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Old 23-08-2009, 18:47   #1
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Weird Vibrations

Just got back from a lovely little day cruise to the Coronado Islands with the lady. Everything went well but I almost called the trip off early this morning when we were motoring out and our engine started to vibrate kind of violently.

In the past this has happened when shifting from forward into reverse (or visa versa) in order to put the breaks on but this morning it happened just when putting the engine in forward.

I was a little nervous because recently our diver told us that we had some electrolysis on our strut (here's the thread about this issue Zinc Fish). I dove down there myself to see what was what and everything seemed to be fine. Now I am having second thoughts.

The problem seemed to go away throughout the day particularly at higher rpms, anything over above idling really. What could this be? We haven't run aground or hit anything recently. I took a look under the hood too and our cutlass bearing seemed fine, the stuffing box seemed to be vibrating a bit but no more than you would expect. Is this normal?
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Old 23-08-2009, 20:20   #2
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What do you mean by looking under the hood to check the cutlass bearing?

What kind of engine?
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Old 23-08-2009, 21:14   #3
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Shaft bent? engine mounts? strut ? its a folding prop? many clues , whe need more info about engine brand, shaft and strut config, last repairs? .....
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Old 23-08-2009, 23:16   #4
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You did not mention brand etc, but sounds like the first thing to check is idle speed. Many boats want to idle at 875 or so. Drop that a hundred and they vibrate wildly, especially as mounts get older. Engines drop in speed when put in gear and then start vibrating. Easy to check with an electronic or manual tach. If you don't have one, use boats tach and raise the speed 100 and see if problem goes away. Only takes a few minutes to adjust the idle speed arm.
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Old 24-08-2009, 13:43   #5
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So yeah sorry for the inarticulate post. I must have been in a Sturgeron haze after our sail. The engine is a Yanmar 3HM35F with 700 hours on it. The shaft is set up in a traditional (V drive?) fashion, and we do not have a folding prop. The problem only seems to occur when we are shifting from forward to reverse and back at low water speeds. I would guess that it that it is not the idle speed as it does not happen when we are in neutral. And it does happen when you are "giving her pickle". Also by under the hood I meant under the companionway stairs, excuse the slang.

I am sort of wondering if this doesn't happen to other people's boats when they are trying to reverse directions? Is this just my prop trying to catch water made turbulent by the fact that it is flowing in an opposite direction? As I said before the cavitations go away when the boats motion through the water speeds up.

Another piece of info I should include is that we just broke a belt that runs from our cam shaft to a compressor for the fridge. The problem seemed to be worse before that happened but still exists. We have not replaced that belt because we aren't currently using that compressor. Could our belts be out of line somehow?
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Old 24-08-2009, 18:56   #6
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If you have the standard Yanmar mounts they will allow the engine to shake.

Also the "drag" from additional sheaves on the front of the engine will cause a load on the engine and hence more load at low speed. i.e. lower idle speed
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Old 24-08-2009, 21:19   #7
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Yanmar engine mounts go bad with regularity.
Our Lord Nelson 35 had a bad mount when we bought it, and it was only 1 1/2 years old.

Steve B.
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Old 24-08-2009, 22:55   #8
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Check for cracks where the engine mounts attach to the hull

I had similar symptoms with a previous boat and it turned out to be fairly extensive cracking of the fiberglass flange that the engine's vibration isolating mounts were attached to. The engine mounts damped out the vibrations during normal idling (no load on the engine), but the engine did its best to jump out of the boat when there was some load put on at low RPM. At higher RPM everything smoothed out and there were no apparent problems.

A lot of grinding, epoxy and glass solved the problem.

Craig
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Old 25-08-2009, 01:38   #9
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I had similar symptoms with a previous boat and it turned out to be fairly extensive cracking of the fiberglass flange that the engine's vibration isolating mounts were attached to. The engine mounts damped out the vibrations during normal idling (no load on the engine), but the engine did its best to jump out of the boat when there was some load put on at low RPM. At higher RPM everything smoothed out and there were no apparent problems.

A lot of grinding, epoxy and glass solved the problem.

Craig
Nope the flanges are fine, I just checked. Hmmmm????
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Old 25-08-2009, 03:36   #10
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Personally, the first thing I would be doing is replacing the mounts. It is the most likely cause, and they even may seem good from an external inspection, but once you lift the engine off, it is obvious that they are toast.
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Old 25-08-2009, 10:27   #11
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Personally, the first thing I would be doing is replacing the mounts. It is the most likely cause, and they even may seem good from an external inspection, but once you lift the engine off, it is obvious that they are toast.
Yikes! This sounds like a bit of an involved fix. We were planning on going out for a sail today. Do we need to fix this right away?

What are some ways we can check their condition without replacing them? Are there any visual indicators?
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Old 25-08-2009, 10:56   #12
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Biggest problem with replacement of engine mountings, is the need to re-align engine to shaft afterwards.

Have you looked at the engine while it is doing its vibration, suggest you do this while closely observing the mounts.
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Old 25-08-2009, 11:38   #13
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And what, if they are bashing the engine around they need to be replaced? feel free to pretend I'm an idiot and explain it to me
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Old 25-08-2009, 13:15   #14
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Engine mountings are designed to absorb vibrations and hold the engine and gearbox within reasonable alignment of the shaft

Initial problems will range from just the vibration at slow revs , through sufficient vibration to break equipment mounted on the engine, to damage to the stern gear and seal.

damaged stern seal will definitely give your bailing arm a good workout! (although this is at the extreme end of broken mounts).
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Old 25-08-2009, 18:49   #15
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Thanks for the dummy explanation. Me thinks the boat will be staying in the slip for the remainder of my stay on her.
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