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

Robert Wilson

Robert Wilson

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How critical are the angles/positioning of the engine?
I ask because when my Kubota goes back into Khamsin after "work" I shall be putting-in a new shaft, cutlass and prop.
Most likely I shall have to refurbish or replace the engine mounts which very likely could alter the angle/approach of the engine and coupling.

Is it "suck it and see", or is there a methodical way to make sure all is correctly aligned?

Many thanks for advice, as usual,

Robert
 
Robert Wilson said:
Is it "suck it and see", or is there a methodical way to make sure all is correctly aligned?
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It's not "suck it and see"! Firstly , you need to position the propshaft centrally in the stern tube, then offer the engine up to the coupling. Fiddle around with the engine mounts until it's a very close match. Then use a feeler gauge to make sure that the gap between engine and coupling is uniform. If you're fitting new mounts, you may wish to repeat the process a couple of months after commissioning.
 
Bearing in mind that engines bounce all over the place on their mountings, and that in particular they will rotate slightly in them when applying torque to the propeller shaft, just how critical does alignment have to be? How much can the flexible coupling deal with?
 
JumbleDuck said:
Bearing in mind that engines bounce all over the place on their mountings, and that in particular they will rotate slightly in them when applying torque to the propeller shaft, just how critical does alignment have to be? How much can the flexible coupling deal with?
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Most boats don't have a flexible coupling.
 
pvb said:
There's some flexibility in the stern gland.
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Also when afloat, the alignment should be checked again - but I suspect Roberts boat is one of those solid immovable ones which wont flex.

I think the longevity of a stern gland is related to how good the alignment is and how much an engine moves on the bearings.
My 2003T hardly moves at all with the engine running - and the last Volvo stern gland lasted 20 years and was only changed because I thought it a bit old!

So Robert, this is something well worth doing properly.
 
For a slow-revving sailboat engine, I'm usually happy enough once I get the engine into position so that the bolts between gear-box and shaft slide into place without effort. That's assuming the bolts are a close fit without visible signs of wear, as they should be. (Perfectionists will lift the shaft end very slightly to allow for the tiny bend of the unsupported shaft).

However, be sure to do this when the boat is on the water and not ashore, when the hull will have flexed from its normal position. If you need to run the engine immediately on relaunch, use it as gently as possible and then undo the bolts: leave it 24 hours and re-align.

P.S. You may find you need to push the shaft down a centimeter or two during this process. Be aware that this may well disturb the packing of a traditional stern gland. I usually repack the top two rings afterwards; it is quite practicable to do this while afloat. (Also, if you have a Spurs type rope-cutter fitted, the locking 'V' will separate and until it is repositioned the cutter will be both noisy and useless).
 
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A flexible coupling allows the engine to bounce around more freely, but must still be properly aligned. Any mislagnment will cause side pressure on bearings and glands, which accelerates wear, and absorbs power. The gearbox output shaft bearing for example will take the full weight of a misalgined engine, and it takes surprisingly little misalignment to cause problems. Flexible bearings can actually be at lot more difficult to align properly because they are - er - flexible. Bear in mind too that the movement in the stern gland can allow the shaft to drop slightly out of alignment with the cutless bearing and shaft log under its own weight. When aligining a flexible coupling you need to be sure the shaft slides neatly into the coupling without catching. This can be quite difficult to get it right. But even a few thou misalignment can cause accelerated wearto the gearbox bearings, the gland, the cutless and the shaft itself. Its straightforward enough, but requires a lot of perseverance, time and patience. It cant be hurried!
 
There's flexible couplings and flexible couplings. Some can happily accommodate substantial misalignment. Others, and the R&D is one of them, tolerate very little: their main function is to inhibit engine vibration from reaching the hull. R&D make no secret of this.
 
Keep the responses coming gentlemen, and thank you for a lot of info so far.
I don't think Khamsin's hull flexes much, lay-up is an inch or more - but I shall take heed.
She doesn't (at present) have a flexible coupling. So, if I read the foregoing correctly, I should ensure the shaft slides easily into the coupling - then keep checking over a number of hours' use - adjusting the mounts if/when necessary?

Sounds easier said than done :eek: but logical
 
Robert Wilson said:
Keep the responses coming gentlemen, and thank you for a lot of info so far.
I don't think Khamsin's hull flexes much, lay-up is an inch or more - but I shall take heed.
She doesn't (at present) have a flexible coupling. So, if I read the foregoing correctly, I should ensure the shaft slides easily into the coupling - then keep checking over a number of hours' use - adjusting the mounts if/when necessary?

Sounds easier said than done :eek: but logical
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The idea that a hull changes shape when in the water is a hangover from wooden boat days when engines were usually bolted solidly to the hull and prop shafts were in tubes supported by bearings at both ends. Such boats did indeed change shape as the hull planking and ribs absorbed water so there was pressure on the fixed engine but not on the shaft. So realignment was often necessary.

This is rarely the case with GRP boats, particularly of the shape and layout of yours. So getting it right ashore is usually fine. However easy to check when afloat (if you have reasonable access to the coupling) by just undoing the bolts, sliding the shaft back enough to get feeler gauges in.

Flexible couplings are not essential as modern stern tubes (as yours will be) only have a bearing at the aft end, so the whole engine/shaft assembly can move. Hence the importance as pvb said earlier of centralising the shaft in the tube before you do the alignment. Otherwise the shaft could run off centre and hit the side of the tube as the engine vibrates.

You may need to do a bit of juggling to get the shaft wedged central while you do the alignment, then remove the wedge and slide the stuffing box back. It can also be tricky to adjust the mounts as movement is in 3 dimension which can be a bit disorientating, particularly if you are hanging upside down in a confined space!. However working logically through the process usually leads to success.
 
Thank you Tranona, sets it out in clear detail. The thought of three-dimensional movement/alignment is rather daunting but I shall have assistance from the local guy. As to the hanging upside-down, just getting in will be a challenge, then working with tools at odd angles!

Good prompt re chocking the shaft before dismantling!
 
Robert Wilson said:
Thank you Tranona, sets it out in clear detail. The thought of three-dimensional movement/alignment is rather daunting but I shall have assistance from the local guy. As to the hanging upside-down, just getting in will be a challenge, then working with tools at odd angles!

Good prompt re chocking the shaft before dismantling!
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bear in mind if changing the mounts the new ones will be "taller" as they havnt had an engine sitting on them for 10 yrs.
always centralize the shaft before aligning :encouragement:
when i changed my mounts i was fortunate in that the threaded rod in the mount could be unscrewed, i used timber folding wedges to support the engine unscrew the threaded rod, slide out the mount & refit the new, one at a time. Then realign the engine.
 
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Tranona said:
The idea that a hull changes shape when in the water is a hangover from wooden boat days when engines were usually bolted solidly to the hull and prop shafts were in tubes supported by bearings at both ends. Such boats did indeed change shape as the hull planking and ribs absorbed water so there was pressure on the fixed engine but not on the shaft. So realignment was often necessary.

This is rarely the case with GRP boats, ...
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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?
 
It is always difficult when physical access is constrained and it can be laborious and tedious when hanging upside down working out which mount and which way to adjust.

I recommend you take a small white board with you to the boat that shows a small drawing that identifies each mount and a circle representing the coupling. Use a bit of masking tape or even better white paint as a reference mark on the coupling edges. Record the gap at 4 points around the couplings.

Extricate yourself back to the saloon and enter the readings on the drawing and visualise how to adjust the mounts to improve alignment. Record each planned adjustment and then execute. Deep breath dive in and take a new set of readings, record and extricate back to the saloon.

It is an iterative process and reference to the drawing, planning and visualising the situation helps you to learn 'cause and effect'. You should be aiming for good alignment certainly within 0.001/0.002 inch and better if you can. Flexible couplings are there not to accommodate static misalignment but to absorb the in service cyclical misalignment experienced in operation and rough weather.

For a small engine, first time DIY I would allocate a Saturday morning.
 
sailorman said:
bear in mind if changing the mounts the new ones will be "taller" as they havnt had an engine sitting on them for 10 yrs.
always centralize the shaft before aligning :encouragement:
when i changed my mounts i was fortunate in that the threaded rod in the mount could be unscrewed, i used timber folding wedges to support the engine unscrew the threaded rod, slide out the mount & refit the new, one at a time. Then realign the engine.
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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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