When comparing a prop with the same diameter and pitch a 2 bladed prop will have a better top speed in most applications verse a three bladed prop. While running a surface drive a three bladed prop will have two blades in contact with the water at any given time. This will increase the total amount of thrust (acceleration) and increase the total amount of drag. (reduce RPM's)

When you have a 2 bladed prop with a total pitch of 2 inches and you add a third blade to it, the total pitch of the prop will remain the same - 2 inches. In other words the added blade will only consume more power at a loss of speed because of the added resistance of dragging the extra blade through the water.

Why do full scale boaters commonly use a 3 or 4 bladed prop on their full scale boat as opposed to a 2 bladed prop?
Again it goes back to thrust. A 2 bladed prop would need to be much larger in order to produce the same amount of thrust as a 3 or 4 bladed prop outputs. Most outboards would not be able to hang such a large sized prop. Also, as you increase in diameter, prop pitch speed increases. As you approach high values close to the speed of sound the prop nearly loses all of its efficiency making it useless.

In conclusion:

2 blades
- higher top speed
3 blades
- increased thrust - faster acceleration

Ryan