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Old 31-08-2015, 15:19   #1
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Bad vibration course of action

I have a really bad vibration that has gotten progressively worse over the last six months (during a trip down the icw to the Bahamas). The boat is out of the water now and the cutlass looks okay. I've aligned the engine myself and it doesn't seem to help.

I'm thinking pull the shaft and prop while the boat is out of the water and have them checked/balanced, I might replace the cutlass just to get that out of the equation.

If that doesn't fix it, I'm on to engine mounts. Anything else I should be thinking of/looking at? One mechanic I had look at it said he thought it was idling rough but that that might have something to do with the engine mounts. But that if it didn't then clogged injectors or a bad mechanical fuel pump could cause a lot of vibration anyone have, any experience with this?

Anyone have any other thoughts or input?


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Old 31-08-2015, 15:28   #2
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Re: Bad vibration course of action

Crack open one injector line at a time to see if there is a cylinder that doesn't change the way it is running. If so, swap that injector with another and see if it follows the injector or if the problem remains with the cylinder. That is a good place to start.
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Old 01-09-2015, 03:28   #3
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Re: Bad vibration course of action

You can get clues from the symptoms:
If the bad vibration increases with speed then some form of out of balance (which could be mass imbalance or exciting force imbalance e.g. engine or prop) or mis-alignment is indicated.
If the bad vibration is at one or more fixed speeds so that there are peaks of vibration in the speed range, then some form of resonance is occurring whirling/lateral vibration, torsional vibration or engine shake on its mounts.
If the vibration is still there (but it may be reduced in amplitude as the engine is unloaded) when you run through the speed range out of gear, then you have eliminated the drive coupling, shaft assembly, cutless and prop.
If the vibration is still there when you trail the prop out of gear while moving ahead (e.g. sailing) then it will likely be the shaft assembly, cutless, prop either one or combinations.
That should be a good starting point and hopefully you already know the answers to most of the questions already.
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Old 01-09-2015, 10:56   #4
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Re: Bad vibration course of action

Quote:
Originally Posted by unbusted67 View Post
I have a really bad vibration that has gotten progressively worse over the last six months (during a trip down the icw to the Bahamas). The boat is out of the water now and the cutlass looks okay. I've aligned the engine myself and it doesn't seem to help.

I'm thinking pull the shaft and prop while the boat is out of the water and have them checked/balanced, I might replace the cutlass just to get that out of the equation.

If that doesn't fix it, I'm on to engine mounts. Anything else I should be thinking of/looking at? One mechanic I had look at it said he thought it was idling rough but that that might have something to do with the engine mounts. But that if it didn't then clogged injectors or a bad mechanical fuel pump could cause a lot of vibration anyone have, any experience with this?

Anyone have any other thoughts or input?


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Focusing on the driveline - prop side of things:

On the alignment, how close did you get, axial and radial between the transmission flange and the propeller coupler face ? A good target is 0.002 inches.

Do you use a drive line rubber coupler piece, or just steel to steel ?

You aligned it in the water I'm assuming. So in the slip died down, in gear, with you below looking from the transmission - coupler perspective at around 1500 engine rpm, a bad alignment or bent shaft condition would clearly be visible as a combination of wobbly shaft as it enters the shaft gland, along with an engine moving up and down on the rear mounts at the shaft RPM, not the engine RPM. That would be the key, and would help differentiate the engine from the parts behind. Might be handy to have mental knowledge of your transmission reduction ratio to make sense of what you witness.

A bent blade can cause this too, but totally different signature, as everything at 1000 engine RPM in gear will appear ok, but at full throttle, the non-symmetric thrust pulses will wreak havoc down under.

A loose cutlass may in time be the result of all this, versus the initial cause.
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Old 02-09-2015, 10:18   #5
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Re: Bad vibration course of action

Wow thanks. Excellent advise guys.


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Old 02-09-2015, 10:33   #6
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Re: Bad vibration course of action

'Bad vibration' is not enough to go on. When does it occur, when does it not occur? What engine, what transmission, what prop, what shaft length, diameter, how supported?

For example, I'm currently chasing a bad vibration on a Shock 35 with a Yanmar engine/Kanzaki transmission that occurs only when in forward gear above 1500 rpm, and only after 10 minutes of running. In this case (after eliminating a whole bunch of stuff) the signs are pointing to a problem in the transmission thrust bearing.
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Old 06-09-2015, 20:19   #7
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Re: Bad vibration course of action

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Originally Posted by donradcliffe View Post
'Bad vibration' is not enough to go on. When does it occur, when does it not occur? What engine, what transmission, what prop, what shaft length, diameter, how supported?

For example, I'm currently chasing a bad vibration on a Shock 35 with a Yanmar engine/Kanzaki transmission that occurs only when in forward gear above 1500 rpm, and only after 10 minutes of running. In this case (after eliminating a whole bunch of stuff) the signs are pointing to a problem in the transmission thrust bearing.
Universal M40B, Hurth ZF15, not 100% on shaft length, I think my diam is 1 1/4". vibration seems to only happen in gear but as noted above a mechanic was noticing we were idling rough, whether or not that was because the engine was idling on worn mounts is TBD.

The engine jumps around a lot in forward and reverse at low RPM and produces a big jolt when you put it in gear with maybe two inches of deflection. The vibration modulates when you crank up the rpm. By the end of the trip I was unable to attain cruising rpm: 2300. It got so bad at one point that I sheered off the alt mount. I just limped into Green Cove Springs and had the poor old girl hauled out. Now here I sit ashore worrying my brain about it.

I should also mention I replaced a damper plate on the engine before this trip and I'm not sure what caused the previous failure but it looked to have been pretty catastrophic for the previous plate so other things could have been damaged. I do not have a record in a log or in my brain about whether or not I had much vibration before I replaced the plate.
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Old 07-09-2015, 03:39   #8
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Re: Bad vibration course of action

2 inches of deflection is an awful lot. I assume and hope that's at the top of the engine and not at the mounts. Either way, it's too much and will do damage. It could well be why the damper plate went. When you say damper plate, is that the drive line damper betwen the gearbox and shaft or some form of top steady? Sounds like new engine mounts are a must, even if they did not cause the problem then they must have suffered damage. The damper plate will also need to be checked given it's rough short life. Also given the amount of movement you have reported, I would suggest checking the shaft has not been bent, disconnect shaft and rotate it in situ checking for radial and axial variation.
The large jolt you reported when going into gear is abnormal. If you go into gear at idling speed, there is neglible hydrodynamic load from the prop and just the inertial load accelerating the shaft and prop up to speed, so you should not get much more than a small clunk
As to the cause, it could be uneven cylinder firing, just a large misaligment causing the engine to sag damaging the coupling and or shaft, or a damaged prop. You really need to a systematic check through the drivetrain:
- Check prop
- check shaft radial and axial runout at gearbox
- check shaft is not bent
- check shaft line from gearbox flange through to cutless bearing
- check shaft to gearbox coupling, run out of gearbox flange
- check engine mounts
- check engine misfiring
You will probably need engineering support for some of this
Good luck
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Old 16-09-2015, 17:51   #9
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Re: Bad vibration course of action

Quote:
Originally Posted by Rapanui View Post
2 inches of deflection is an awful lot. I assume and hope that's at the top of the engine and not at the mounts. Either way, it's too much and will do damage. It could well be why the damper plate went. When you say damper plate, is that the drive line damper betwen the gearbox and shaft or some form of top steady? Sounds like new engine mounts are a must, even if they did not cause the problem then they must have suffered damage. The damper plate will also need to be checked given it's rough short life. Also given the amount of movement you have reported, I would suggest checking the shaft has not been bent, disconnect shaft and rotate it in situ checking for radial and axial variation.
The large jolt you reported when going into gear is abnormal. If you go into gear at idling speed, there is neglible hydrodynamic load from the prop and just the inertial load accelerating the shaft and prop up to speed, so you should not get much more than a small clunk
As to the cause, it could be uneven cylinder firing, just a large misaligment causing the engine to sag damaging the coupling and or shaft, or a damaged prop. You really need to a systematic check through the drivetrain:
- Check prop
- check shaft radial and axial runout at gearbox
- check shaft is not bent
- check shaft line from gearbox flange through to cutless bearing
- check shaft to gearbox coupling, run out of gearbox flange
- check engine mounts
- check engine misfiring
You will probably need engineering support for some of this
Good luck

I am estimating that two inches from about 600 miles away so I may be exaggerating. And yes I was referring to the drive line damper between the fly wheel and the trans.

- check shaft radial and axial runout at gear box
- check shaft line from gearbox flange through to cutless bearing
- check shaft to gearbox coupling, run out of gearbox flange


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Old 09-10-2015, 13:55   #10
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Re: Bad vibration course of action

Quote:
Originally Posted by unbusted67 View Post
I have a really bad vibration that has gotten progressively worse over the last six months (during a trip down the icw to the Bahamas). The boat is out of the water now and the cutlass looks okay. I've aligned the engine myself and it doesn't seem to help.

I'm thinking pull the shaft and prop while the boat is out of the water and have them checked/balanced, I might replace the cutlass just to get that out of the equation.

If that doesn't fix it, I'm on to engine mounts. Anything else I should be thinking of/looking at? One mechanic I had look at it said he thought it was idling rough but that that might have something to do with the engine mounts. But that if it didn't then clogged injectors or a bad mechanical fuel pump could cause a lot of vibration anyone have, any experience with this?

Anyone have any other thoughts or input?

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Yep been there got the T shirt this year and learned the hard way.

Removed and renewed Cutlass bearing -better but still vibration!
Fitted new shaft and another cutlass bearing - better but still some vibration
Changed 'all four' engine mounts and re aligned shaft etc - problem solved !
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Old 24-10-2015, 19:24   #11
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Re: Bad vibration course of action

Just went through this myself. Prop was not balanced and cutlass bearing was bad. Once the prop was tuned and balanced and cutlass changed, no more vibration whatsoever. I have a drive saver on my shaft to transmission connection. If there is hesitation and vibration going forward but not in reverse, then it's likely the thrust washer in the transmission. A rough idle is a completely different issue.


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Old 24-10-2015, 19:26   #12
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Re: Bad vibration course of action

Is the shaft true? You could have picked up a line running down the ICW.
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