat in feet.

e.g. for a WLL of 49 feet the maximum speed is 1.37 x √49

= 1.37 x 7 = 9.6 knots (maximum hull speed for WLL of 49 feet).

This is an approximate guide, as the factor varies with different shaped hulls.

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