right prop for a 90-horse on a pontoon

I have a friend who bought a used 2004 22' Crest pontoon with a 04 Mercury 90 horse 2-stroke. The prop that was on it when he bought it was pretty badly beat up, so he bought a new one. I was appalled by the lack of performance when I first went aboard...at full throttle the tach said we were turning 4800 rpm, but we were barely hitting 12 mph. I'm guessing that the dealer he bought the prop from sold him whatever he happened to have, and suspect that the diameter may be too small for the thrust needed on a pontoon. I don't recall exactly, but I believe he has a 12x15 now. I'm thinking he needs a little more diameter, and a little less pitch. Opinions?

A 13 " will bump the tach up to about 5200.-----------------As for the speed , it depends on the boat. Not all of them are speed boats.

You need a prop with at least 14 inches in diameter. Pontoons run best with the largest diameter prop possible. A Crest is a heavy pontoon, not the fastest but with the right prop you should get about 22-23 MPH. If you don't get 5000 RPM you don't make 90 hp. I too believe a 13 pitch is right for this boat. As for speed, your speedo might be wrong too. Pontoons typically don't get good readings because they're slow. If you have a speedo that only goes to about 35 you could get a reading but your speedo tubing must be in good condition (not pinched of smashed) from the speedo all the way to the engine. If it's hooked up to a pick-up mounted to the pontoon you might not get a good reading.

Thanks for the tips...speed was by GPS, I never trust boat speedos.