All things aside, the propeller fitted to that engine when it left the factory was a 10-3/8" x 13" 3 blade aluminium, part number 48-43586A40. That said, it doesn't mean it is the right prop for the boat you have it on. Someone may have changed it for whatever reason.... As racerone said, post up the boat details and we'll see what we can do about getting you the right prop....
Here's the thing about props. The pitch of the propeller (the distance the propeller would travel in one revolution with no slip) ultimatley determines how fast the engine will run at wide open throttle (WOT).... More pitch, less revs.... Obviously that is also governed by the load that engine/prop is trying to push. So, mostly, the load is pretty much the same, so the prop is the easiest to change. The manufacturer will specify a 'Maximum RPM'. For your engine,, that's 5000-5500rpm. If the WOT revs are too low, that overloads the engine,
at all revs, and can lead to a destroyed engine, detonated pistons etc. Too high RPM isn't as bad, so long as you don't run over the RPMs for a long time. My boat spec is 4400-4800, but the prop I have runs me to about 5200. Great for the boat, and in the weather I predominately boat in, not a problem... Props usually come in 2" pitch increments, and each 1" of pitch changes the WOT RPM by about 150-200 rpm. My preference is to pitch right at the very top end of the specified rev range*.... (a couple of hundred over isn't going to hurt).... And if you pull any sort of water sport toys (tubes, skis etc), fit a prop 2" less than the 'right' prop....
In a nutshell, think of it like the gears in your car. Only on an outboard, you get one to play with and it's not so easy to change 'gear' on the fly.
Yes, I and many like me have multiple propellers of different pitches, but we select the prop for the day and that's what we get to play with.....
HTH,
Chris.......
* Except the 2+2 engines (100hp, 115hp, 125hp). Because a lower pitch prop will make the transition between 2 and 4 cylinders too 'jerky'.... You need to prop those engines in the lower half of the recommended rev range.
