Prop Suggestions for Pontoon

[mayorjones]

I have a 21' Sun Tracker pontoon with a 50 hp Merc (I don't believe it's a big foot). Anyway, I am having a lot of cavitation problems, and have read through here and other similar sites that a standard 3 blade prop isn't the best choice for a pontoon, so I think I'd like to try a four blade. It currently has a 12.5x9. RPM wise it's pretty good - don't think we need to change much there. The motor is mounted as low as possible (I think?), but it still cavitates quite easily, even when a majority of the weight is in the back and the motor is trimmed all the way down.

Any suggestions on what I should buy? This is all very new to me, so I apologize for my ignorance.

Thanks!