Prop shaft alignment

[cyrilslugg]

I have replaced the prop and shaft and discovered that at least one of the engine (Yanmar 1GM10) mountings has come loose, which I realise now is probably the reason why the shaft was so badly worn in the first place. My local grease monkey tells me that I now need to spend a 'couple of hours' aligning the engine and shaft, but is reluctant to tell me what (if anything, other than common sense) is involved. I'd like to do the job myself. Anyone know of a document that says how to do it?
Failing that, all suggestions on how to do it would be appreciated.

 

[eyupdougdown]

Get yourself some feeler / slip guages for a start.

 

[alahol2]

When I did mine (having fitted a new engine) the first step was to ensure that the shaft and coupling would slide forward and engage exactly with the coupling on the engine, the bolts sliding in without any binding. I have a flexible coupling, so the next step was to bolt up then measure, using feeler gauges, the deflection of the coupling at 90 degree spacings, without turning the shaft. If I remember correctly the maximum difference was about 10 thou, preferably 5. It took an awful lot of jiggling to get it right. If 3 of your feet are still tight, you may find you can get it close enough by only adjusting the foot that is loose. Wisdom has it that this should be done (or at least re-checked) when the boat is afloat. Good luck.

 

[cyrilslugg]

Thanks guys. The boat is afloat and I'll try your suggestions, especially as the remaining 3 mountings look on the face of it to be secure.

 

[theia]

you need to get the prop central in the shaft tube first. it's not easy to explain but - wrap a piece of of flexible pastic 'tightish' around the shaft so that it does not overlap. then wrap insulating tape around that to build up a 'sleeve', (this is inside the boat), the flexible plastic allows you to slide this 'sleeve' up and down the prop shaft. keep going until this 'sleeve' is a snug fit in the shaft tube. the prop shaft should now be central. you can now begin to align your engine with the prop shaft coupling using feeler gauges. align the engine sideways first then you can secure the mounts before aligning it vertically using the adjustment nuts. it sounds right when you've done it before but i don't know whether you can understand what i mean!

 

[cyrilslugg]

I think I get it! - and it sounds a pretty logical idea. Thanks. I'll try it out next Saturday.

 

[jerryat]

Hi theia,

Yes, that's a suitable system if you're ashore, but not possible when afloat surely? I assume Cyril's boat has a stern gland which would prevent 'centering' the shaft as I assume you are describing.

It is difficult to explain, but alahol2 is just about spot on IMHO. It is though, for obvious reasons, more difficult with a flexible coupling, but aiming for circa 5 thous. is as good as most can get. With a metal to metal connection, it is simpler is one respect, but a real b**ger in another, in that the gap is more clearly defined. /forums/images/graemlins/frown.gif

Quite correct to rotate the shaft (holding the two sections firmly together) and insert the feeler gauge at each point. The hard, boring thing is the adjustment of the engine mounts to close the gap. It is seldom as simple as just changing one, as that can muck up the next point etc. Hence I suspect, your original friend's comment re a couple of hours! /forums/images/graemlins/crazy.gif

If you've got the common threaded vertical adjustment on the mounts, you are very lucky. My Bukh mounts require a selection of shims to be slipped round each engine bolt to achieve adjustment. That means lifting the engine each time to add or remove, so you can imagine what that does to one's previous 'perfect' measurement!!! /forums/images/graemlins/mad.gif

Just take it slowly and methodically and get it as close as you possibly can. It is tempting to think 'that's near enough' when your in a cramped postion at the rear of the motor, but you'll give your gearbox rear bearing and shaft a much longer life if you do. /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

Very good luck,

Cheers Jerry

 

[nedmin]

As a very rough guide(this will only tell you if youve got it wrong!).Feel the friction needed to turn the prop.shaft,then feel the friction needed to turn the g/box coupling.when you think you have got them in line(not a five minute job)bolt up and you should be able to turn as before.If its very hard to turn start again!!! One of the problems is letting the engine "sit" on the mounts and not the threaded section of the mount try ing to pull the rubber apart.

 

[Birdseye]

Its better doing the alignment afloat since the shape of the hull is different ashore / in water. Mind you, chose a calm day since you will be loosening off your engine mounts

I dont know your boat so you need to make sure what I say is safe and wont do any damage before trying it. Make sure your shaft cant slip out of the sterngland when the coupling is released..

What I would do on my boat is:

1/ split the coupling at the flange. Undo the engine bearer bolts on the beds and slide the engine slightly forward ie toward the centre of the boat. Support the shaft coupling to prevent droop is there is any sign of it doing so.

2/ First alignment check is - when you slide the engine back toward the coupling, does the centre of the gearbox flange align with the centre of the shaft flange? Adjust the height of the mopunts until it does.

3/ Loosely re bolt the engine mounts to the bearers. . Fit the bolts loosely into the coupling flanges but do not tighten. Do you have any gap between the flanges of the gearbox and the shaft? Measure with a feeler guage at 90 deg intervals. Adjust the engine mounts until the gap is the same all round. Your target is no more than 1 thou difference. Once you have achieved this, bolt up. The re-check. The undo and realign since bolting up will move the alignment a bit.

Note: If you have a flexible coupling, you can do the third item with the coupling tightened up and it will be far easier. All you do is to get a slip guage to fit the gap between two opposing bolt heads in the coupling, and then rotate the shaft through 90 deg increments. The idea is to adjust the mounts until the slip guage fits between the same boltheads in exactly the same way as you rotate the shaft through the 90 deg stations.

Its quite easy to do, but time consuming. Just remember the relative distance between the engine mounts and the coupling. So, for example, if the gap at the top of the coupling is less than at the bottom, and if the distance from the rear mount to the coupling is half that of the front mout to the coupling, you need to lower the front mount by twice as much as the rear mount. If you dont do that, then you will mess up the alignment you did in stage 2.

 

[cyrilslugg]

The boat is a MG Spring without a flexible coupling - so sounds like the the harder of the scenarios. I can't answer some of your questions because I haven't been to the boat for a week or so (me London, boat Southsea) and haven't thought to notice the coupling details before. As you can probably guess, I originally thought the job would be fairly straightforward - but nothing ventured etc.


Many thanks for all this valuable info. I think I'll take sandwiches and a sleeping bag as well as the feelers and spanner, and if it all fails I suppose I can always bolt on an outboard!


Cheers guys