Displacement hull speed

I was on a forum a little while ago where a poster stated that his production monohull yacht had a water line length of less than 24’ but could achieve a speed of 7.5 knots. This is, of course, impossible without defying the laws of physics. Such a boat would have a hull speed of no more than 6.5 knots. He probably needs to check the accuracy of his speed log.

Let’s recap the displacement hull speed law: As a boat moves through the water it creates a wave. As the boat moves faster the wave increases in length until it eventually reaches the waterline length of the boat. At this point the boat can go no faster without climbing up the face of its own bow wave. Considerable power is required to do this – well beyond that available to the typical sail boat.

The formula for theoretical displacement hull speed is:

Speed (knots) = 1.34 x √LWL in feet

Example: LWL is 25’. Hull speed is 1.34 x 5 = 6.7 knots.

Some lightweight flyers, even if they do have displacement hulls, can slightly exceed this theoretical figure; a constant of 1.4 instead of 1.34 brings these boats into the catchment area, so to speak.