I have an early 90's Evinrude 70 hp on my 16 ft. flat bottom boat. The other night on the way in from the pass, the motor and boat performed normally at low (less than 2500 rpm) but whenever I went to give the boat more throttle the engine speed increased while the boat's speed did not. It was almost like the prop was slipping or something. The motor revs but the prop will not spin any faster. I eased it into the ramp at slow speed not wishing to break anything... have not had it out since. Any ideas what this could be??