I have a 21ft pontoon boat with a 90 hp mariner. My max rpm is 5600. When I go to full throttle the motor speeds up but the rpm doesn't change. Do I need to try a differnt prop? My elevation is around 6500 ft if that makes a differance or not. Thanks for your help.

you mean the RPM goes up and speeds does the same?

yes. said it wrong. thanks