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Old 10-20-2014, 11:26 AM   #1
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Strange noise from back end of boat

Coming home from a 3 day trip Saturday, I slow down to about 3 kts, put engine in neutral, and this noise, kind of high pitched to low pitch happens. I assume it was the prop spinning down to a stop; not sure where it comes from, possibly stuffing box? I've heard it before, months ago, but not recently. Last time it happened was during close quarters maneuvering, as in forward, then reverse, then repeat same. No leaks or wobble from shaft passing thru stuffing box. Any ideas?
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Old 10-20-2014, 11:38 AM   #2
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Greetings,
Mr. GC. Well, high to low pitch definitely, as you have surmised, has something to do with the drive train and the prop winding down IMO. Sooooo.....cutlass bearing, stuffing box, transmission. Don't discount the stuffing box so readily AND it's the easiest to check. Could be running dry. Is it warmer than usual?
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Old 10-20-2014, 11:54 AM   #3
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RT: No, it drips as it's supposed to, and runs cool, I check it regularly when underway. Hopefully not transmission. Next time out I'll get down by the stuffing box and listen, also open the engine room door and see if it might be transmission. Thanks
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Old 10-20-2014, 11:59 AM   #4
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Do you have a shaft bonding brush? They can be noisy as you describe.
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Old 10-20-2014, 12:19 PM   #5
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Sounds like the shaft is "singing" in the strut bearing. At slow speeds the shaft can wipe the bearing dry and you get that singing noise. There is a technical name for it. But I forgot what it is.

Does the noise sound sort of like when you run your finger around the rim of a wine glass?
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Old 10-20-2014, 03:28 PM   #6
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Capt. Bill: yes, that's the sound except its not constant, it goes from high to low to nothing. And the prop, or something, has sung for years. Is this bad or nothing to worry about?

Retriever: I do have a shaft bonding brush, except its not a brush, it's a hard, black rectangular piece attached to an aluminum strip and sits on top of the shaft. Had not thought of that as being the problem, I'll lift it up out of shaft contact and see what happens.
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Old 10-20-2014, 03:42 PM   #7
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Greetings,
Mr. GC. Now there's a thought. Mr. R. may have hit upon the source. That hard black rectangular piece could be a block of carbon and I could easily see it "singing". Try the easy cheap stuff first. Let's not jump to seclusions just yet....You could give the block a spray of graphite. That will lubricate it without destroying electrical conductivity.
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Old 10-20-2014, 03:46 PM   #8
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Good advise RTF, will do. I think you're right, it is a block of carbon.
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Old 10-20-2014, 04:01 PM   #9
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Quote:
Originally Posted by Gulf Comanche View Post
Capt. Bill: yes, that's the sound except its not constant, it goes from high to low to nothing. And the prop, or something, has sung for years. Is this bad or nothing to worry about?

.
Some boats just sing. It can be an alignment issue. But unless the bearings are wearing out prematurely or you are seeing other issues that could be caused by bad alignment it's not a big deal.

I've never heard of a shaft brush singing. But anything is possible.
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Old 10-20-2014, 04:15 PM   #10
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Capt. Bill: That's the train of thought I'm going with, vibration is pretty much the same as it's always been, Perkins 4-236 and/or drive train vibrates at low rpm, just fine at 1600. Now, the singing shaft brush is easy to prove or disprove.
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Old 10-20-2014, 05:09 PM   #11
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Sometimes cutless brgs and shaft brushes make noise at super low shaft rpm. Go roll the shaft by hand, if it turns easily then alignment can't be far off. You might even hear the noise while rotating.

Next time under the boat go see how much slop is in cutless bearings.
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Old 10-20-2014, 05:38 PM   #12
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Thanks Ski, will do.
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Old 10-20-2014, 05:49 PM   #13
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Sometimes the anti siphon loop in my cooling system makes a high pitched whistle that is audible depending on rpm. I find noises are often something rattling loose on the boat at certain rpm such as a tool in my toolbox etc. I would tie up the boat and run it under load at the dock until I had isolated the noise.
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