Prismatic Coefficient

[tspeer]

Originally Posted by BigCat

You guys are mean. In simple terms, the PC is how tapered a boat hull is under water. More volume in the ends = a 'blunter' boat, with a higher percentage of its underwater volume in the ends = a high PC. Less volume under water in the ends means less boat area underwater in the ends, ie. more tapered under water. A PC of 1.0 is not tapered in the ends at all- each slice of the loaf is the same area underwater. A PC of .49 is very fine, ie. very tapered - like an ocean kayak. ...

There's another interpretation of PC, and that's how big the maximum cross section is for a given volume (which, of course, means a constant weight for the vessel). A PC of 1 has the smallest maximum cross section area. As the PC drops, the maximum cross section has to increase to make up for the volume lost at the ends (or anywhere else).

I think this interpretation helps to make sense of why higher speed craft tend to have larger PCs. One would think that blunter ends would increase the wave drag, and they might, but there's a benefit to be had from the reduction in maximum cross section. At some point, the benefit from one is outweighed by the penalty from the other, hence there being an optimum PC that is a function of Froude number.

[Mat-C]

Originally Posted by tspeer

There's another interpretation of PC, and that's how big the maximum cross section is for a given volume (which, of course, means a constant weight for the vessel). A PC of 1 has the smallest maximum cross section area. As the PC drops, the maximum cross section has to increase to make up for the volume lost at the ends (or anywhere else).

I think this interpretation helps to make sense of why higher speed craft tend to have larger PCs. One would think that blunter ends would increase the wave drag, and they might, but there's a benefit to be had from the reduction in maximum cross section. At some point, the benefit from one is outweighed by the penalty from the other, hence there being an optimum PC that is a function of Froude number.

Tad eluded to something similar elsewhere.... do you have a reference, or an equation for determing the optimum PC?
Thanks

[abhishek]

prisamatic coefficient = block coefficient of finess/mid ship area coefficient