I have a 2009 Four Winns 22' Funship with a dual prop stern drive and a 5.0 litre engine with 173 hours on it. At low speed everything works fine. When I accelerate the engine revs up, but the boat doesn't accelerate. It feels like a car with a bad clutch. The boat has new(spring 2018) Volvo Props. I took the stern drive off and have it in my basement. I've tried to reproduce the slippage problem, but have had no success. Any ideas or suggestions?