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That Sinking Feeling (story In Pictures)


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18 hours ago, Vaughan said:

We haven't seen photos of the engine mounting, or the propellor end, where I assume the shaft is running in a cutless bearing, supported by a P bracket or a skeg.

I also assume that this boat has a short shaft, with no Plummer block to support it. 

This is a good example of why what we call the "inboard bearing" is not a bearing at all, but a water seal, known as a "stern gland". The shaft is not designed to "run" in the stern gland. Which means it is always vital that the engine is properly aligned to the shaft.

In a GRP boat, this alignment should always be done when the boat is floating, and not out in a boatshed.

By the way, that looks like a most excellent braized repair to a casting which may not be available any more!

I can't let this go without saying a big thanks to Vaughan for this information. It's extremely usefull to those of us with big old grp boats.

A hopefully constructive note on forum posts: in the future, this thread will be harder to find due to it's title; it is just the kind of thread people with engine mounting problems would be looking for. 

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OBSERVATION:-

The main engine quite literally pushes the boat along, with the thrust transferring up the prop shaft to the gearbox via our R&D Flexible Coupling and via those softening old engine mounts to the hull, so that's another reason why they should be in good order.

As recent retiree, trying to hold onto the boat and keep it running with all the associated costs on a limited budget, I do worry about the cost of engine removal etc.

 

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It was quite a shock to discover the shaft log damage, and the purpose of this thread was to alert other (non-professional) boat owners to both look out for this and be reassured that it's not the end of the world if it does.

====

NEW OWNERS

I think that the forum could do with a dedicated "new owners" section

There must be plenty of new owners out there who have previously owned boats with outboard engines (devoid of all the paraphernalia associated with inboards), or never owned a boat at all. Not forgetting that, of course, outboard engine powered boats have their own issues.

19 years ago, I was in the former category and I well remember that very steep initial learning curve that tapers off as years go by. Simple questions like "what's a calorifier?" and "how do you winterize?" are distant memories and basic knowledge now.

I am probably not alone in having had to repair of modify so much over the years that there's hardly an item on the boat that I don't now have intimate knowledge of (with the sole exceptions of the INSIDES of the main engine, anchor winch and Eberspacher, which now needs servicing by the way).

BUT, I never expected to be dealing with the main fabric of the boat.

The point is that there must be other owners out there that can contibute with similar threads. I would like to read them!

 

 

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1 hour ago, TeamElla said:

The main engine quite literally pushes the boat along, with the thrust transferring up the prop shaft to the gearbox via our R&D Flexible Coupling and via those softening old engine mounts to the hull,

What is actually happening is that the thrust of the propeller is coming up the shaft and being taken up by the output shaft (and its bearings) in the gearbox!  This is why ideally, a prop shaft should have a Plummer block in between the stern gland and the engine, to take up the forward thrust.  So many boats do not have these.  If you want a photo of one, I know that Griff on Broad Ambition has one fitted.

Luckily, your PRM gearbox has a thrust bearing on the rear of the output shaft, and can accept the forward thrust.

Your problem, if the engine is actually moving forwards and backwards when you change gear, is that the output shaft is splined on the end, where the flange coupling fits over it, and this movement will wear the splines away.  So one day, you will go astern, the shaft will move back and slip into the worn area on the splines.  All of a sudden, no astern gear!  And a gearbox strip down, to repair the problem.

So it is important to look at your engine mountings to try and reduce or prevent any longitudinal movement.  The flexible couplings are there to take up engine vibration.  They are not there to let the engine wander about on its beds!

Something to talk about with a boatyard, I suggest.

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51 minutes ago, Vaughan said:

They are not there to let the engine wander about on its beds!

Thanks Vaughan,

I don't think that we are anywhere near that stage yet on Ella, but your advice is appreciated (and will be acted upon!)

Rgds,

Steve

 

 

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This is why ideally, a prop shaft should have a Plummer block in between the stern gland and the engine, to take up the forward thrust.  So many boats do not have these.  If you want a photo of one, I know that Griff on Broad Ambition has one fitted.

Herewith prior to the engine and gearbox installation 

Griff

 

BA NBN 857.JPG

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23 hours ago, Vaughan said:

This is why ideally, a prop shaft should have a Plummer block in between the stern gland and the engine, to take up the forward thrust.  So many boats do not have these.

Thanks to Griff for the picture.  I do like the idea of the plummer box.

 

"So many boats do not have these"

I should say so

Ella is a Meakes Madeira 27CC (centre cockpit) which has an Appleyard Lincoln ELYSIAN HULL and a different Meakes of Marlow superstructure.

There must be a huge number of Elysians on the Broads (but maybe just the centre cockpit ones will have a similar prop shaft arrangement) and the BMC1500 / PRM100 pairings also appear to be common in Broads based boats.

 

23 hours ago, Vaughan said:

Luckily, your PRM gearbox has a thrust bearing on the rear of the output shaft, and can accept the forward thrust.

More by design on behalf of the boatbuilder than luck of course but something to think about if someone were to replace their gearbox

 

STERN GLANDS

We originally had a traditional "stuffing box" style stern gland which was solidly fixed to the end of the shaft log (complete with a threaded nut to compress the "rope like" stuffing and a remote greaser). My decision to change to a "Packless shaft seal" PSS gland took away the "greased up" stuffing that was between the prop shaft and the inner wall of the shaft log. This change left nothing but river water between the prop shaft and the wall of the shaft log, so it's demise was probably hastened as the engine sunk down on it's mountings and it came into contact with the prop shaft.

Maybe that's not such a bad thing though, as otherwise the gearbox bearings would have suffered the abnormal sideways load for longer before it was discovered.

 

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  • 7 months later...
On 26/08/2020 at 13:52, Vaughan said:

This is why ideally, a prop shaft should have a Plummer block in between the stern gland and the engine, to take up the forward thrust.  So many boats do not have these. 

Hi Vaughan, just seen your quote ref Plummer blocks ..........after being on my boat for a whole week (it was great to be on the river again - ended up visiting Loddon 3 times in a week!) I've come to learn I have a Plummer block on Karizma, and the grub screws were loose, so I tightened them up! - turns out, some say they should be left loose so the shaft has a little 'bow to stern' movement if needed. do you agree? or should the grub screws be tightened to the shaft?

Steve

plummers block .jpeg

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The whole idea of the Plummer block is to take up the forward thrust from the propellor and stop the thrust from being taken by the gearbox and the flexible engine mounts.  So there should be no forward movement of the shaft.

The Plummer block also aligns the shaft with the inboard bearing (which is not a bearing, but a gland) and stops it from being worn oval by side pressure from a badly aligned shaft.

Shaft alignment with the engine is still vital, but having the Plummer block makes it much easier.

It is possible someone left the grubscrews loose the last time they had to draw the shaft back, to get the gearbox off and change the flywheel thrust plate.

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7 hours ago, Vaughan said:

The whole idea of the Plummer block is to take up the forward thrust from the propellor and stop the thrust from being taken by the gearbox and the flexible engine mounts.  So there should be no forward movement of the shaft.

The Plummer block also aligns the shaft with the inboard bearing (which is not a bearing, but a gland) and stops it from being worn oval by side pressure from a badly aligned shaft.

Shaft alignment with the engine is still vital, but having the Plummer block makes it much easier.

It is possible someone left the grubscrews loose the last time they had to draw the shaft back, to get the gearbox off and change the flywheel thrust plate.

Thanks for the clarification. I guess I did the right thing :default_beerchug:

This forum has a wealth of knowledge :91_thumbsup:

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I have a few BMC spares like mounts and manifolds, I also have a shaft log like the damaged one, I struggle to throw stuff away which might be handy even if I don't run stuff myself !

 

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