engine revs but boat doesn't go

[rtracy]

This is the first season with my new outdrive. Boat is fine with 2 people but if I am pulling a tube and have 4 people in the boat when I am trying to plane it acts as though something is slipping. The engine revs but I can't get it to plane. I have to work the throttle to get it to plane and once I am planed it's alright till I slow down.
I pressed the hub out of the prop to see if it had spun but it doesn't look like it was spun. Could the 21 pitch prop be causeing this? Any idea?
My boat is a 87 Glasstream, 18 foot with a 350ci engine.

Thanks, Ray

[Technical Support]

It sounds like your engine coupler is slipping. It must be just starting to fail. If it seems like it is getting worse, then I would suspect it for sure. Typically the coupler will hold until you reach a certain amount of power, then it slips. As time goes on the amount of power it will hold before slipping will get lower.

Tech Support

[rtracy]

How can I check to make sure thats what it is. I have had the boat out a couple different times and it doesn't seem to be getting any worse. I don't know if the coupler is mounted with rubber around it but if it is it seems I would have burnt it up by now. I put the boat in gear and turned it against the stop and it was turnning the engine over. Is that a good enough check? It didn't slip at all. I had a buddy watch while I was turnning it. Sorry for all the questions I just want to make sure before tearing things apart.
I also checked into the prop and the engines max RPM is 4600. If I calculate 21 multiplied by 4600 I come up with 96 but at full speed I am only doing 55 MPH. Could I be getting cavitation? I don't know what degree prop the boat came with stock. I have a Quicksilver 48 11070 A40 21P on it now.
Thanks, Ray

[Technical Support]

if it is not getting worse it shouldn't be the coupler. There is no way of checking it until it lets go completely. Usually at that point you will smell burning rubber.

You might want to cunsult a prop chart to see if you are using the correct prop. Mercury has one on their website and I would guess Michigan wheel and Solas also would.

Tech Support