Engine Alinment

[purplerobbie]

All four mounts on my engine where broke and i have just replaced them. i have been alining the engine and it is more or less done.
When i run the engine out of gear there isnt much vibration but when i put it in gear i get a slight wobble?
I have tried to stop this but i cant.
moving the nuts on the engine mounts as little as a quarter of a turn move it out slightly which looks like it would wobble more.
The engine is on flexible mounts but the prop shaft is mounted straight to the gearbox.
A few people have said there should be some sort of flexible mounting here?
The other thought is the prop may have a slight bend in it?

Any thoughts?
Rob

 

[FAITIRA]

The engine will vibrate/move more, in gear Rob, the shaft should ideally, with flex engine mounts, have a flexible coupling in it to compensate for the engine moving on it,s flex mounts, relative to the prop shaft in it,s "fixed" stern bearing.

 

[purplerobbie]

I will get a flexible mount for between the gearbox and prop and see what that does.
I might have another look at the Alignment just to make sure.
I'm probably just being to picky again

Rob

 

[tazzle]

Agree with above - you will need a flexible coupling if you've installed flexible mounts.

 

[TigaWave]

If you are fitting a flexible coupling, there are two types. Most are there to allow for the movement of the engine on its mounts and not to allow for shaft mis alignment. Vibration could be bent shaft, prop damage (even very slight will cause vibration) or bearing wear either stern tube or cutless.

Talk to Lancing marine if you have any questions regards the mounts or couplings. They are a customer of ours but we don't get involved in couplings ourselves.

 

[wotayottie]

You dont have to have a flexible coupling with flex engine mounts - few boats are supplied as standard with a flex coupling and all have flexible mounts these days.

You will always get a different level of movement with the shaft connected - resonances will be at different frequencies etc. Someone once told me that the way to check that you were in alignment was to sail the boat with the engine in gear but not turned on. As the shaft rotates, watch the movement of the engine. The less movement, the better the alignment - this way of checking eliminates the rocking due to firing pulses.

Make sure the engine is sitting equally on all 4 mounts. You can have it perfectly aligned with all the weight on 3 mounts and it will vibrate like a good un. So with shaft disconnected and top mount nuts loose, put a crowbar under the engine and ease it up wards checking that it leaves both mounts at the same time. Do the same on the other end of the engine.

 

[Bilgediver]

The other thought is the prop may have a slight bend in it?
.......................................................................................

This is quite possible if the boat has snagged a pot line or something.

Try undoing the shaft coupling from the engine and checking that it mates to the gearbox flange face to face . then torn the shaft 90 degrees ..180 degrees.......270 degrees without turning the engine and see if the gearbox flange still lines up in each position.

I have seen cases where it didn t and either because the shaft was bent or in one case because the hole bored in the coupling for the shaft was not at 90 degrees to the flange face!!!!!!!!!

Running in either of these conditions would create fatigue problems in the coupling bolts.

 

[Billjratt]

Are you in or out of the water? Boat shape may change. The flange/feeler guages method above is the sure way to mate engine to shaft.

 

[Tranona]

You need to check if the inboard end of the shaft is rigid, that is with a bearing in the stern tube. If you have a flexibly mounted stuffing box then there probably is not and the shaft is only supported by the outboard cutlass bearing. This is quite normal with flexible engine mounts as the whole shaft can move with the engine. The extra wobble you get when you go into gear could be as suggested the half couplings are not fully aligned. Follow bilgedriver's suggestion and check the gap with feeler gauges. Should be no more than a couple of thou difference. The shaft could be slightly bent, but you can only really check by taking it out.

If you do go down the flexible route, consider the Bullflex type from Vetus as these allow up to 2 degrees misalignment. Remember with any flexible coupling the shaft will move back so make sure you have clearance behind the prop.

 

[Pye_End]

Even a flexible coupling should have a well aligned engine.

Many boats do not have flexible couplings, and am not convinced by them unless the shaft is well supported.

Prop shaft well worth checking to see if straight. Is the cutless reasonably tight? Is the prop balance ok?

Does it wobble in gear but in low revs - ie with very little load on it? If it does then probably not the prop, but guess either the shaft or the cutless - assuming the alignment is good enough. Can't see why the alignment would change until a reasonable load is put on it.

 

[LittleShip]

Another vote for the feeler gauge between the flange method. This should be no more that 2-3 thousands of an inch. Checking the top gap and then the bottom will tell you if your lined up, if it is and you turn it 180 degrees and it is out of line again the shaft is bent. You check side to side movement the same way.

I have fitted a R&D coupling between the shaft and gearbox and it seems to improve the vibration problem.

Good luck with this one
Tom

 

[chrishelen]

Agree,final adjustments must be when the boat is in her natural element,supported by water,if she is ashore and not chocked corectly then you will get a false feeler gauge reading at the flange faces,even a few thou will make a differance,hope this makes sense but it comes from bitter expieriance. /forums/images/graemlins/confused.gif

 

[purplerobbie]

When it is ticking over and i put it in gear the back of the engine has a slight wobble on it.
When i increase the revs it goes.
The stern gland is the stuffing type with a pipe off it. There is hardly any movement in the shaft when i tried to push it around
It could be the prop i suppose or a bent prop shaft.
I just thought it should run the same whether in gear or out?
thinking about it will wobble more in gear as is it spinning a prop and shaft?

She is in the water at the moment

If i look down on it it looks right. if i pull them together there isnt a gap they mate up ok there is no catching.

I would like to fit a centaflex coupling but i'm really not sure on the space and i would probably need to shorten the shaft.

Rob

 

[Pye_End]

Would have thought either shaft or engine alignment. Presume the flexible mounts are the right ones?

You say the alignment is 'more or less' right. Presume you have feeler guaged and know how close you are? I would start here, and if you know that you are really close with it and it still wobbles then next time she is out you probably have to have the shaft checked. Can't see it being the prop if it is wobbling at low speeds. I wouldn't necessarily put a flexible coupling in purely to solve this problem, although it is worth considering on its own merits.

 

[ianabc]

All of the above seem to be excellent idea
BUT
we used a dial attachted to a sharp probe, the probe resting on the shaft.
Turning the shaft slowly allows the (much magnified) deviation to show on the 2 inch dial.

Such an instrument is retoinly used to check shaft allignment.

A proper dial gauge cost approx. 200 pounds, an enginer was with me examoining the knock offs from China and could not tell the differece between the Chinese copuy and the expensive St##### original.

Gave us 2/1000 of an inch accuracy and no vibration.

Next time we'll remember to tighten the lock nuts directly after allignment!

 

[doug748]

Any details about just how the DTI is used? Now they are available for about £15 it could be a worthwhile investment.