Hull speed formulae are boogus when looking at beach catamarans. The hullspeed formula that is so often thrown about is really a misintepretation of the Froude law which actually only links wave length to wave speed.
For some boats the Froude law coincided with the maximum speed they can attained given a limited drive due to some factor. Of course all sailboats are limited in drive their sails can produce because of stability considerations.
Now Froude's law applied to beach cats as well it just that the commonly encoutered misintepretation of it is not rightfully applied to these designs. Hence the parados that catamarans can sail faster then "their maximum hull speed".
I can tell everybody out there that debunking the "max hull speed formula" is like fighting the multi-headed hydra. For some reason this "error" is immortal and ever since its rise to fame it has dominated discussions where. I'm done with it. If you don't understand the true meaning of Froude's law and the true nature of the "max hull speed" formula that is derived of it, then there is simply no use in continueing the discussion. It is like arguing that the earth is round with a midevil christian fundamentalist who thinks it is actually a flat pancake because he can see with his own eyes that it is flat. Sorry about this, but that is just the way things are.
If not planing, then in that power-sucking semi-planing regime. But not displacement.
Read Bethwaite very carefully again and then see where his "modes" are just names he gives to a limited series of specific displacement craft. He doesn't include catamarans for example and later on in the book he actually writes that cats are "different" and don't seem to have the forced displacement mode (or whatever he calls it). So his model is only applicable to a specific type of sail craft and it falls down when looking at more different craft.
And that Stealth F16 is definatly still in displacement mode. It may have a component of dynamic lift somewhere. Either from the angled foils or from some pressure build-up under the hull, but 80 % of its weight is still carried by archimedes law. Therefor it is a displacement craft. I'm sorry
Wouter
