Is this excessive shaft movement?

[sailingjupiter]

I have just replaced the old stuffing box on the boats shaft with a PSS seal. All fitted OK. The shaft is 3/4 inch into a 1.5 inch stern tube so there is plenty of space around the the tube.

When I test fired the engine whilst out of the water, the shaft was banging around into the stern tube with the engine at tickover (1GM10). Was making quite a noise. With more revs this stopped as engine vibration reduced.

The old stuffing box used to surpress this as it held the shaft tight to the stern tube. The new PSS seal doesn't hold this part of the shaft tight at all as it relies on the rubber bellow pressurre against the stainless rotor.

At the prop end the cutlass bearing appears OK. No excessive play. Am I looking for a problem that isn't there? How much movement is expected at low revs? I'm a little concerened as the PSS seal relies on pressure between the steel rotor and the carbon face on the belows to stop water getting through. If this is vibrating excessively is there a chance of water coming in?

Any replies greatly appreciated.

 

[Norman_E]

It sounds as if the engine mounts have gone soft with age, allowing it to dance around on tickover. Probably worst when the engine is cold. If this is the case the only real cure is to replace the rubber mounts. I am assuming that the tickover speed is correct. A temporary palliative might be to speed up the tickover a bit.

 

[Eygthene]

In the 1GM10 Operating manual (page 23 7-3 (3)) "Avoid resonance range operation. WARNING Depending on the driven machine, engine vibrations will be excessive in a certain speed range due to resonance of the engine with the engine bed. Avoid engine operation in this range."

From my experience, having fitted the softer mounting of the shaft seal like yours and a generous clearance in the stern tube, with an engine only 3 years old, this resonance does occur near to tick-over, but stops as the revs are raised towards normal operating speeds. The engine mounts are fine.

 

[earlybird]

Don't know about the 1GM10, but the newish 2GM20 on my previous boat vibrated a great deal on tickover, and considerably more if the shaft was detached at the gearbox flange, indicating that the shaft was restraining the engine movement. I would expect the single cylinder engine to be at least as bad. I raised the tickover speed slightly over the recommended figure to reduce this. The stuffing box was of conventional type however.
I would not be happy if a PSS seal experienced a lot of movement, I suggest you try a faster tickover, bearing in mind that it might affect gear selection.

 

[PuffTheMagicDragon]

All engines have a particular speed at which they vibrate excessively; so, as has already been said, try to avoid this range. The rigidity of the mountings vis-a-vis the mass distribution of the engine has a great significance on this resonance

However, besides checking the state of the mountings, I would also check very carefully to see if one or more of them are twisted and also that they are all carrying their fair share.

Some resonance will still remain, whatever you do; you can only lessen it as much as possible.

 

[scottie]

With engine mounts - shaft coupling - stern gland there is a convention that you only have 2 out of 3 flexible other wise there is nothing stopping the the whole lot moving all over the shop

Was your old stuffing box flexible ?

One way round it is to use a R+D type coupling disc as they are stiff enough

 

[sailingjupiter]

Old stuffing box didn't appear flexible. I think that it stopped the shaft jumping as much as it is now. I don't really know what type the stuffing box was as there was no "packing" in it - just the grease I kept putting through the greaser. It was flush around the shaft. Had a lock nut and the short thick rubber tube connecting it to the shaft tube. Definatley non of the packing rope inside I was expecting. It didn't look like some should have but wasn't there. It looked designed this way.

I think there is some kind of flexible mount between the gearbox and the coupling as there is a plastic (or something looking like it) ring between the two.

Do I have 3 out of 3 flexible as mentioned above? Am I going to have trouble on launching???

 

[catmandoo]

By changing the fixing arrangement of the shaft , engine , couplings cutless bearings etc you have altered the natural frequency of the whole assembly .

The speed at which the shaft assembly resonates is called the critical speed . Most manufacturers try to design it out by having the natural frequency above operating speed - High tuned or well below operating speed - low tuned . Its sort of like a guitar you changed the position of the frets ie the particular part of the shaft causing the problem is now a little longer between supporting points by removing the stuffing box thus resonating at a lower note .

How far is the seal now from the cutless bearing ?

As the other posters say your tick over seems to be on this frequency , try adjusting the tick over speed upwards Is there any way of shortening the distance between support points ( bearings )? Also might be different when you are in the water when water fills the space around the shaft under your new mechanical seal and [prop thrust increases the length further . How much longitudinal movement have you got?

If the new seal is in the middle between the cutless bearing and the next engine side earing then the shaft may now have a greater deflection at that point ( sort of like a skipping rope ) and the rubber boot gets a lot of movement How much deflection can the seal take ? or even how much can the cutless bearing now take as it could now not be parallel when resonating - skipping rope effect . A word of warning - don't run it dry . It relies on water to lubricate it

Incidentally did you run the system when you were out of the water before you changed the seal ie how did you know that it was not already noisy out of the water?

 

[scottie]

The plastic ring is a R+D coupling and the short hose is the flex part of the stuffing box

have you checked the alignement since you refitted the coupling?

 

[sailingjupiter]

Thanks for the ongoing replies..

If it helps the boat is an MG Spring around 25ft. Yanmar 1GM10. I did look at another MG Spring but the shaft exits the bottom of the hull with a protruding stub that holds the shaft tight. On my boat there is basically just an elongated hole (1.5inch diameter) so its flush bottomed. P brackect is maybe 30-40 cm after that before the folding prop.

The whole shaft is probably not much more than 1 metre in length. The new PSS is less than 10cm from the shaft coupling so maybe changing is didn't make much difference to the movement the shaft originally had. Maybe I'm just listening extra carefully as I changed the stuffing box. It was always noisy and there was always a knocking sound either in or out of the water. I guess I never looked too closely into where the noise was coming from before. I can now say it is the shaft knocking into the shaft tube at tickover.

I did run it in gear breifley at the weekend. I first thought it was the shaft that was bent hence the knocking, but picking up revs the knocking stopped.

Maybe in water it won't be as loud or noticable. Maybe its as it always was. Maybe the PSS has had no effect and will perform to expectation as it can handle the movement.

Hopefully I'm just paranoid!

 

[vyv_cox]

A couple of points that might help:

It is not a good idea to run a PSS seal out of the water, the carbon face is machined very accurately to be flat and smooth. Running it dry could destroy it in a matter of seconds.

Most single cylinder engines are pretty rough on tickover. All the Yanmars seem to rely on very soft mountings to keep their noise emissions down, the combination of these two factors resulting in a lot of movement, and possibly clashing of shaft to tube at tickover.

One thing to check is that the shaft is aligned centrally within the tube when at rest. This can easily be checked by sliding the seal forward. If not, then the engine will need to be re-aligned with the shaft after it has been centralised.

Your old packed gland had a greater damping effect than the PSS has, possibly accounting for the observed change.

 

[Bodach na mara]

As Vyv says, check that the shaft is central in the tube. I always wonder why the instructions for lining up engines never mention this. Probably to do with the older installations having rigidly mounted stuffing boxes and not the type of floating box on a rubber hose that is often used now. I found that there was only about 1.5 mm clearance all round when the shaft was central, so if it is off-centre ( and even if it is centred) a bit of vibration of the engine on its mounts can have the shaft banging on the tube. That used to happen to me when motor-sailing on port tack!

I would have replaced the mounts but the local engine specialists looked at them and said that they were good for a while yet. Mind you that may have had something to do with the fact that they could not locate a supplier for replacements.

 

[cliffordpope]

It seems to me you have mixed up two different systems.
A traditional stuffing box surely serves two purposes - to keep out water, but also to provide a virtualy rigid bearing.
That allows you to have a flexibly mounted engine with a flexible connector between engine and shaft.
If you take out the stuffing box bearing you then have a system with 3 points of flexibility, which is bound to thrash around. (Apart from the noise it can't be very good for the cutlass bearing.)
I'd say you need either a solid connector, or a rigidly mounted engine.

 

[PeterGibbs]

QUOTE: When I test fired the engine whilst out of the water, the shaft was banging around into the stern tube with the engine at tickover (1GM10). Was making quite a noise. With more revs this stopped as engine vibration reduced.
__________________________________________

Good advice hereabouts on engine mounts - first port of call.

Slack in the stern tube comes up here frequently. How to describe an acceptable amount of lateral movement? One test is rumble noise when under way - quite a lot of boats would exhibit this, but it's not an acute problem. Leakage at the hull seal is the other obvious clue. If you are concerned when under way that the stern gear is unbalanced or the cutless failing, you may, within reason put a hand on the rotating shaft at lowish revs: providing you can do this safely you will learn quite a lot and get a feel for what's normal on your boat.

I worked on the basis that 1mm movement by hand in the cutless bearing was ok but 3mm was definitely not - on a 25mm shaft. Remember also that the cutless is water lubricated: if you turn the the shaft any length of time dry you will expect a grumble or two!

PWG

 

[doug748]

Blimey, your head must be spinning.
Firstly I am sure your Spring was designed and built with flexi engine mounts, coupling and stuffing box. Perhaps your experience or a chat to a fellow owner could confirm this? The amount of "give" in the flexible coupling you describe is quite small though, and (as you say) the flexible sleeve which attaches the stuffing box to the stern tube did give some element of support. Do the sensible checks suggested re engine mounts, but expect them to be ok. As you imply you may have had a rumble, at low revs, all the time but just never noticed it, afloat with all the panels in place.
At the end of the day you are going to have to try things at sea. If it helps I ran a PSS seal for 100 hours last year with a poorly aligned engine (don't ask), it coped very well indeed; these seals are not as feeble as at first they might appear.

 

[earlybird]

I am sure that the PSS seal, (I have one on my present boat ), will be fine if the shaft is spinning reasonably concentrically. This would be the case even if the engine is misaligned. However, what I would not like to see it subjected to for lengthy periods of time, is excessive radial movement of the shaft, caused by the engine shaking excessively on its mountings.

 

[Bodach na mara]

Re the stern gland, I have had boats with the gland screwed on to the end of the stern tube (i.e. "rigid") and with the engine on rubber mounts. I was always a bit wary of that arrangement, as I could not see how you could have engine movement coupled to a rigid shaft. I have also sailed on boats with the whole thing rigid and found that the engine vibration shook the whole boat. The standard arrangement on a Westerly is for the shaft supported only at the rear bearing, which was originally white metal, but many are now Cutless type. The engine is on rubber mounts of a type that is not to be found just now. They are rectangular blocks of rubber on a three-hole mount on the bearer and with a single stud for the engine mount. They are about 25mm thick (bearer to mount face), which is a lot less than anything available now. The gland floats on a short rubber tube to take up any engine movement. It has worked for over thirty years.

 

[cliffordpope]

I have only ever encountered the traditional arrangement: rigid stern gland, rigid or flexible engine mounts, and a car-type prop shaft with a pair of universal joints. No alignment problems, no whipping. The engine can be angled or offset as required independently of the shaft angle or bearing.

 

[Tranona]

Expect that the knocking noise is the shaft hitting the side of the stern tube and that the old stuffing box had sufficient support for the shaft and resilience of the hose to keep the shaft central. Your new seal does not provide any support. You already have a flexible coupling which takes out some of the (considerable) movement of the engine, so not much more you can do there. I would be tempted to go back to the old stuffing box. Unfortunately, because of your shaft size you can't use the Volvo seal which does give support.