Last year I bought a 2005 15 foot proline boat with a Johnson 48 HP outboard and it worked great last year. I tried to take it out for the first time this year and it doesn't want to excelerate. When I put the ears on it when out of the water the prop does spin faster when I move the throttle forward. Howver, in the water, moving the throttle forward has no impact on moving the boat forward faster. A friend recommended I fill the second half of the tank with high octain gas (the other half was from last year). I also just put Sea Foam in the tank and changed the spark plugs. No effect! Any ideas?