Thanks to you, both, for replies. I did the mark thing and ran it hard in a tank and it ran fine w/ no slippage. I checked marks afterwards and they did not move. Maybe it was cavitation at the time it happened. I'll take it out or sea trials in the near future and see it it happens again. (It's the kicker for my 18'). If it happens again, I'll replace the prop Thanks, again. Dennis G