stern gear

[colin loach]

just spoken to a vetus engineer and he said there should be a small hole in the weed hatch to let it breath water is fed into the joint due to the pressure of water finding its own level. there must be some water in my joint other wise it would of seized up . I will now drill a small hole the same size as the pipe fitting.

[ChrisPy]

Sorry if it sounds bit nit picking but cavitation produces steam bubbles which condense and leave a vacuum which then collapses, no air coming out of solution.

of course you're right, I was thinking 'vapour'.

 

actually the 'voids' in the water are formed from water in vapour phase, which is not quite the same thing as steam. Steam contains liquid water droplets which is why it is visible. Water vapour is like gas - invisible. I'm not sure if 'steam' is produced under cavitation conditions.

 

well, we may as well be pedantic now we've started.

 

*****************

 

I'm not convinced that, at narrowboat speeds and loadings, the shaft seals and glands need much in the way of cooling by water flow. Surely as long as there is water present at the seal then lubrication exists.

 

 

 

Has anyone ever actually witnessed their gland steaming as a result of overheating through lack of lubrication?

Edited August 10, 2009 by ChrisPy

[colin loach]

liverpool boats are now collingwood boats and there should be a hole of 4mm at the top for air ventilation.

[Albion]

I'm not convinced that, at narrowboat speeds and loadings, the shaft seals and glands need much in the way of cooling by water flow. Surely as long as there is water present at the seal then lubrication exists.

 

I'm sure this is correct as the rotational speeds are fairly low but there is still a fair bit of work/energy being dissipated through the shaft system. My concern was that if, on launch, the stern tube assembly trapped some air which couldn't get out then the seal and possibly part of the bearing could/may be running in air and wearing faster than it should. If water has found its way in there completely then there probably hasn't been any damage done. This could have happened due to the face seal not being absolutley air tight initially or the tube linking to the weed hatch top plate not being perfectly sealed by its clips for example.

Roger

[WotEver]

Has anyone ever actually witnessed their gland steaming as a result of overheating through lack of lubrication?

 

I would imagine that would be quite painful...

 

... coat, door, yes I know...

[colin loach]

I hope the water as made its way in there must be quite a bit of pressure on the gland and if the air compressed a bit then water should of entered the gland. I have driven the boat for 300 hours with not any problems so fingers crossed.

[Albion]

I hope the water as made its way in there must be quite a bit of pressure on the gland and if the air compressed a bit then water should of entered the gland. I have driven the boat for 300 hours with not any problems so fingers crossed.

 

The pressure won't be huge Colin, it is purely the water height difference between the water level outside the hull and the lowest part of the stern tube. On an LB maybe about 12 inches max. Fill a 12 inch tube with water and you will be able to experience the same pressure by keeping your finger over the lower end, it will hardly be noticeable.

 

It is very likely that any air has dissipated by now, as you say, but drilling the hole in the weed hatch lid is still a worthwhile exercise otherwise every time the boat comes out for blacking etc you'll always have to double check that the tube has refilled OK before you head off with any significant revs.

Roger

[blackrose]

It is tempting to think that but there are two arguments why I don't think that is the case:-

1. If you want the flow of water to be sufficient to lubricate the stern gear bearing/seal why make the hole in the lid of the weed hatch so small?

2. On a properly designed weed hatch there is a baffle plate fitted in the aperture in the uxter plate, even when the seal is at the top of the weed hatch up-stand) to prevent splashing and noise from the prop. The prop is the only thing that could generate the pressure that you suggest and yet it is (should be) baffled to prevent that.

I know some builders didn't use a baffle plate and LB may have been one of them.

 

There is another possibility, of course, and that is that LB didn't really know what they were doing at all and simply found somewhere to stick a pipe that they had to attach to the stub on the bearing to prevent leakage. There may be no more science in it than that.

Roger

LB weedhatch covers do have a baffleplate fitted - mine does anyway.

I think you may be correct that LB didn't really know what they were doing and just thought that enough water would flow through their system to lubricate the gland. They may have been right too.

 

In any case, my test showed that when the prop shaft wasn't spinning no water came through the gland (although the gland would still have been immersed in water), and when the propshaft was spinning water came through from the gland into the tube. Whether it would have exited through the weedhatch (say 6" above) is another matter, and whether the whole arrangement works when the boat is moving forward has still to be tested.

Edited August 10, 2009 by blackrose

[Albion]

snipped

and when the propshaft was spinning water came through from the gland into the tube. Whether it would have exited through the weedhatch (say 6" above) is another matter, and whether the whole arrangement works when the boat is moving forward has still to be tested.

 

The difference betwen when the shaft is stationary and rotating is probably due to the centrifugal slinging of the water from the shaft and seal face I guess. It's an interesting engineering exercise to see what happens when under way though

Well, for me it is, but then again, I don't have to do the testing.

Roger

[colin loach]

I have been told to drill a 4mm hole by lb and vetus say to drill a 8mm hole. I will take top pipe off and drive forward and see if the pipe blows or sucks, all so do the same in reverse.

[Albion]

I have been told to drill a 4mm hole by lb and vetus say to drill a 8mm hole. I will take top pipe off and drive forward and see if the pipe blows or sucks, all so do the same in reverse.

 

I'd just drill the biggest hole that you can through the fitting that's welded to the lid. It won't matter really what size it is but, just in case it's true that the water flows through it, then the bigger the better. I suspect it won't and that it's only an air bleed but time will tell.

Roger

[colin loach]

i think your wright i will drill the 8mm hole. let you know what happens on saturday many thanks Colin