60 HP Merc "Big Foot" @ higher RPM range?

[aceoky]

I'm posting this for a lady I know, she had recently bought a 21ft Pontoon with the mentioned motor on it, all was good for awhile (it's IIRC ) a 2004 Merc supposed to be very low hours on it. Runs great, but the other day she tells me at a bit over 4,000 RPMS it "revs up, RPMS rise but boat slows like you dropped the throttle back fast)

She has this boat on a nearby lake (no trailor) so taking it in to be checked will be a task.....she may be able to get someone qualified to look at it, but was hoping I might be able to find some knowledgeable folks here who could "warn" her of what it most likely is and the expense involved...

She ran the boat back in (about five miles she thinks) @ 3000-3200 RPM with NO hiccups at all, so I'm thinking it only does this (or did it) at higher RPMS (WOT runs about 5000-5500 RPM IIRC)

Thoughts? Opinions?

Thanks in advance for any input on this.

[fxnrxn]

It would be nice to take a ride and see if the motor is cavitating. That could account for the revs and drop in speed. Could be a simple fix if it's just a mounting issue. I scuba dive with a group and our Zodiac with too many "skinny" guys in it exhibits exactly the same characteristic that you are describing. Cavitation is the cuprit in our situation. I'm not a pontoon expert, but it may be a loading issue causing cavitation. Mike R.

[aceoky]

Thanks for the fast response, I really appreciate it as she's very concerned.

[Tim_Kelly]

Possibly the prop slipping on the hub too?

[perchjerker]

has she tilited up the motor and looked at the prop? Might be something simple like weeds or a piece missing from the prop since it sits in the water it may go unnoticed

[aceoky]

has she tilited up the motor and looked at the prop? Might be something simple like weeds or a piece missing from the prop since it sits in the water it may go unnoticed

That was the first question I asked her, she said she did and there was nothing visible on or around the prop. She did mention it had a couple "nicks" on the prop when she bought it, which makes me "think" maybe the prop IS slipping on the hub. (thanks Tim_Kelly)

If it is slipping, what would she need to do to fix it (and she wants an idea of costs involved if possible-ball park ideas would be great) and also her main concern is , (obviously) is it "dangerous" to the engine to run it like that (It doesn't seem to slip (or whatever) at lower RPMS)

She didn't think she could get anyone for a couple weeks as they're all busy winterizing houseboats etc on the lakes here right now :banghead:

IF it were your boat, would you just "sit it out" or use it carefully until someone could repair it?

It's cold enough now here I don't think she'd be running that fast at all anyway, but IF it could damage it, even at < half throttle I'd want her to know this. As usual the walleye are beginning to "heat up" on that lake right now, talk about bad timing.:sad:

Thanks to all for your thoughts, I (and she will) appreciate it very much indeed!

[walleyeguide]

That was the first question I asked her, she said she did and there was nothing visible on or around the prop. She did mention it had a couple "nicks" on the prop when she bought it, which makes me "think" maybe the prop IS slipping on the hub. (thanks Tim_Kelly)

If it is slipping, what would she need to do to fix it (and she wants an idea of costs involved if possible-ball park ideas would be great) and also her main concern is , (obviously) is it "dangerous" to the engine to run it like that (It doesn't seem to slip (or whatever) at lower RPMS)

She didn't think she could get anyone for a couple weeks as they're all busy winterizing houseboats etc on the lakes here right now :banghead:

IF it were your boat, would you just "sit it out" or use it carefully until someone could repair it?

It's cold enough now here I don't think she'd be running that fast at all anyway, but IF it could damage it, even at < half throttle I'd want her to know this. As usual the walleye are beginning to "heat up" on that lake right now, talk about bad timing.:sad:

Thanks to all for your thoughts, I (and she will) appreciate it very much indeed!

just pull the prop off and see if it has a rubber hub or the flo-torque hub.
if the hub is plastic it is flo-torque,it likley is not spun---would be broken into pieces,stripped if it impacted bottom etc.
if it has a rubber hub it more likely be spun.
the solution is to replace the prop ,or send it out and get it re-hubbed.
if it has flo-torque and its not busted up there are other problems.
frank

[aceoky]

Thank you Frank! :cheers:

[yarcraft91]

The symptoms sound like what the motor on my 22' Sweetwater pontoon boat does. In my case, it is caused by prop cavitation. Very small amounts of weeds on the lower unit will cause it.

[staylor]

..they love to ventilate at speed when the hull comes out of the water. If it was the prop hub and it spun with the motor at 4000 rpm then the hub would be failed completely- and you would then be lucky to get to 1500 rpm without it spinning......that is unless you've got the plastic flow torque hub and the previous owner did not torque the prop to the factory torque spec, and this will cause the prop to do all kinds of weird things. As posted by others, pull the prop and inspect the hub. Then re-install with the proper torque if its OK. Next, experiment with different motor running angles to see if this cures the problem. Next step is to try a prop with no dings, and maybe one in a lower pitch since too much pitch can also cause the venting problem on pontoon boats. If one of the "dings" is a section of the leading edge that's bent over then this alone can be the problem. I also recall a pontoon rig where the "dings" ended up to mean that the owner had somehow managed to actually wear away the diameter of the prop so that it looked fairly good, but was 1 inch smaller in diameter then it was supposed to be.
Doug

[T Mac]

Sounds like Venting to me, too.

[aceoky]

..they love to ventilate at speed when the hull comes out of the water. If it was the prop hub and it spun with the motor at 4000 rpm then the hub would be failed completely- and you would then be lucky to get to 1500 rpm without it spinning......that is unless you've got the plastic flow torque hub and the previous owner did not torque the prop to the factory torque spec, and this will cause the prop to do all kinds of weird things. As posted by others, pull the prop and inspect the hub. Then re-install with the proper torque if its OK. Next, experiment with different motor running angles to see if this cures the problem. Next step is to try a prop with no dings, and maybe one in a lower pitch since too much pitch can also cause the venting problem on pontoon boats. If one of the "dings" is a section of the leading edge that's bent over then this alone can be the problem. I also recall a pontoon rig where the "dings" ended up to mean that the owner had somehow managed to actually wear away the diameter of the prop so that it looked fairly good, but was 1 inch smaller in diameter then it was supposed to be.
Doug

Thank you so very much, hoping the weather will "stabilize" soon so we can find out. Been very cold (below freezing here) which is very unusual for this part of the country especially this early.

I can't thank you fine folks enough for your help in this matter! What an awesome site here!