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That Sinking Feeling (story In Pictures)

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40 year old engine mountings

Engine gradually (and un-noticed for years) lowered down until the prop shaft is silently rubbing on the side of the stern tube


Hole in stern tube (in our case a "shaft log" version of a stern tube) which is fitted completely inside the boat  

See our story in pictures..................

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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

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 forwa

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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Thanks for the replies:-

We are shimmed for now, but long term we'll be looking at new mountings

The problem with new mountings is that they are not made to the same pattern and none of the new ones will fit in the available space to the engine mounting brackets (so it will be an engine out job!)

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33 minutes ago, annv said:

Hi suggest you fit a bridge tell/tail to top of shaft to be able to monitor any movement in future. John

Thanks for the suggestion John,

Actually we have a novel way of checking this:-

The boat has a GRP hull

The shaft log obviously sits mostly inside the boat, although it equally obviously has river water inside it

However, it has never been electrically bonded to other hull fittings and has never suffered any galvanic corrosion through the boat's 45 year life. As a result it is electrically isolated from (or when in the water has a very high electrical resistance from) the prop shaft, so an ohmmeter (fluke multimeter) connected between them both on the "buzzer" range gives a tell-tale sound if they connect. It's easy to make it touch by gently pushing sideways on the shaft.


There is a problem though:-

I took the shaft log out and refitted it exactly as it was removed (and went on to permanently glass it in before the prop shaft was re-fitted).  There was no room for rotational movement within the many securing bolts, so I knew that it had been fitted back in the exact fore/aft position from which it had been removed.

BUT the engineer that I paid to carry out the engine shimming has advised that the shaft log is slightly out of true with the prop shaft. The reduced distance between the prop shaft and the new thicker wall of the shaft log, combined with them being slightly out of true with each other makes for very little room for the shaft before it touches.

I can address this when the engine is re-mounted as it needs to move further to port than is possible by tilting on shims alone, but in the meantime the position is critical and I have now noticed a rattle when the engine is at or near tickover. It disappears with higher revs or when under way in all but the slowest speeds but I have proven that it is due to the engine's movement / vibration on it's old mountings allowing it to lightly touch on inside of the shaft log. I can stop it simply by putting slight pressure on the shaft with my finger but it's irritating though not harmful at the moment.

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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!



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