I have a21 ft pontoon with an Johnson 90hp want to try and get little more top end speed current speed is 24mph at 5200rpm with a 14x11 prop. With 2 people and very light gear. I had a 13.75x13 prop and was getting 5000 rpm and 24 mph. GPS. Any ideas on this. had motor gone over and they said everything is fine, also installed a water glide this summer but didn't give me the increase speed we hoped for.