Prop hub slippage?

[DickR]

I'm into my second or third season on a replacement prop for my 17' Mercruiser Alpha One I/O boat. The prop is a Michigan Match 3-blade aluminum, and since I put it on, including a few trips out in the boat this spring, it has been fine, a proper match for the engine/boat.

Yesterday evening, near the end of a nice after-dinner cruise at around 3000 rpm, the engine suddenly surged in speed while boat speed dropped. I immediately dethrottled and went to neutral. The engine was running just fine. I had heard no thump or anything else to suggest a collision with anything. I put it back into gear, and the boat moved into headway speed. On advancing the throttle to come back onto plane, the engine speed again surged. I returned to the dock at little more than headway speed.

Visual examination of the prop did not show anything unusual, such as evidence of any collision with something.

I am not aware of anything in the drive train that is like a car's clutch and that could exhibit slippage under load. It is my understanding that in the hub of the prop there is some structure that is designed to slip if the prop does hit something, to protect the outdrive from damage.

So, help me diagnose my problem. Am I about to get a new prop to solve the problem, or could there be something else wrong so that a new prop would be a waste of money?

I do still have the old prop, which I replaced because the hub was worn and exhibited too much play but which otherwise performed adequately. I could put that one back on again to see if the boat at least gets up on plane and runs normally. If that is the result, I guess a failed prop is the answer. Your thoughts?