It is cheaper and easier to build that way, plus it allows easy fitting of the rudder pintles. Ever tried to allign rudder pintles on a curved catamaran stern, I did !

My point here is of course that such shapes can also be the result of pretty non-spectacular considerations. A while back we covered the reason for the rakes back bows and that too was pretty mondain.

But I have no desire to endlessly discuss any virtual hull theories, they are all simply wrong. The reason for it that the pressure at the stern plane is still very much the ambient airpressure. As a result there is no significant difference whether the hull has a virtual extension or not. Again I refer to aerospike rockets engines to see a real example of how a virtual lengthening should look like. Here the zone between the exhaust jets is a (gass) spike of much increased pressure pushing against the flat between the rocket engines. In effect the rocket base is entlarged with a spike made up from compressed gasses. Theoretically this spike can also be made of metal but this works just as well. Actually it works better as you have less weight in the engine and less trouble preventing the metal from melting. This is a true example of a virtual extension. Water cleanly seperating from a stern is really not. I may have some secondary drag reducing effects but nothing major. It certainly doens't involve Froude'a law much. I leave it at that.




How do you know that they are not ?

Additionally, maybe the dynamic pressure zones around a cat hull are too low to result in significantly lifting or suction forces. Afterall the hulls are very long and slender and water is only accellerated out of the way relatively gently. This can also be seen in Gareths plots. Overall wave-making drag is not a very big factor. If you make plots like that for mono-hulls you'll see the wave-making drag to be alot more dominant. Rememder Miss Nylex crew found a wave-making drag to be only 15 % of the total boat drag, the pressure forces will be accordingly small. Besides on a sail boat you also have a mast and sail trying to push the boat over its bows with a leverage that requires two people to be at the very back of the hull. If anything, the suction downward at the stern is helpfull !

later !

Wouter