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Old 25-12-2008, 21:40   #1
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serious prop shaft vibration

Came home from last trip about a month ago, everything fine. Pre trip check out for xmas trip - started the yanmar and it ran like a dream, put it into forward at idle and the motor tried to jump out of engine compartment, out of gear and into reverse and all OK, back into forward and applied a little revs and that is OK. So it's only low revs in forward the motor doesn't like, vibrates and shakes prop shaft something fearsom.
Could be build up on the prop???? Any help greatly appreciated.
Looked through a thousand threads and didn't find what I wanted.
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Old 25-12-2008, 22:22   #2
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Some engines jump around badly if the engine idle speed is too low, especially after the mounts start to get old. Check out the RPM's. (When you put it in gear, the revs drop and it "jumps" more. That is the first thing I would check. Lots of other possibilities, but go for the easy one. Probably want to see idle 850 or so, minimum, but check your manual.
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Old 26-12-2008, 06:46   #3
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vibration

check the shaft coupling. In forward gear the thrust from the prop will force the shaft forward against the coupling and may be the source of your trouble.
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Old 26-12-2008, 10:26   #4
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First, you're sure there's nothing wound on the prop or shaft? After looking for something snagged on the prop (hope the water's warm, BTW!), try these two checks: 1) grab a solid piece of the motor and try to pull the motor around. If it moves easily, either mounts or vibration dampers are in bad shape. 2) Try pushing sideways on the prop. If it moves and then hits with thump of some sort, the cutlass bearing is probably worn out. If the engine is tight on its mounts and the cutlass bearing isn't sloppy, do a full engine / prop shaft alignment. It should a) ensure everything's in line and b) reveal loose bits (coupling bolts, for example). It's possible the transmission, particularly the output shaft bearings, is starting to go. How's shifting in general? Also, how many hours on the engine?
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Old 26-12-2008, 12:46   #5
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Do you have a folding/feathering prop?
Maybe it threw a blade.
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Old 26-12-2008, 13:45   #6
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Um, even with a plastic prop, such as those made by CDI and a few other sources, the imbalance due to a missing blade would show up when reversing sharply as well as when moving forward (the report is vibration forward but not in reverse). In the case of a bronze prop, a lost blade would be hard to miss.

A friend with a CDI-made feathering prop, lost a blade and was able to limp into port, but there was no mistaking something was wrong.
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Old 26-12-2008, 15:50   #7
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Thanks everyone for the help so far. This morning will try the engine mounts etc. but they were the first thing I checked but will have another look. It does idle slow but up to now has been perfect and can't think of anything that may have changed. Thought of checking shaft for something wound on it but all was good a month ago and hasn't been anywhere since.
Will let you know if I find anything. Thanks again.
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Old 26-12-2008, 16:07   #8
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If you have let diesel drip on those Yanmar mounts they are shot.....diesel is the enemy to that rubber. The one cylinders do jump around at lo-lo revs.
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Old 26-12-2008, 19:47   #9
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Have a diver go look at your prop, shaft and cutlass bearings. Inspect for yourself your engine coupling and engine mounts. I would not use your engine any further until you are certain of the problem.
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Old 26-12-2008, 21:23   #10
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What size engine do you have?????
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Old 26-12-2008, 21:52   #11
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It's a 3JH3E 35 hp.
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Old 26-12-2008, 22:06   #12
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Just been and checked things again. Mounts appear to be OK, if you grab a solid part on top of the engine and shake it hard there is very little movement. One worry I noticed while turning the shaft by hand was the seal part of the manecraft shaft seal is off centre ie it doesn't just spin it goes up and down on the seat. Might need to go to the yanmar pro.
I love sailing boats when everything works.
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Old 27-12-2008, 01:59   #13
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I'm not sure about which part of the shaft you're talking about but I suspect it's the assembly on the inside of the boat that keeps water from entering the boat, past the prop shaft (e.g., shaft seal, stuffing box). A little more detail would help here. Nonetheless, nothing on the shaft or around the shaft should move up and down (which I take to mean perpendicularly to the shaft, not parallel to or along the shaft's axis).

My guess is that once the "up and down" bit is sorted out, things will go back to normal. How big of a job stopping the up and down movement is, is happily left for the boat owner to resolve.

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In troubleshooting a problem, (toning things down for a family audience): "Assume is the mother of all [screw]-ups". Really, assume nothing. If you can't prove the condition of all related items, they remain suspect.
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Old 28-12-2008, 09:59   #14
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Shaking the engine won't tell you much.

Other than it shakes.

Get a block of wood and a pry bar and see if you can raise the engine.

The forward mount on the starboard side (or the one under a fuel filter is usually the first to go.
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Old 28-12-2008, 10:18   #15
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Um, well... I humbly beg to differ on all points. On a Universal M-40, we managed to kill a port aft bracket (complete fracture). Actually, which side goes first depends on the direction of engine rotation. A solid push on the side of the rocker cover showed a great deal (such as daylight between the piece bolted to the block and the piece on the damper's stud). Lifting the engine with any sort of lever could produce all manner of excitement from sheared off accessories (oil pump, for example) to bent oil pans (the joy of finding there is now negative clearance between the freshly reshaped pan and a crankshaft counterweight can be ...ah... stirring). Or not. I'll pass on the try, thanks.

For the record, the broken bracket showed up when a new damper plate was torqued onto the back of the engine, as part of a transmission replacement. The damage was confirmed by pushing on the motor while the tech put a light on the bracket in question.

Added: Pushing the motor around by hand isn't a definitive test for a cracked mount or bad isolator, but if the motor moves a lot, it's a good sign all is not well. Conversely, if the engine goes thump as it's shut down, even if shaking the motor doesn't show anything obvious, I'd still look long and hard at the motor mounts.
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