Prop Shaft Vibration

[oilyrag]

Hi Guys

I have a '73 vintage GRP yacht originally fitted with a 12hp Dolphin. It has been re-engined with a Yanmar 1GM10 and the original stern gland replaced with a PSS Seal.
Problem is now that the long unsupported length of the shaft (1500mm) coupled with the fact that the PSS offers no support to the shaft, means that at various engine revs the shaft whips around and knocks on the inside of the stern tube.
It would appear that the solution is to provide some form of support to the shaft where it enters the stern tube (as it had with the original stern gland) but still allow the shaft to move with the flexibly mounted engine.
Anyone out there experienced a similar problem and found a solution?

 

[Seapepper]

I have a similar vibration problem but using a conventional seal with a 1GM10. I think it might be a lack of concentricity in the two flanges connecting the gearbox to the prop shaft, or a lack of concentricity in the shaft in the coupling. Although I haven't tried this yet, it might be possible to cure it by more even tightening of the allen screws connecting the coupling to the shaft. My shaft seems to be a rather slack fit in the coupling and uneven tightening of the allen screws could cause the shaft to be not centred in the coupling.
Turn the shaft by hand and push one finger in the gap in an aft engine mounting. You may just be able to detect a slight movement in the mounting, showing that the shaft is causing the engine to wobble. If you think of this the other way round, then if the engine drived the shaft, then the engine will cause the shaft to wobble, or vibrate.
Hope this is of help. I will follow this thread with interest.
Cheers, John.

 

[oilyrag]

Thanks John

I don't beleive it's due to lack of concentricity with the coupling as the knocking occurs with the engine out of gear and the engine idling; this can be cured by increasing idle speed until the engine is running a bit more smoothly.
The shaft (new) is a good sliding fit in the coupling and it takes only about 1/4 turn on the allen screws to tighten to max with a normal allen key.
The problem then manifests itself at certain revs and can again then be eliminated by varying engine speed to a point where the engine is running smoothly.
Incidentally I have installed new engine mounts (genuine Yanmar) and the problem is as bad as with the original mountings.
I could live with the knocking but what concerns me is the possibility of the shaft failing due to fatigue.
Awaiting further input with interest.....
Mike

 

[JimMcMillan]

I had a similar problem on a conversion to the the same engine using a flexible bearing to the prop shaft and I eventually discovered that althougth the shaft slipped into engine coupling smoothly and appearred perfectly aligned(it had been fitted professionally)the originall shaft which was used was very slightly bent where it exited the coupling.new shaft cured problem.Just a thought..

 

[jeanette]

I too am having problems with the alignment of the shaft but I think my problems are to do with the thrust bearing.

Apart from the noise and vibration I find that the movement causes the shaft seal to fail when the engine is in gear. After all if the shaft is not in line, then the seal will struggle to keep the water out.

I am about to get the problem fixed but I was curious to know if you experienced similar problems?


Jeanette

 

[pvb]

Hold on a minute!

You just said that the "knocking occurs with the engine out of gear". If that's the case, it can't possibly be the prop shaft vibrating, can it?

 

[Plum]

Assuming your shaft is central in the tibe at the inboard end, then the only conclusion to reach is that the clearance between the shaft and the inside of the tube was not specified to cater for a shaft without an inboard shaft support. The flexible coupling between the gearbox and the shaft must be of the correct type. If you have an inboard support bearing then the flexible coupling must allow for some axial (plus angular) movement and if you have no inboard shaft support the flexible coupling must be of the type that does not allow any axial movement (just angular). Assuming you have the right coupling, then I recommend you go back to the old inboard bearing and stuffing box. If you intend to add a bearing between the seal and the gearbox coupling, which is not normal, you need to consider whether you want this bearing to take the prop thrust loads or you want it to be a sliding bearing so that the prop thrust gets transmited to the gearbox as now.

 

[Twister_Ken]

Harmonic Vibration

If this happens in certain narrow rev bands then presumably there is some harmonic at work which excites the vibration. In which case an intermediate bearing which altered the harmonic length of the shaft might offer a cure. But you'd only expect a harmonic if something was out of true. So if you're sure the shaft and the gearbox/shaft joint are true and balanced, maybe the prop is out of balance? Has there been a bit of prop damage?

 

[oilyrag]

Hmmm... possibly, it's a new shaft and same problem with old shaft but I suppose one shouldn't assume that the new shaft isn't slightly bent.

 

[oilyrag]

Re: Hold on a minute!

yes, you can see the shaft vibrating as the engine (single cylinder diesel) vibrates around on it's flexible feet and the knocking, or perhaps more accurately tapping, can be stopped by placing a hand on the shaft.

 

[oilyrag]

Thanks Jeanette

think we have different problems but, if it's of any help to you, the PSS seal I have fitted hasn't leaked a drop of water since it was installed despite the shaft vibrations. The design of this seal means that it is self-aligning and could easily cope with far more distortion than would occur in any installation.
Unfortunately for me, I believe that the total lack of support that this type of seal gives to the shaft is exacerbating my problem.

 

[oilyrag]

Re: Harmonic Vibration

I'm pretty sure it's simply a case of engine vibration causing the shaft to flex

 

[oilyrag]

Yes, I'm almost certain that shaft/tube clearance is the problem and a larger diameter stern tube has been suggested as a cure. I was hoping for a less drastic solution but haven't yet ruled this one out.
An Aquadrive unit has also been suggested but apart from the cost, I do not see how one could be installed properly whilst hanging upside down through a cockpit locker and I'm not at all certain that it would cure the problem.
I'm interested in your comments about the flex coupling, it currently has the 'standard' yanmar flex coupling which is not designed to accept any shaft mis-alignment and therefore I assume does not allow for any axial movement.
I am considering a Vetus 'Bullflex' coupling which, Vetus claim, absorbs or eliminates most of the vibrations transmitted from the engine to the stern gear while maintaining perfect shaft alignment. I'm a bit of an old cynic and, to me almost begs the question 'how does it know?'!
Still, if anyone has any experience of this type of coupling it would be nice to hear some comments.
I know that failing all this the obvious solution is to go back to the old stuffing box but I love my new PSS!
Ho Hum....

 

[RJD]

Re: Hold on a minute!

The Volvo drip free inboard stern gland may provide the answer - it has a water lubricated cutlass type bearing within the gland prior to two lip seals that prevent ingress of water.
The bearing aligns the prop shaft centrally within the tube and may well dampen the shaft movement. It is a fairly rigid arrangement but allows the shaft to move a little. Minimum shaft diam is 25mm (I suspect there are no imperial arrangements)
Food for thought...
Another consideration is...is the engine sat squarely on the flex mounts. Easily checked by lifting the front of the engine and measureing gaps at front mounts with a feeler gauge. If the engine is out on the mounts it adds the harmonics..
TonyP

 

[oilyrag]

Re: Hold on a minute!

Hi Tony

This one had crossed my mind but is currently on my 'reserve list' as my shaft is 1". I'm told this doesn't matter at the 25mm Volvo seal can accomodate the oversize but I'm also concerned that the rigid nature of this seal might not stand up to the amount of vibration I seem to have - better to have tapping than a boat full of water! maybe I'm being a bit too pessimistic though.....

I haven't tried measuring the gap at the front mounts and will certainly give this a whirl.

Thanks

Mike

 

[Plum]

The Vetus Bullflex coupling has a removable centering ring so it can be used in either application. However, as you say that the shaft hits the side of the tube even with the engine out of gear and at idle (due, I assume simply due to the engine rocking on its mounts) then it is unlikely that a change to the flexible coupling will make any difference. So, unless you want to rip the whole lot out and fit a larger stern tube (how much larger would be required to guarantee you never get the problem again???) you need to fit a centering bearing (eg. the old one!) and then fit a flex coupling that allows axial movement (eg. Vetus Bullflex with centering ring removed)

 

[oilyrag]

Hi Plum

I'm sure that you are right - reluctance to make a retrograde step with regard to the type of stern gland seems to be an insurmountable problem and compromise must be the order of the day.

Thanks for your input
Mike

 

[jfkal]

Am using the R&D marine coupling which solved exactly the same problem. Howeverf be very precise with the engine alignment. It worked for me by:

Finding the center of the sterntube with the shaft. Hold it in place by a small wooden bracket. Then a align the engine coupling face to the shaft face.
My yard had it "perfectly" aligned before with the shaft sitting at the bottom of the stern tube and the engine fully bolted down. Guess what, they are still waiting for me to pay them......

 

[charles_reed]

The problem is that your Dolphin had relatively stiff engine mounts which made sure the shaft stayed in approximately the same axis, as well as being a very smooth twin cylinder.
You've now changed to a single-cylinder engine on extremely flexible mounts and the shaft is wandering around as thought it were a planetary egg-beater.

There are two possible solutions, which one you choose depends on the room between stern-tube start and gearbox end.
If you have plenty of room, fit a c/v bearing such as the Halyard - you can then forget about shaft/engine alignment.
I had insufficient room and was able to mitigate (but not cure) the problem by extending the length of the cutless bearing from 75 to 120mm.
I'd be very wary of replacing the old stuffing box (unless you have a very oversized electric bilge pump).

 

[Robin2]

I have a similar set up in my boat - 1GM10 and PSS seal at the inboard end of the propellor tube and a cutless bearing at the outboard end. I also had (?) the same problem of the propellor shaft knocking against the tube.

I have fitted a bearing on the shaft between the PSS and the engine. This bearing (ball race in a casting that looks like a mantle clock) is mounted flexibly to a bulkhead so that it does not take any thrust loads but it seems to provide sufficient damping of the movement of the prop shaft to stop the knocking.

So far I have only had it working for a few hours of engine use and I can't say it has definitely solved the problem for the long term.

If you are interested in more details send me an email and I will try to send you a sketch.