Thread: Your hull type
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Old 08-20-2013, 02:01 PM   #170
Tony B
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City: Joe Wheeler State Park, Al
Vessel Name: Serenity
Vessel Model: Mainship 36 Dual Cabin -1986
Join Date: Jul 2011
Posts: 1,251
I am of the school that there is really no such thing, other than a marketing ploy, of a semi-displacement hull. I think that a "semi-displacement" hull is really a slight modification of a planning hull. It allows one to market a planning hull with either a less than optimal planning hull or a planning hull with not enough power on board for a true 'plane'.
From some of the data I have seen gathered by folks while on long trips, the semi-planing hull does not quite match the data of a full displacement hull at theoretical hull speed.
At hull speed or just Below it, a FD hull will show a really small difference of speed increase if you almost double the RPMs. All of my sailboats, when run at hull speed will almost double the fuel consumption to increase speed 1 kt above hull speed.

On my Mainship Aft Cabin Semi-Displacement hull, the increase of fuel, RPMs and speed remain fairly linear before and some beyond the theoretical hull speed. Then somewhere before planning, the fuel economy gets better because unlike a FD hull, the SD hull starts to squat and then rise slowly into a plane creating less water to push and then into a plane with greater fuel economy as not much of the boat is pushing water.

I believe that hull speed calculations are relatively meaningless on anything other than a full displacement hull. I say 'relatively' because they have some merit.

AS THD said, every hull design is a compromise. Even amongst FD hulls - wide vs. narrow.
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Cruising the Eastern U.S. Inland Waterways and Gulf Coast. Presently on the ICW in Louisiana and heading Back to Texas.
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