The Grand Compromise...Semi-Displacement

In looking for a power boat, it often seems the choice comes down to planing versus non-planing (displacement) hull forms. While it is true that a planing hull can operate at a displacement speed, which can be numerically described as a function of waterline length, the difference is most easily thought of in terms of whether or not you have gotten "over the hump" and onto a plane. A good semi-displacement hull just rises slightly, there is no "hump."

A 50' waterline length planing hull can operate around 10 knots or so before it tries to climb over the bow wave and plane, but a 30' waterline length hull gets to only about 7 knots. Pretty slow for most people, who also find that their ideal speed often falls somewhere between the full displacement slow-poking speed and the 25 knots plus of the planing express cruiser, with its thirst for fuel. Modern planing express cruisers are way ahead of their forebears in fuel efficiency, in particular the MJM boats, which utilize sophisticated (and expensive) construction to achieve light but strong hulls, modest beam, and modern propulsion. But not everyone wants to go 25-35 knots.

My magic number is 16-17 knots or 100 miles in 6 hours, roughly the length of the Jersey Coast. And I want to be able to do it in a 40-50ish foot boat slicing along into a 15 knot wind, not bouncing along on the top of the water. Top speed in the low 20's is fine. I also want to throttle back to 10 knots and make a pleasant run offshore from Norfolk to Block Island.

The Wizard of Beals

William "Pappy" Frost, who built fishing boats on Beals Island and later in Portland, gets most of the credit for developing the Maine lobsterboat hull, the classic semi-displacement powerboat form. His boats showed that you could power a boat through a wide range of speeds without ever having to climb over a hump with its resulting drop in efficiency. The relationship between fuel burn and speed is much more linear than in a planing hull. His great grandchildren, the Lowell Brothers, are still designing and building boats in Maine. There is a link on the last page to their website. Their view of the form is naturally centered around their family, but there are many others, with names like Bunker, Ellis, Hodgdon, Lash, Beal, or Rich with similar contributions.

Many modern motor yachts have been built with this style hull, several on actual lobster hulls are pictured on the Powerboats I Like page. Others are pictured here. See also the Links page, for a photo of a 1938 Hinckley powerboat that beautifully embodies this hull type.


© Peter Bass 2013-17