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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel > Bumping/Thumping Noise-Advice Needed
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


Thread: Bumping/Thumping Noise-Advice Needed Reply to Thread
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Topic Review (Newest First)
06-28-2011 04:03 PM
FishFinder Fixed it! The correct answer: a blockage somewhere in the raw water cooling was causing the white smoke and loud banging (which I believe was backpressure?). After testing the injectors (were fine), I decided to dismantle the entire exhaust system and clean the cooling passages. Reinstalled and smoke/banging were gone. Have run it for about 20 hours since and all is good ... or at least I am back to the old problems and no new ones.

Thanks for the responses.
06-20-2011 03:44 PM
Faster Your exhaust hose likely makes a big loop up under the cockpit or coaming before going to the exhaust fitting, in many cases this hose is not secured. As you pump the 'slugs' of water through that hose might be giving a 'kick'. This could be hitting the side of the cockpit well and those are the thumps you hear.

Our boat does this occasionally, but relatively quietly. If you can view the hose underway you'll know soon enough.... FWIW
06-20-2011 03:44 PM
FishFinder Good to know it's probably not a drivetrain issue. On the muffler -- do they have baffles? I was under the impression they were just empty inside and were fairly foolproof. I could take the hoses off I suppose and look inside.
06-20-2011 03:29 PM
arf145 I agree--if it isn't happening in gear at the dock, that makes drive train issues less likely. With mine, you could see the excessive movement in the shaft, and hear it, any time it was in gear.
06-20-2011 02:11 PM
kd3pc 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
06-20-2011 01:55 PM
FishFinder 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
06-20-2011 11:47 AM
SVCarolena 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.
06-20-2011 11:45 AM
arf145 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.
06-20-2011 11:17 AM
BarryL 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
06-20-2011 09:34 AM
TQA Check your engine mounts! Sounds like one or more may have failed.
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