propeller question on 1980 9.9 Johnson

[John Glenn]

My stepdad (94) gave me a 9.9 Johnson, which has performed very well in all regards. Last time out, running at full speed, the engine RPM went way up with a whine, and the prop slowed way down. I switched to neutral, and shifted again into forward. Prop turned fine at low RPM, but as it was given throttle, RPMs increased but prop rotation did not. Shifted into reverse and forward fine from neutral, but prop speed remained lower than RPM should have turned it.

Other than taking it to repair, any suggestions on what/where to look for the problem? Thanks for any help.