Hot Stern Gland

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BigStu

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Hi all

just had the stern gland of my old Crabber MK1 serviced. New packing, rubber, and new prop shaft.

went out for a shake down and it is very hot to touch- can’t keep a hand on it for long.

I presume it all needs to bed in a bit. Just leave it or should I back off the nut?

what’s the worst that could happen with a hot gland.... ooo err!
 
It should not run hot. Backing off the nut may fix it but I have had a similar problem when using PTFE packing, which seems reluctant to return to its original thickness when backed off. PTFE has very low thermal conductivity, so can run very hot if a little overtightened. Graphitised flax does not suffer this problem, in which case backing off the nut(s) should fix it.
 
Well, I have had this problem several times now and have finally worked out what I think the problem is. Having repacked the gland and set it up barely under any compression, I have been surprised to find it very hot by the time I have motored the four miles from the yard at Totnes to Dittisham where I could pick up a mooring.

The first time a little pile of wax had formed under the gland, so I repacked the following year. Again despite the packing not being under any compression it happened again, and then again this year after replacing the packing with one of the fancy GFO packings.

As far as I can tell, when first set up the packing is tight enough to prevent air escape and as the stern tube is some four feet long with a down incline I have concluded that the problem is an airlock in the stern tube. Once the gland has been backed right off to let water come out and then tightened back again, while the gland is back at exactly the same compression as before, it runs dry and cool.

So my best guess is that you had an airlock in the stern tube and as the packing was new and not yet run in, it ran warm but with no water cooling or lubrication to dissipate the heat it got hotter and hotter. Probably after being in the water for a little time the stern tube will have filled with water and the problem will go away, but I would back the nut off a bit anyway - however if you mark it I suspect when you have eventually tightened it you will be back where you started from.

I may be wrong and this wouldn’t be the first of my theories to bite the dust - perhaps a proper engineer will be along in a moment to comment.

Andy
 
Amlov said:
I may be wrong and this wouldn’t be the first of my theories to bite the dust
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I quite like your theory. Is an air lock in the stern tube possible? I think the theory is plausible but have never come across it before.
By how much would the air in the shaft be compressed at 1 metre depth?
 
I like the theory too but in my case I changed the packing afloat each time, so water was running out of the gland as I pushed my three turns of packing in. I did this twice with PTFE with the same result, very hot gland, whereas with traditional packing all was well.
 
Any chance that the packing is grabbing the shaft and rotating with it- generating the heat in the housing rather than in the shaft (from which it would more readily have dissipated)? Worth a try to put on turns in the opposite direction?
(this branch is getting thinner and thinner -- if the above is preposterous, it was , of course, a joke)
 
No more than you would expect really but I have found that pumping too much grease into the sterngland causes it to run hot, since the water that would have had a cooling effect is displaced from the stern tube.
 
vyv_cox said:
I like the theory too but in my case I changed the packing afloat each time, so water was running out of the gland as I pushed my three turns of packing in. I did this twice with PTFE with the same result, very hot gland, whereas with traditional packing all was well.
Click to expand...

+1

Over the years I've replaced packing afloat on a few occasions, don't understand why people worry so much. Another cause of overheating can be over greasing stopping water getting to the packings. I would suggest OP backs the nut off a good bit until excess drips and then just nip up until they just stop.
 
Poignard said:
No more than you would expect really but I have found that pumping too much grease into the sterngland causes it to run hot, since the water that would have had a cooling effect is displaced from the stern tube.
Click to expand...

That's maybe possible, but I can't see that there'll ever be much circulation of the water, so don't think it would be cooling much.
 
pvb said:
That's maybe possible, but I can't see that there'll ever be much circulation of the water, so don't think it would be cooling much.
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Yes, you wouldn't have thought so but it's what I have found by experience in my boat. Less grease, less heat.

And the bearing/gland is not tight as the prop freewheels easily under sail, even at low speed through the water.

Also better for the environment! ;)
 
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