Seal between stern tube and hull

[WotEver]

Hi all,

 

How does the bronze (?) Stern Tube attach to the hull? Is it a flange on the outside? Mine's leaking slightly between the tube and hull (in other words, behind the greaser) and I was wondering whether the boat has to come out of the water to fix it.

 

Cheers,

Tony

[blodger]

Hi all,

 

How does the bronze (?) Stern Tube attach to the hull? Is it a flange on the outside? Mine's leaking slightly between the tube and hull (in other words, behind the greaser) and I was wondering whether the boat has to come out of the water to fix it.

 

Cheers,

Tony

Mine is doing likewise so went into drydock for blacking and to fix. Staff suspected welding around the boss and swim join so shotblasted and welded. However on refloating leak persists. There's a thread on here where I asked advice and several contributed knowledgably. I am going back into dock to have sterntube renewewed. Apparently, there is a stop in the boss that tube shouls be pulled hard against but often tube is backed off for alignement of bolts on the cross tiebar that stuffing box bit is affixed too, hence leak. I had bearing end renewed 6/7 years ago introducing this problem which became evident substantially 2 years ago. The K99 grease gets flushed out so that it can even leak at the thread join in the sterntube eventually. Hence, I am going for a full replacement this time. A bodge offered is to back of the tube, space permitting in drydock and clean out and pack the stop with marine goo. Apparently the dock has done this on a few boats recently!

[Chris Pink]

Hi all,

 

How does the bronze (?) Stern Tube attach to the hull? Is it a flange on the outside? Mine's leaking slightly between the tube and hull (in other words, behind the greaser) and I was wondering whether the boat has to come out of the water to fix it.

 

Cheers,

Tony

 

The bronze bearing is screwed into the steel part welded into the hull.

 

Unless the fabricator gets the spacing spot on it's not possible to screw up the bearing up completely and still have the bolts into the web aligned.

 

I have seen one upside down because that's where it fitted best.

 

As to a fix in place I would try winding some oakum around the joint and packing it in.

 

Other than that, when docking remove the bolts wind up the bearing until tight then make up a packing piece to go behind it.

 

When manufacturing the trick is to assemble the whole thing before welding the boss into the hull.

[WotEver]

Thanks both.

 

The stern gland was only fitted 18 months ago and since I've posted this question the boat yard who fitted it has offered to see if they can sort it.

 

I'll post back with the result if I remember

 

Tony

 

The bronze bearing is screwed into the steel part welded into the hull.

 

Unless the fabricator gets the spacing spot on it's not possible to screw up the bearing up completely and still have the bolts into the web aligned.

 

I have seen one upside down because that's where it fitted best.

 

As to a fix in place I would try winding some oakum around the joint and packing it in.

 

Other than that, when docking remove the bolts wind up the bearing until tight then make up a packing piece to go behind it.

 

When manufacturing the trick is to assemble the whole thing before welding the boss into the hull.

 

Thanks for the explanation - I understand now. So, wouldn't a few turns of oakum behind the flange (between flange and hull) be a 'standard' part of fitting any stern gland? I'd have thought that if you have little to no chance of the flange lining up when the tube is screwed right in then it would be 'normal' to use some compressible waterproof packing. Like oakum.

 

Is there a reason that's not a great idea?

 

Tony (again)

[Chris Pink]

Thanks for the explanation - I understand now. So, wouldn't a few turns of oakum behind the flange (between flange and hull) be a 'standard' part of fitting any stern gland? I'd have thought that if you have little to no chance of the flange lining up when the tube is screwed right in then it would be 'normal' to use some compressible waterproof packing. Like oakum.

 

Is there a reason that's not a great idea?

 

Tony (again)

 

My experience (once!) is that the assembly is made up then offered to the hull, tacked in place, then the bronze bearing is removed while the tube is welded in properly.

 

So it shouldn't then leak. And we didn't use any packing (which would take some space) and it doesn't leak. The packing suggestion was a repair suggestion.

 

The trouble now is that you can't tighten the bearing without the web bolts going out of line so packing or sealing is the only repair option.

 

Was it a new bearing in an old stern tube?

[alan_fincher]

I've been told (more than once!), that a heavy duty PTFE tape should be used on the threads between the brass stern tube and the steel boss that's welded to the hull.

 

Given that, (rightly or wrongly!), installers try to end up with the inner boss looking tidy, and the greaser inlet at the top, you obviously can't rely fully on the fact it is going to screw in right to the end.

 

When ours was leaking I successfully bound around the joint with hemp impregnated with Fernox LS-X leak sealer, which held it 100% until the boat was next on the dock, and it could be sorted properly.

[nb Innisfree]

PTFE tape doesn't work very well with large diameter threads. When I fitted ours during self build I coated threads in silicone, been no sign of a leak in over 4 years afloat.

[rjasmith]

PTFE tape doesn't work very well with large diameter threads. When I fitted ours during self build I coated threads in silicone, been no sign of a leak in over 4 years afloat.

 

I used a liberal coating of Marineflex on the threads of the steel sleeve and both ends of the bronze bearing sleeve.

 

It had previously been assembled dry with the lined up shaft in place to allow welding of the steel sleeve and steel steadying plate that the gland box fixes to with the two bolts. Then after everything had cooled off I undid it all (and took the shaft out) to apply the Marineflex to the threads. Finally screwed it all up again and got both threaded bronze sections to close up fully with the bolting flange in line with the holes in the steel plate. OK I did have to file the steel a bit!

 

Hopefully it won't leak but the boat's still in the garden of course so I've yet to find out!

 

Richard

[Timleech]

PTFE tape doesn't work very well with large diameter threads.

 

That may be true in theory, but it's one of the better options for assembling this type of sterngear IF it is properly tightened, it does work. It makes dismantling when the bearing comes to be renewed a realistic prospect whereas any sort of setting goo is likely to make it very difficult if not impossible. This is because the thread is so long, as well as being in a difficult position to get a lot of heat into the steel boss to soften whetever compound has been sitting there for several years. Also using heat will expand the brass tube inside the steel boss and be counterproductive in that way.

I regularly see leaking tubes as per the OP as a result of assembling for appearance rather than durability.

If I fit a replacement tube I expect to have to cut out the support plate and refix it, or redrill the holes. I consider myself and my customer to be lucky if neither of those is needed!

I suppose in an ideal world one could assemble dry as a test, check the position of the stuffing box and what fraction of a revolution it needs to go to line up neatly horizontal - then dismantle, put the tube in a lathe and skim off a calculated amount from the end of the tube based on the thread pitch, and then do a final assembly with the sealant of choice. Frankly some of these things are such a nightmare in terms of access that by the time you've got the old one out and the new one in once you've done quite enough grovelling and moving pipe wrenches a tiny fraction of a turn over & over again that the prospect of doing it all again as well as persuading the customer that the time was justified really doesn't hold much appeal!

 

BTW plumber's hemp is probably easier for most people to find than oakum, and is made for exactly that type of job. Use it with grease or some sort of non-setting compound.

 

Tim

 

I used a liberal coating of Marineflex on the threads of the steel sleeve and both ends of the bronze bearing sleeve.

 

 

Richard

 

Ouch!

Marineflex is an adhesive as well as sealant, dismantling that when the tube needs renewing will be fun/impossible

 

Tim

Edited January 29, 2010 by Timleech

[alan_fincher]

Hopefully it won't leak but the boat's still in the garden of course so I've yet to find out!

You could, of couse, fill it with water, then check carefully around the outside of the stern gear !

 

 

BTW plumber's hemp is probably easier for most people to find than oakum, and is made for exactly that type of job. Use it with grease or some sort of non-setting compound.

Yes,

 

As I said, it worked for us.

 

I used Fernox LS-X, largely because that's what I had, but also because with care it can be made to adhere to something that is already leaking.

[rjasmith]

(snip)

 

Ouch!

Marineflex is an adhesive as well as sealant, dismantling that when the tube needs renewing will be fun/impossible

 

Tim

 

Oh well - it probably won't leak then!

 

Mind you I wouldn't have thought Marineflex has a very great shear strength and I've got a pair of 36" Stillsons!

 

Hopefully renewing the tube is going to be quite a few years away - I've still got a lot of fitting out to do yet!

 

Richard

[WotEver]

Was it a new bearing in an old stern tube?

Yup. New bearing and gland all in one go.

[Chris Pink]

Yup. New bearing and gland all in one go.

 

In which case it is unlikely that the new bearing would mate with the old stern tube so as to put the web fixing bolts and the greaser in the right position so it will be a little slack.

 

Remedies are two; Tim's solution above, or sealant.

 

Maybe the boatyard did use sealant and this has failed?

 

Either way it is a dry dock job unless you can wind some hemp or oakum around behind the bearing in front of the stern post.

 

 

by the way, what's the difference between plumber's hemp and oakum, never having seen the former.

[Timleech]

by the way, what's the difference between plumber's hemp and oakum, never having seen the former.

 

Plumbers hemp is finer, softer and not 'tarred'.

 

Tim

[WotEver]

In which case it is unlikely that the new bearing would mate with the old stern tube so as to put the web fixing bolts and the greaser in the right position so it will be a little slack.

 

Remedies are two; Tim's solution above, or sealant.

 

Maybe the boatyard did use sealant and this has failed?

 

Either way it is a dry dock job unless you can wind some hemp or oakum around behind the bearing in front of the stern post.

Well, the boatyard that did the job is going to take a look at it, so maybe (hey, I can hope, can't I) they'll slip it if required at no cost.

 

T