Low boat speed only

[justbob]

2001 Sea Pro 21wa with a 4.3 Mercrusier. Headed out to go fishing, boat ran fine at low speed (5-6mph). Cleared the 5mph zone and throttled up but the boat wouldn't pick up any speed. Engine RPMs would come up but no boat speed. Prop is fine, no rubber hub to spin. Can put the outdrive into gear and turn the prop by hand and the engine will turn over. Am I correct in assuming that the coupler is slipping and needs to be replaced?

[freezerfiller]

Hi Bob and welcome to the forum. I don't know much about mercruisers, do they have a keyway on the propshaft? If so, it can be sheared off causing that issue. Maybe someone with a little more outdrive knowledge will come along in a bit.