Engine revs w/o the boat accelerating properly

[Coastal Denizen]

Hi, I have a 99 Merc 2 cycle 4 cylinder 125 HP outboard on a 17.5 ft Bayliner. Had been running fine until last week when while planing off the engine would rev w/o the boat accelerating normally. I figured the prop hub was bad. Got a brand new 3 blade alum. prop and am having exactly the same problem and its getting worse. Don't hear any gear grinding etc. and shifting is normal. Sometimes advancing the throttle fairly slowly it will not "slip" or whatever and it planes off OK. I just tried a WOT run, got 5400 RPM and 46 MPH with zero "revving" sound and that is as fast as it's ever run. It seemed to run perfect at WOT. But you slow down and try to accelerate back on plane and it slips or revs or whatever. I have looked for something near the lower unit that could be causing ventilation or turbulence but don't see anything. Have moved the depthfinder pickup all the way up to make sure it is out of the way. I'm stumped. Any ideas?

 

[nwcove]

Re: Engine revs w/o the boat accelerating properly

try lowering your motor as low as it will go on the transom.....just to rule out what you think it is....ventilation. check your gear oil for metal filings just in case ! it does sound like air getting around the prop under certain situations. what happens on a wot holeshot? if there is something broken, or slipping it should show up on the holeshot.

 

[Coastal Denizen]

Re: Engine revs w/o the boat accelerating properly

I checked to make sure the motor had not shifted on the transom, its still bolted securely. It has power tilt/trim and I always start with it all the way down. But I did try starting with it up a little, made no difference. I will check the magnet in the drain plug. But it just does not seem like it could be gears slipping without tons of noise and bad shifting. Tnx.

 

[nwcove]

Re: Engine revs w/o the boat accelerating properly

not sure of your setup, but can you lower the motor on the transom? (not tilt and trim.....but drop it a hole or two on the bracket?) im more familiar with omc's but had a few that needed more water because of hull warpage from sitting wrong on the trailer. does it "slip" at wot from a holeshot?

 

[Coastal Denizen]

Re: Engine revs w/o the boat accelerating properly

After getting on plane and then running WOT it does fine. No "revving" sound and normal WOT RPM and MPH. Slips or revs bad on a hole shot w/ shoving the throttle as if pulling a skier (but I have only been pulling the kids on a tube lately.) I've had this boat & engine for 3 summers w/ no hint of this problem before. Engine position has not changed. Have only been running 3 blade alum props recently after the original 4 blade plastic prop broke. But no problems with the alum prop for the first few days then the problem started all of a sudden. Possible hull warping I had not though about. The boat is kept in a lift and sits on wooden bunks but could sometimes be slightly out of position. Any ideas what to look for as to possible hull warping? And does that happen with fiberglass? Tnx

 

[nwcove]

Re: Engine revs w/o the boat accelerating properly

if it revs/slips on a hole shot id rule out any problem with the hull, it shoudnt get air to the prop out of the hole.....unless you changed pitch when you upgraded the prop? then you probably would get a better top end speed, but could cavitate on hard acceleration.