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Old 10-15-2019, 11:49 PM   #1
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Velvet Drive Mystery

I know there has been a lot written on this already but my issue has a bit of a twist.
Here goes, I have twin Lehman 120's driving BW Velvet Drive 10-13 transmissions. When I was on the flybridge, docking at a secluded outstation a few weeks ago I noticed a strange noise that appeared to be coming from the forest. I would describe it as a high pitch metallic hum. I thought that we were going to be attacked by banshees. Once docked I did a little trouble shooting. When the port engine was in reverse there was a rattling sound coming from the transmission. When I walked to the stern of the boat I could hear the metallic hum but not in the engine room. Quiet in neutral and forward. The engine idles at 600 RPM in neutral according to my laser tachometer. The transmission fluid is at the proper level. Of course when I got home and tried to recreate the fault it had vanished. I would like to think that it has miraculously fixed itself and I don't have to take anything apart but I suspect that if I do nothing it is going to bite me at the worst possible time.
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Old 10-16-2019, 12:26 AM   #2
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You might want to get the tranny fluid analyzed. If it was something coming apart it should show up there. I’d start there before pulling the tranny. Might want to check both so you can compare
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Old 10-16-2019, 02:56 AM   #3
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"Rattling sound" could be a failing damper. The "banshee high pitch metallic hum" is way harder to guess, but the chance of 2 different transmission faults at one time isn`t great.
A banshee (/ˈbænʃiː/ BAN-shee; Modern Irish bean sí, baintsí, from Old Irish: ben síde, baintsíde, pronounced [bʲen ˈʃiːðʲe, banˈtiːðe], "woman of the fairy mound" or "fairy woman") is a female spirit in Irish mythology who heralds the death of a family member, usually by wailing, shrieking, or keening.
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Old 10-16-2019, 06:44 AM   #4
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How long since your damper plates have been replaced? American Diesel's recommended service life is 2,500 hours.
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Old 10-16-2019, 11:39 AM   #5
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VD's do often rattle at very low rpm's especially when they have a lot of hours on them. Of course dampers that are getting worn out can make all kinds of noises. Its tough if you can't replicate the sound now. If the transmissions have high hours on them (4-5Khrs) one or both of them may be getting worn. According to American Diesel (and my own experience with exactly the same transmissions) they're due for rebuild by 5K hours.


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Old 10-16-2019, 11:42 AM   #6
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Another thing to check for low RPM rattles or other noises / play is the reduction unit. Particularly the stationary plate that holds the ring gear from rotating. The teeth on it can wear and cause rattles / slop. Both of mine failed within 2 years with around 1400 - 1500 hours on the transmissions. The first one showed some symptoms before it went, the second one was seemingly just fine until it stripped the teeth off the plate without warning while cruising, causing a total loss of drive on that trans (and still no funny noises).
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Old 10-16-2019, 12:33 PM   #7
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Reverse puts in play a planetary gear set and those can make some noise. Are you sure the noise is new, and worse than the other engine? Compare. And yes there can be gear rattle and cutless bearings can squeal and props can sing. Gots to do a little sleuthing to narrow it down.
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Old 10-16-2019, 04:00 PM   #8
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Thanks All, lots to think about. Can't do an oil analysis as I did my annual oil change last week. The engines are just coming up to 4000 hours. I've put 1000 hours on her. I suspect the dampers have never been changed. Probably a good place to start. It may have rattled before. I could only hear it when in the engine room.
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Old 10-16-2019, 04:11 PM   #9
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Quote:
Originally Posted by GordC View Post
Thanks All, lots to think about. Can't do an oil analysis as I did my annual oil change last week. The engines are just coming up to 4000 hours. I've put 1000 hours on her. I suspect the dampers have never been changed. Probably a good place to start. It may have rattled before. I could only hear it when in the engine room.
Properly cared for, your damper plates will last a very long time. Improper care, not reducing rpm before shifting, may shorten that time significantly.
My experience is this:
hours at time of purchase:3000. I know nothing about the history that precedes me
hours at inspection of both damper plates: 4500
hours at subsequent inspection of port side: 6000
present: 6500

Each inspection revealed no indication of concerns.

Not saying it isn't your problem, just saying I would inspect carefully before throwing them out.
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Old 10-16-2019, 04:12 PM   #10
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You changed your engine oil last week? Or transmission oil?
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Old 10-16-2019, 05:10 PM   #11
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Damper plate lifespan definitely varies. As a general rule, the rougher an engine is at idle and/or the lower the idle speed is set, the shorter the damper plate life (as it's having to do more work).
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Old 10-17-2019, 03:54 PM   #12
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Thanks Koliver and rslifkin. Gkesden I changed the engine oil and the transmission oil. I should have made that more clear on my post.
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Old 10-17-2019, 07:19 PM   #13
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If you have an intermediate bearing on the shaft between the transmission and the shaft log it could be the source of a 'screech".
A really useful tool to have on board is a cheap mechanics stethascope available from Harbor Freight for about $5.00. It has a long metal proboscis in place of the medical "horn" and can really help pinpoint odd noises.

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Old 10-18-2019, 12:35 PM   #14
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Thanks Pete. Do you mean something similar to a constant velocity joint to allow the drive shaft to bend. If that is the case, I don't. Just a one piece 15 foot long drive shaft.
The stethoscope sounds like a good idea. I will get one.
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