Aligning engine and prop-shaft/cutlass-bearing ???

Robert Wilson said:
Would you expand on that please? Centralise: as in side to side, up and down or both ie 360°?

I shall be renewing the cutlass bearing as well and wonder if there is any tolerance there, or does it also have to be "dead-in-line" with the coupling and so on?
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There have been several threads on this in the past. Perhaps the easiest way to do it is to make up three or four small wooden wedges. Push them between the stern tube and the shaft, ensuring that the gap is equal all around. Then move the engine on its mountings until the flanges are absolutely parallel in every sense, up and down and side to side. This assumes no top bearing on the stern tube, with a 'floating' gland attached with rubber hose.

I have not studied the whole of this thread but warn against making everything flexible. There are three possibilities, engine mountings, coupling and cutless bearing, all of which can be flexible. In most modern installations the shaft does not run in a bearing at the top of the stern tube and in this case there is no requirement for a flexible coupling, as engine movement is accommodated by the cutless bearing. If there is a bearing at the top of the tube the shaft will be fixed, in which case a flexible coupling is needed to allow the engine to move without destroying the bearing. If all three are flexible the shaft is effectively floating and can rattle against the stern tube.

There is quite a bit of info about this on my website. In my case, the bottom photos of a Colvic Northerner on the Sterngear page, the shaft is fixed using non-flexible bearings top and bottom and there is a flexible coupling between gearbox and shaft.
 
AndrewB said:
My old steel boat definitely 'hogged' enough when ashore to upset the alignment. Don't know about GRP, but have heard owners complaining of cabin doors that won't shut properly ashore, is this not due to flexing?
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That may well be the case where bulkheads flex. However the engine sits on substantial beds, usually on the thickest part of the hull, either above or just aft of the keel. This is particularly the case with the OP's boat which is the more traditional V shaped hull at that point. Would be really worried if there was any movement in that area.

Even modern boats where there may be flex in other parts of the boat, the engine beds are substantial and usually part of the grid structure. As explained above the shaft (if it is shaft drive) is free to move in the stern tube, so provided the alignment is correct it will not change relative to the hull when in the water. If it is a saildrive, all this is irrelevant - one of the advantages as no alignment is required.
 
Robert Wilson said:
Would you expand on that please? Centralise: as in side to side, up and down or both ie 360°?
..
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A good tip is to make a dummy bearing to fit the shaft and be a snug fit inside the stern tube. You may be able to cut up an old cutless bearing for this or if you know someone with a lathe, it is easy.

Better still if you have a 1in shaft you can use a roll of electrical tape (the i/d of the roll is often 1"", check it out). Unroll the tape until it just fits inside the tube and bobs your uncle.

To roughly get things right, a short length of shaft (maybe 9in) held it the coupling allows you to put a steel rule across the two for checking. If you have good light behind and a clear eye this is pretty accurate.
 
OK, now time to think about what is required:
You need the gearbox output flange centre to be on the stern tube centre line (to minimise the risk of shaft hitting stern tube walls and reduce wear on the cutless bearing)
You need the gearbox output flange to be perpendicular to the stern tube centre line (to minimise wear on the coupling and bending load on the prop shaft)
Flexible couplings are not a substitute for universal joints!

Now for first wild guess, centre the prop shaft in the tube and wrangle the engine into place, the coupling should slide easily onto the bolts. Hereafter the prop shaft should only be supported by the cutless bearing and gearbox flange, since, when you cease to support the prop shaft by other means, it will sit on the engine and move it. But do watch out when adjusting the mounts that you dont go too far and press the shaft against the tube (shouldnt happen since all adjustments will be to centre it)

Now get some measurements. You need cutless bearing to front of stern tube, cutless bearing to rear engine mount, cutless bearing to font engine mount, gearbox flange to rear mount, gearbox flange to font mount (all as seen in side view) and thread pitch on the mounts and finally diameter of the gearbox flange.
So, at this point the gearbox flange and coupling should be on the same centres, since they have machined registers to align them. However it is unlikely that they are parallel and that the shaft is centred.
I will only describe vertical alignment (since its easiest to implement, number of turns on a nut as opposed to a hair or a gnats with a pinch bar)
To fix an off centre in stern tube: measure top, bottom, port and starboard clearance at the front of stern tube., calculate offset by subtracting the gap at the bottom from the gap at the top (+ and - is important here).
you will need to adjust the rear engine mounts up by:
offset x cutless bearing to rear mount / cutless bearing to front of stern tube.
you will need to adjust the front engine mounts by:
offset x cutless bearing to front mount / cutless bearing to front of stern tube.
you can decide the number of turns by your measurement of thread pitch.
To fix the coupling not being parallel: use feeler gauges measure gap between gearbox flange and coupling at the top, bottom, port and starboard, calculate offset by subtracting gap at the top minus gap at the bottom, again + and - are important
adjust the rear mounts up by:
offset x flange to rear mount / flange diameter
adjust front mounts up by:
offset x flange to font mount / flange diameter

The idea is that you pick centres for the engine to move round such that one set of adjustments does not affect the other measurement. if you measure and adjust accurately, combining the 2 stages, it is (theoretically) possible to go at each mount only once. No machinist would ever do that though, refining a desired measurement slowly is much more their thing.
 
Lots of advice, but until we know whether the OP's stuffing box, or patent seal, is rigidly fixed to the sterntube, or is hanging on a bit of rubber hose, it is impossible to be specific.
 
Remember that the rule that normally only 2out of 3 elements should be flexible from engine mounts, coupling, and stern gland although often R&D couplings and considered rigid.
If using a flexible coupling the engine alignment should carried out before it is fitted if possible.
 
NormanS said:
Lots of advice, but until we know whether the OP's stuffing box, or patent seal, is rigidly fixed to the sterntube, or is hanging on a bit of rubber hose, it is impossible to be specific.
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My stern tube is literally just that, a tube glassed into the hull. Without crawling in and measuring I would say that the tube is about and inch and a half internal diameter and the shaft is one inch diameter.
The stern gland is a "rubber" bellows type tube jubilee-clipped onto the stern tube, with a carbon bush which interfaces with a stainless steel collar clamped onto the shaft.
I can't off-hand remember the name or type; PSS or similar.

My current coupling is rigid, not flexible.
As previously stated I shall be renewing shaft, cutlass bearing and prop (to a three blade)

I hope that helps.
 
Right, well we now know that the shaft can move around, and should indeed be temporarily supported inboard, so that it is in the centre of the stern tube, before offering up the gearbox coupling to the shaft coupling. Plenty subsequent advice given already.
 
You might find (as on my boat) that there isn't enough room to slide the stuffing box forward so that you can insert wedges. I found that the rubber boot did most of the work anyway. I used a piece of string to take the weight of the shaft. You then adjust the engine mounts so that the two flanges can come together. Final adjustments using feeler gauges at four positions between the flange faces should continue until you have differences of less than one thou per inch of flange diameter. I found it helpful to have a bolt in finger tight. There wasn't any change in the alignment after launching my old GRP boat.
 
Robert Wilson said:
My stern tube is literally just that, a tube glassed into the hull. Without crawling in and measuring I would say that the tube is about and inch and a half internal diameter and the shaft is one inch diameter.
The stern gland is a "rubber" bellows type tube jubilee-clipped onto the stern tube, with a carbon bush which interfaces with a stainless steel collar clamped onto the shaft.
I can't off-hand remember the name or type; PSS or similar.

My current coupling is rigid, not flexible.
As previously stated I shall be renewing shaft, cutlass bearing and prop (to a three blade)

I hope that helps.
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With a new shaft and a new cutless bearing situated in the stern tube I think you will find that the shaft will be pretty much centered in the tube. You would just have to lift the inner end a little to compensate for the weight of unsupported shaft + half coupling.
 
How did it go Robert?
I did not replace the mounts one at a time. had to lift the engine to remove old mounts and insert new. Stud too long and set in rubber. Marked and measured old mounts positions and repositioned the new ones by it.
Used different mounts for rear / front.
 
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