boat revs up like a clutch slipping in a car

91 glastrom volvo penta 230 boat works great for a while then it will rev up acts like a clutch slipping in a car just recently got boat i changed lower unit oil i have no manual but info i found said it uses same oil as engine 10 30 so i used delo 15 40 also found when working on it was end play in prop so i tightened prop is there supposed to be endplay in prop or is it a torque spec and what is it the oil i drained smelled like gear oil is there a clutching system in the lower unit do i need gear oil in it what could cause it to slip any help would be much appreciated

It seems to me that your prop is spinning on the hub. If this is the case, you will need either a new prop or get yours re-hubbed.

thanks for input i am auto tech after i removed prop it appears as it has a rubber laminated hub similar to a harmonic balancer on automotive aplication i think it was slipping then would cool down then restick itself for a bit then would get warm again and start slipping under a heavy load