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.
The waterline length on some boats, particularly those with long overhangs, increases as the boat heels, so they go faster heeled that upright.
Monohull boats of the type most of us sail have displacement hulls with sail plans or auxiliary engines of inadequate power to get the boat up onto the plane and so have to conform to the displacement hull speed rule. Planing power boats, of course, are unhindered by the rule and can roar off into the distance leaving we sailors bobbing in their wake.
The well known, perhaps infamous, McGregor 26, a sort of hybrid, a power boat with sails, uses a 60HP motor to get it’s very light weight onto the plane and achieve a speed of 24 knots. (pictured)
But, generally speaking, when someone in the club bar tells you his Slug 22 cruises at 8 knots you’ll know he’s unaware of the displacement speed formula.