earling that should say hydrofoil instead of hudrofoil on my earlier post.
Once you have that info you can use this formula to calculate how low the pitch can go:
Current pitch TIMES RPM at top speed DIVIDED BY RPM maxumum rated EQUALS your new pitch.
Eg. 19 X 4200 / 4600 = 17.3
So, IF you are running 4200 now and your engine can run 4600 you could get a 17 pitch without any risk of over revving the engine.
Once you know the Maximum RPM of the engine you could also just put a piece of red pinstriping or tape on the tachometer to remind you of the Redline for your engine and run whatever pitch you choose. Just throttle back a bit when you get to the redline so you don't over rev.