If you go up in the number of blades, you need to go down in the pitch.
What is your goal? The mercruiser gasoline engines all have a max rpm of 4400 - 4800 rpm's. At the very least you would need to know what the max rpm is Wide Open Throttle (WOT)
I agree with Mike, you really need the following information to move forward:
1) Max RPM at Wide Open Throttle?
2) Engine size?
3) Carb or Fuel injected?
4) Outdrive model
5) Outdrive gear ratio
6) Boat stats(Length, width, weight)
I'd run through the calculator and see what you come up with.