How much of a pitch change do I need?

[2cyclemercman]

I am looking for a 8 pitch 3 blade prop for my 06 Mercury 50HP Bigfoot 4 stroke outboard. I have a 10 on it now but the RPMs at WOT are way down.

 

[Elkins45]

Final report: Lowering the motor back to the original (lowest) position and replacing the 15 pitch prop with a 12 gave me 5100 rpm, a 3 mph increase and full steering control with no cavitation in turns. All is well.

 

[achris]

This is what we need to see a photo of. Tilt the engine down and get the anti-cavitation plate level with the bottom of the boat, the get down and take the photo level with the anti-cavitation plate.