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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 06-20-2011
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Bumping/Thumping Noise-Advice Needed

Could use a little advice here -- I've got a 1980 Volvo MD7A that I'm worried about. When in gear, there is often a low tone thumping or banging sound that seems to be coming from the rear of the boat, but I can't pinpoint where. The thumps are rhythmic, about 3 or 4 a second. The thumps will vary in volume, will be loud for a few seconds then quiet down a little. I've heard these thumps before, noticed them most when in waves (figured it was water covering the exhaust port, which is right at the waterline and easily submerged). But the thumps seem to be worse now and happening all the time. I can feel the thumping in the hull, it's almost like someone is hitting the hull with a rope or something. Increasing throttle increases the volume of the thumps but rhythm stays the same.

The engine itself seems to run fine, plenty of power but now it is generating grey/white smoke at higher RPMs which I did not notice before. I just hauled out by the way, and cleaned the prop, shaft, installed new zincs, oil change, impeller change.

Please advise, I'm about to leave on a week long trip. Is this potentially damaging to the engine to run it this way? My gut tells me this is an exhaust issue ("water hammer?") Thanks!
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Last edited by FishFinder; 06-20-2011 at 03:49 AM.
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Old 06-20-2011
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White smoke usually indicates steam or unburned fuel.Most likely bad injector.Other possibility low compression,however this is usually a gradual process.LOL enjoy trip.marc
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Old 06-20-2011
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Does it make the noise when the engine is running while in the slip or just while under way?
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Old 06-20-2011
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check the alignment?
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Old 06-20-2011
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Check your engine mounts! Sounds like one or more may have failed.
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Old 06-20-2011
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Hello,

IMHO your cutlass bearing is worn. That allows the shaft to move around and you can hear a thumping sound as it hits the strut.

If you have a fixed blade prop you can test this. pick a day when can sail quickly, say 5 kts or so. Turn the engine off and leave the trans in neutral. check the shaft to make sure it's still turning. If you hear the thumping, place the trans in reverse. When the shaft stops turning the thumping will stop. Now place the trans into neutral. Do you hear the thumping when the shaft is turning? If so, it's the cutlass bearing that is worn out.

Good luck,
Barry
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Old 06-20-2011
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Assuming this is happening when in gear, I'd be thinking something in the drivetrain, including engine mounts, as someone else suggested. If it happens even when not in gear, engine mounts would be #1. I had a thumping noise when in gear after installation of new shaft and new cutlass. Turned out the shaft wasn't true--the wobble caused the thumping sound. I would bet that a bad cutlass bearing would do similar.
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Old 06-20-2011
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Like others have said, it sounds like either a bad motormount (replaced ours this year), allignment, or (most likely) a worn shaft bearing. If you can get your hands on the prop underwater, try to move it around. Also take a look at the shaft while it is turning, in gear, under load, to see if it is bounding around. Lots of things can cause these types to symptoms, so it is difficult to diagnose without more info, but at least you have a few good places to start.
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Old 06-20-2011
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OK thanks for the responses. I just did some tests --thumping does not happen with boat in gear at dock. Does not happen in neutral. I got boat up to speed, and put engine in neutral (prop was spinning) and no thumping. The thumping DOES seem directly related to the exhaust port being underwater. Hit a wave, and when the stern pushes into the water, thumping gets loud. Over the wave and stern raises above water, thumping disappears. The stern "squats" when underway and this pushes the exhaust port underwater sometimes. Yesterday I had people on board (I usually sail solo) and this may have pushed the stern down a bit more than usual, hence the thumping was a lot worse. Not sure how to fix this ... seems like it would be bad for the engine (backpressure)?

I posted some videos I took today of my engine running while underway, would love if you guys could have a look and let me know if you see anything awry:

MD7A_Video
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Last edited by FishFinder; 06-20-2011 at 01:59 PM.
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Old 06-20-2011
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It appears that your boat has a water lift style muffler/mixer...if so...you may be looking at a broken baffle as you indicate the periodic thump gets worse when the thru-hull is submerged...that would put more back pressure on your exhaust system and perhaps cause something loose in the waterlift/mixer to do the thumping...I think that is what I see in your rear engine segment...

Contrary to the others, I do not think it is shaft or bearing, as it does not happen at the dock...were something out of whack there, one would think you would see it any time the engine is running
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