The advantage of two blades over three or four is that there are fewer blades cutting the water and creating resistance. The disadvantage is that they are slow out of the water and can not carry heavy loads because of the smaller blade area. They should also be balanced because there are only two blades. The bronze blades are thin and sharp.
In real words testing two years ago, this two blade bronze prop GPS's at 52.9 MPH versus 51 MPH with a Ballistic 21" three blade stainless steel and 49 MPH with a 20" Laser II stainless steel. Every boat and motor is different, and only by testing several can you find out the best.
This 21" Bronze prop was running 5,400 RPM at 52.9 MPH. The newer 115 HP Mercs have better porting and breathing and can turn higher RPMs and the 21" may still be the best prop for the 1988 115 HP Mercury, maybe 56 or 57 MPH at 5,800 to 6,000 RPM for a short distance, not sustained. If RPMs are too high, have a 23" pitch 2-blade bronze to try.
It will be interesting to try the props, even compare MPH between the four identical ones.
We'll have to have a prop testing day.
Doug, they could be refitted for the OMC motors. A prop shop was going to fit a OMC Raker to my Mercury.