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At low speeds (below roughtly 6 knots) the wave system around any 16-20 hull is such that wave-making drag is negligiable. In effect any energy lost in the bow wave is won back at the rear of the hull. This is the scientifically correct intepretation of the Froude law, from which the (errornous) max hull speed law is derived. So at these low speeds the drag from the hulls is almost entirely created by wetted surface drag. In effect relatively long hulls are more draggy in this speed spectrum then shorter hull, that is under the assumption of equal displacement.


I used to think this too. I now do not believe it to be true. There is some very good drag prediction software called michlet, specifically for long slender hulls like catamaran hulls. It appears to be well validated, and I have done a little experimental validation on it myself. I am happy with its predicted results. It is by using this sotware that I have come to those conclusions.

If you know of a better resource for drag prediction of slender hulls I’d like to hear about it, or better still experimental data.

So I will stick to my guns on this one when I say that on beachcats longer and lighter is always better for low drag at the speeds that count and that at 6 knots the wave drag is far from negligible.

In addition when you say that for a 107kg boat 5m is better than 5.5m I would say that that only holds true for speeds above 20knots, below 20 knots 5.5m is better. Although to be honest there is bugger all in it.

Gareth
www.fourhulls.com