Hull speed

The MacGregor 26 is no more, apparently, although a very similar boat is supposedly going to be built by one of the younger family members to fill the void. The MacGregor 26 is a boat that can divide opinion just as surely as any forum anchoring thread. Is it a power boat that sails or a sailing boat that planes at high speed? Well, both actually. It’s claimed that the M26 can achieve 24 MPH under power – with a 60HP motor strapped to the transom! 

Fans of the boat say it sails well, too, but just what the weight of a 60HP outboard on the back does for trim I’m not sure.

Anyway, it was not a boat I’d personally consider but it found a ready market with those that felt its competitive price and split personality suited their needs, and weren’t put off by its odd appearance.

Before anyone rushes out and straps a 60HP motor to the transom of their 26’ heavy displacement cruiser in the hope of matching the M26 for speed, 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 displacement-hulled sailing 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.

Most monohull sailing boats have displacement hulls and are constrained by the displacement hull speed law of physics. And then there’s the MacGregor 26.