>>>"I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions"
I agree, I think that the structural integrity of the hull will be the biggest limiting factor of speed for your average Hobie 18. You can continue to add more power (well, assuming the boat has enough flotation to support the motor), but eventually the forces generated by the flowing water will crush the hull. Just like with wind, the force of the flowing water increases as a function of velocity squared, so if you double the velocity, you quadruple the forces on the hull.
>>"the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed."
I seriously doubt the hull is going to catch fire/burn as a result of hydraulic frictional forces. Fiberglass hulls regularly travel through water at high speed- the water acts as a coolant. Again, the hull collapsing will probably be the main issue. Just a guess here, but I would think you will start to have structural issues around 50knots, maybe even sooner- especially in a turning situation.
