1969 Merc 500 issues

[Nebssor]

My 1969 Merc 500 runs great, but isn’t performing well. Starts and runs great, and when I lift the throttle while in neutral the engine achieves good RPM. But when I put it in gear, it doesn’t achieve near the same RPMs and struggles getting my 14’ fiberglass boat up on plane. Haven’t got actual speed or RPM, only going off sound of engine. Changed plugs and put new water pump and housings in this summer...??? Ideas??

 

[michaud500]

Wrong prop?

 

[Nebssor]

Removed the prop that was on it yesterday and will be replacing it. I’m unsure as to the “correct” prop for that motor, but it is same one that was on it when I purchased it. This problem began end of last year before I put it up for winter, before it began it was much faster, probably close to the 25ish mph it’s supposed to go. Inside of the prop is a rubber bushing holding the spline, and when I removed it there was some “threads” of rubber inside. Does anyone know if that age prop has a replaceable hub? Or will new prop with hub in it be only way to purchase? Haven’t had any luck just finding a hub.