Box Keel

In some designs the box keel s midsection is boxy/rectangular, with a flat bottom, ( to increase the total surface of the semi-planing hull ???), and in some others it is U shaped, with a round bottom, ( to increase the total volume of the semi-displacement hull ???). Should the box keel s longitudinal axis remain horizontal while the boat is moving forward ?

 

"Should the box keel s longitudinal axis remain horizontal while the boat is moving forward ?"

In other words would a "Water Ski" style bottom be efficient at creating lift , and further reduce power required at speed?

Good question as these boats seem best below SL 3 and cruise even slower , but there IS dynamic lift there.

Some thinking is that 3/4 or so of the displacement should be in the box keel, which would give a good area to generate lift.

The newest navy tri seems to be a box keel with training wheels (amas), to create a wide platform for choppers to fly off.
Wonder if it would be faster with a bit of main hull also creating lift at 60K, and no training wheels.?

FF

 

There are many elements in a box keel design. Lets point out some of them;

1. Distribution of volume between the box keel and the rest of the hull.
2. Length of the box keel. Should it run over the whole length of the hull or not ?
3. Form of the box keel. Should the whole hull remind a slender catamaran hull placed underneath a planing hull ? Or two superimposed planing hulls, the beamier one placed over the other ?
4. Underwater nose and tail shapes of the box keel. Bulbous, pointed, vertical,etc.
5. L/B ratios of the box keel and of the whole hull.

The whole concept seems very interesting, but I can not find any systematic evaluation even of the few existing variations , ( and no systematic series of course...) Why is that so ? I believe that the main reason is the limitations of the existing computational tools, that are not so advanced to handle such complex hull forms...The low CG of these hulls might be an advantage even in the case of future faster motorsailers, with powerful heavy engines, batteries, fuel and water tanks, ( even some of the lead ballast in the case of a swing keel boat like the Southerlys) placed into the box keel.