Wouter,
This is a good explaination of what I've always agreed with...that standard cats & tri's don't really plane. They are displacement hulls that have sufficient power to weight ratios to overcome the bow wave limit.
What performance benefits would you get from a boat that had most of its bouyancy on some kind of bulb(s) that were completely below the water surface? The actual "hull" could site above the water surface supported by vertical fins. This would eliminate huge amounts of drag from the water/air interface...and it the reason why submarines go faster while submerged.
The laser dinghy has roughly 1/3 hull ratio and it too is not held back to 5 knots as its top speed.
There is only 1 true constant in Froude's law and that is 1.34. All the other constants are just tricks to make the formula fit the real life data where indeed the formula is not applicable anyway.
Froude's law only decribes the speed at which a wave on the watersurface travels in relation to its wave length. It has nothing to do with "theoretical hullspeed" apart from the fact that HEAVY boats in comparison to their waterline length are unable to travel faster then the wave system around them. In this situation the wave length of the wave system is then the same as the waterline length of the hull. Basically the hull is trapped between two wave crests and has insufficient power to force itself out of this craddle.
LIGHTWEIGHT boats tend to have plenty of power in relation to their size and as such can just overpower this limitation and accellerate to higher speeds. This is what catamarans, surfboards, skiffs and dinghies do. This can also be done on a larger scale with for example navy Frigats deploying gasturbines as engines. Another way around this cradle is to go onto foils or start planing. Naturally a navy frigat neither planes nor foils, it is too large for that.
Wouter
