3YM20 muted screeching noise under way

[danstanford]

Sea trialling a B323 when my wife asked what the funny noise was. I was listening to the engine sounds which were great and smooth but after it was pointed out could hear it clearly. Over 2000 rpm it seemed to be less and the engine smoothed out and down at idle I really couldn't hear it, but it has me spooked.
It reminded me of an electric fan with bad bearings and the blades rubbing on the housing but then I should hear it the same all the time.....
Any ideas?
Dan

[shakey doug]

Possibly a singing propeller or tight stern/strut Bearing.

[barnaclejim]

belt slipping makes a screeching noise. particularly with accessories on the engine like high output alternators and fridg. compressors.

[Sailmonkey]

It could be either of the above, or alternator bearings, or water pump.....or nothing.


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[dgz3]

As noted, it could be lots of things. It is often possible to isolate and identify noises like this with a mechanic's stethoscope or even just a long handled screw driver. Put the probe (or the screw driver point) firmly against various parts (alternator, water pump, transmission, stuffing box, etc.,). When you find the culprit you will hear it plainly. If using the screw driver, press the end of the handle against your ear and cup your hand around it. You'll want a big, heavy, long screw driver for this.
Tips: don't put it against anything thats moving (DUH-UH) like spinning shafts or pulleys. And watch your hair, clothing, other body parts, etc.
Check it in gear and out. This may eliminate the rest of the drive train. A squirt of belt dressing or even WD40 will silence a squealing belt.
It's talking to you.

[a64pilot]

I'm betting an alternator, if my batteries are down, I can plainly hear my alternator, it's not bad bearings it's just the noise it makes when it's making significant power.

But,
Pay for a real mechanic to do a real survey if your worried.

[danstanford]

Have now had her out a couple of times and we love almost everything about her! The noise crops up at just over 1000 rpm and seems to almost entirely go away by 2000 rpm. Last night I was cruising into the marina at too high a speed so I dropped down to about 1500 rpm and the noise sounded horrendous like some bearing was about to seize from running dry. I threw her into neutral and the noise went away so it is definitely somewhere in the propulsion system. The PO's said it was always there and they just put it down to some kind of resonance at that rpm range and didn't run her there.
I did notice the water pump">raw water pump drive belt seemed loose but we have a good flow of water at idle and when running so that seems unlikely.
I am going to try the listening method with a length of hardwood and a screw driver but am wondering if someone has an idea how to really test the prop/prop shaft system other than listening?
Dan

[jibstay]

Do you have any belt dust?

My loose belt screeched and caused lots of belt dust.


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[danstanford]

Originally Posted by jibstay

Do you have any belt dust?

My loose belt screeched and caused lots of belt dust.


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Jib, there is no belt dust present nor was the noise present when I revved her up in neutral to re-charge my flat batteries. I didn't go too high but probably 1300 or so and there was not a peep but that speed in gear going through the water is very noticeable. The above suggests that it is not the alternator or belts, but the noise could be a belt....but wouldn't it screech at start up or idle, or over 2000 rpm?

[jibstay]

I'm no expert, but tightening the belt would be a pretty simple thing to do, to see if it solves the problem.

I had some rust on the inside of my alternator pulley, that caused premature wear and loosening of the belt. But the belt dust was a big red flag.


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[dgz3]

Try revving it higher in neutral. Well into the range it makes noise. 1300 rpm isn't much. Don't go crazy, you don't need 5000 rpm. If no noise, then that probably isolates the problem as something aft of the motor. Transmission, coupling, stuffing box, shaft log, etc. Though not necessarily. And some or all of those things may not make noise until and unless under load when the prop is pushing it all forward and maybe to the side if out of alignment.

It's talking to you now. You better figure out what language it's speaking before it tears up completely, possibly taking a bunch of other expensive stuff with it.

And are you sure it's not the belt???

Eliminate that first, one way or another. Have your wife run the boat until it makes the noise. Spray WD40 on the underside of the belt. Between the belt and pulley. If the noise changes significantly you have found the problem. The wd40 will wear off quickly and it won't hurt anything.
A belt may not make dust until it is about to fail. They make dust when over stressed. As with a high output alternator under heavy load with an inadequate belt/pulley combo. High output alternators usually run a double belt system for that reason.
Belts can also become glazed if they are just slipping from being under tightened. If the underside of the belt has a shiny smooth appearance, it won't grip the pulley. And once glazed, no amount of tightening will make it work.
Why not just change it out now? And don't just get the same belt that's on it. It may not be correct. Study up on how to properly tension it. It may need to be tighter than you think. But too tight and you can burn up bushings in pumps, alternators and such.
Water output is no indication of belt tightness. Nor is charge rate.
An alternator belt might screech on start up if the battery is discharged from cranking and the motor is revved until it enters its high charge RPM range. But your regulator may be set not to produce high charge rates at low RPM so as not to stall the motor. It could be the belt is screeching when the alternator reaches full output RPM. Which is probably around that 1500 range.

Or.... you can just wait and see what breaks!
This might be a good time to assess your mechanical abilities. Sometimes, it is cheaper to call in a pro early.
You're in Ontario? Are you hauling out for the winter? You may want to at least diagnose it before you haul. Mechanics can get pretty busy in the spring.

[Sailmonkey]

Is this a screeching or more of a high pitched singing wail?




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[danstanford]

Originally Posted by Sailmonkey

Is this a screeching or more of a high pitched singing wail?




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Laughing at my complete inability to answer that question! The best I can do is say it is just like this noise:

https://www.youtube.com/watch?v=cSksqVs6IgA

...which this Guy thinks is a singing propeller. It has a two blade fixed prop which is likely factory and the engine only has 300 hours on it.

Having heard this video I think that is what it is but most mentions of this issue seem to reference a pretty narrow band while mine covers all the way from 1000 to 2000 rpm and is strong enough that my wife heard it and was quite concerned.

[Sailmonkey]

Ours had a wide band too from about 1100-1600 or so. Lasted roughly 400 hours, slowly diminishing through that time. We ended up in a marina that was electrically active, the slight damage to the prop for the year we were there ended the singing.


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[JohnT]

Chech all the above suggestions - if the problem is not found, then check the following:
- check your engine mounts as you may have a broken mount
- check your stuffing box for any misalignment
- dive on the boat (with engine off) and check any play on the stern bearing

When securely tied to the dock, get somebody to stick the boat into gear whilst you watch the engine and shaft coupling for any forward / aft movement as it goes into forward or reverse.

From the sounds of it, your screech only happens in gear. So the obvious is to check engine mounts, alignment and play on the shaft. From what you describe, it sounds like the engine is moving slightly sideways under torque, resulting in a movement of your shaft at the stuffing box. I do not know what stuffing box you have, but they should be serviced every 500 engine hours. Your boats manual should cover the stuffing box/gland.