I have a 190 22' pontoon boat with a 96 50 hp Mariner 4stroke engine. At full power the engine will suddenly rev very high. After throttling back to idle, the engine will pull the boat ok. Return to full speed and it happens again. I brought the boat home and checked the prop. Cant turn the prop without turning the motor over by hand. Can't seem to get the prop to slip at home. Do I have a prop slipping or does the engine have a clutch that might be slipping? Does it even have a clutch?