WOT RPM. Loosing MPH. Any ideas

'89 Bayline 3.0L OMC I/O

Strangest thing happened the other day. Had been cruising at WOT doing about 34mph with a boat full of 4 people and plenty of gear when suddenly the boat started slowing down and loosing speed. The engine was running fine, not over heating and keeping WOT RPM around 4300ish, but kept loosing momentum. Tried throttling back and up again but made little difference. It felt like the boat was suddenly pulling a tube with eight people on it. Kinda felt like the clutch was slipping out of it. This lasted about 5 min. Then the boat ran strong the rest of the day with no other problems. Any ideas? Didn't run through anything, prop was clean and clear, out drive looked fine. Kinda worried about it happening again, but this time not fixing itself. Any ideas welcome. Thanks.

Re: WOT RPM. Loosing MPH. Any ideas

Could be your prop hub spinning. Mark a line on both the hub and the prop and then run the boat for a bit at or near WOT for 5 - 10 minutes under load. Check the prop afterwards and see if the lines are still lined up with each other. If not, then your prop is spinning on the hub.

Re: WOT RPM. Loosing MPH. Any ideas

If it was the prop slipping, RPMs would have gone higher while speed went lower.

I would start at the tank vent and go through every single portion of the fuel system (including the fuel itself). Also, very few 3.0s came with water seperators, if yours doesn't have one yet, add one.

Re: WOT RPM. Loosing MPH. Any ideas

Could of picked up some seaweed or something else and it finally shed. Don't worry unless it happens again.

Re: WOT RPM. Loosing MPH. Any ideas

If not the prop hub it could be the engine coupler. Hope not, it requires engine pull and a $300 part.

Re: WOT RPM. Loosing MPH. Any ideas

Sounds like a fuel problem as stipulated above.

Re: WOT RPM. Loosing MPH. Any ideas

If it was a fuel problem the engine RPMs would have come down and resulted in loss of RPMs and Speed. He stated the engine RPMs stayed at WOT but the boat slowed in MPH. Only way that can happen is slippage, be it Prop venting, Prop hub or Engine coupler. Of course I could have mis understood it as it seems half the responses think fuel and half think slippage.

Re: WOT RPM. Loosing MPH. Any ideas

Yeah, somehow I missed the line - WOT stayed at 4300 bit.

I would like to change my vote to debris in the water.

Prop hubs and couplers just don't let go a little - and they certainly don't fix themselves...