190hp from 500cc is very close to the specific power output of good 15cc boat motors, and the Honda is on petrol rather than nitro and methanol, so it is doing very well. This would indicate that the specific power outputs of the engines can be maintained for larger cylinder sizes (the Honda having 125cc cylinders).
Drag goes up with the square of speed, so that is why 90 boats are not four times faster than 21's, also the 90 boats are much heavier so the engine has more power absorbed just to get the boat to the same speed. Although I agree that the specific power the good 90's is lower than that for the good 21's. Some of this would be related to the ability to use higher nitro better, but 21's also tend to be development offshoots of the 21 car motors. That is big money stuff, to the point that there are professional drivers! So their motor development has been better.
As far as I know the main reason that there are very few racing model boat engines over 15cc is that most are made in Europe, and their racing does not permit larger than 15cc. Given that the rules in the US and Aus allow for up to 30cc, a few of the manufacturers have done the low cost development option and stretched their 90's out to 100's. Thus they have made engines of larger capacity, but these engines have the port sizes, carb, disc, exhaust header, and pipe sizes of 90's. Hardly optimal.
One problem a manufacturer would face introducing a 30cc motor is their would be little use for it, at least straight away. New hull sizes, pipe sizes, props, fuel tanks etc would need to be made to fully use such an engine. A 120 would not be too far out of reach of current hulls, although tank capacity may be a problem in riggers, and appropriate props may be difficult.
Ian.
