sterntube leak not from stuffing box

[blodger]

How common is it for the stern tube to leak from where it fits into the boss welded into the end of the swim?

 

After two years of putting up with the leak, particularly bad in deep clear water like the Thames, I have been in dock for the usual blacking and to have the sterntube sorted. I went to the same dock that had fitted a new bearing, prop shaft and prop shortly after I had acquired the boat about six years ago. They welded up around the boss and the swim end weld suspecting weld failure there. Loads of K99 grease was forced through to make sure the bearing end was saturated with grease. On refloating and running the engine the same leak is persisting but slightly less because I am feeding a greaser full through at every opportunity. (Just as I did when it first started).

 

The drydock owner tells me that on a couple of boats recently with a similar problem he has had to back the sterntube out and clean out the threaded boss up to the stop and apply something akin to gasket cement before screwing back up, etc to cure the problem. He proposes to do this for me if more grease does not work which it will not.

 

Anybody had a similar experience?

 

I just want to be able to give the greaser a couple of turns after engine use and not have to be paranoid about the two auto bilge pumps working all the time!

[alan_fincher]

We had a leak where the threaded tube screws into the boss welded into the boat.

 

My understanding is that it is usual to seal these threads with something like a large application of PTFE tape.

 

At least that's what I think got done to ours,and it hasn't leaked since.

 

I think they are sometimes taper threads, but you would need the experts to confirm that, as I'm by no means sure.

[nb Innisfree]

When fitting our stern tube I sealed the threads with silicone, been water tight for over 4yrs.

[soldthehouse]

We had a leak where the threaded tube screws into the boss welded into the boat.

 

My understanding is that it is usual to seal these threads with something like a large application of PTFE tape.

 

At least that's what I think got done to ours,and it hasn't leaked since.

 

I think they are sometimes taper threads, but you would need the experts to confirm that, as I'm by no means sure.

As Alan said PTFE tape or threadlock, loctite or similar. Loctite have a superior range of joint compounds but PTFE is the easiest and least permanent in terms of "at a later date maintainance"

[nb Innisfree]

As Alan said PTFE tape or threadlock, loctite or similar. Loctite have a superior range of joint compounds but PTFE is the easiest and least permanent in terms of "at a later date maintainance"

 

PTFE tape isn't as effective on large threads i.e. more than, say, 1" BSP?

[Guest]

Stag?

[blodger]

We had a leak where the threaded tube screws into the boss welded into the boat.

 

My understanding is that it is usual to seal these threads with something like a large application of PTFE tape.

 

At least that's what I think got done to ours,and it hasn't leaked since.

 

I think they are sometimes taper threads, but you would need the experts to confirm that, as I'm by no means sure.

Thanks for that and others' posts. In retrospect I do not understand why the sterntube was not backed off and re-fitted with some sealant as now advocated and as this forum suggests when in drydock. I was quite clear on where the water was coming in. I guess they were reluctant to believe that a job that they had done was not still effective? I was not aware that there was a stop in the sterntube where a seal could be effected until after the docking. I had assumed that what was needed was more effective PTFing or better of the thread abaft the brass bearing section. I had been prepared to uncouple the shaft etc to facilitate but realised the rudder would have to be removed or hung up out of the way etc

 

The sterntube and indeed prop shaft is quite short and no plumber's block for thrust so the forces on the boss are quite large and must cause wear. I suspect any fix with sealant/ptf would not be permanent and need renewing at each docking if such proved to be the case.

 

It remains to be seen if there is enough space between the stuffing box and prop clamp to unbolt (4bolts) the former from the cross support to unsrew and back out the sterntube. I think there is just enough prop shaft outside the boat. Where the sterntube is threaded into the back of the stuffing box grease emerges at times when you are forcing it. I have considered jubilee clipping packing there to prevent this but presume a proper job would mean that this too should be ptf or otherwise sealed?

 

If the full job I had anticipated now has to be done I am expecting to not have to pay full docking dues but that remains to be seen.

[Timleech]

It remains to be seen if there is enough space between the stuffing box and prop clamp to unbolt (4bolts) the former from the cross support to unsrew and back out the sterntube. I think there is just enough prop shaft outside the boat. Where the sterntube is threaded into the back of the stuffing box grease emerges at times when you are forcing it. I have considered jubilee clipping packing there to prevent this but presume a proper job would mean that this too should be ptf or otherwise sealed?

 

That and your water leak suggest to me that the whole asembly might never have been properly tightened. It's surprisingly common, the bulder/installer wants to keep it looking smart by having the stuffing box nicely aligned when another half turn or whatever would have ensured it was properly tight. Even more so where replacement parts have been fitted, unless you are very lucky the supporting plate will need to be cut loose, and rewelded after the strengear is tightened home. If it's not all fully tight, over time the threads will become slack and start to leak.

PTFE isn't ideal, but I don't think there is an 'ideal' sealant for the very long thread into the steel boss. If you use a thread sealant such as Stag or one of the Loctite products, dismantling at a later stage can be a nightmare/impossible. PTFE tape does win on that front.

 

Tim

[soldthehouse]

That and your water leak suggest to me that the whole asembly might never have been properly tightened. It's surprisingly common, the bulder/installer wants to keep it looking smart by having the stuffing box nicely aligned when another half turn or whatever would have ensured it was properly tight. Even more so where replacement parts have been fitted, unless you are very lucky the supporting plate will need to be cut loose, and rewelded after the strengear is tightened home. If it's not all fully tight, over time the threads will become slack and start to leak.

PTFE isn't ideal, but I don't think there is an 'ideal' sealant for the very long thread into the steel boss. If you use a thread sealant such as Stag or one of the Loctite products, dismantling at a later stage can be a nightmare/impossible. PTFE tape does win on that front.

 

Tim

Thank you for reitterating my sentiments I thought I was alone on the ptfe being the only real solution to a bad job with loctite being a good seal but poor with respect to dismantling

[blackrose]

Thank you for reitterating my sentiments I thought I was alone on the ptfe being the only real solution to a bad job with loctite being a good seal but poor with respect to dismantling

How about using Marineflex? Much tougher and more durable than silicone, but not as permanent as loctite.

[Glennbrown]

I had same problem a couple of years ago and the threads on the steel boss had corroded.

I dismantled whilst in dry dock and JB welded the stern tube into the boss-problem fixed!!

[blodger]

I had same problem a couple of years ago and the threads on the steel boss had corroded.

I dismantled whilst in dry dock and JB welded the stern tube into the boss-problem fixed!!

Resurrecting thread since I am now back in drydock to endeavour to stop the leak I went in for before Xmas. At first drydock owner/worker thought a bodge with sylglas & Jubille clip would work then aquiesced that water was fair trickling in from betwixt boss and tail bearing. Shaft is now uncoupled and stuffing box off. Tomorrow prop off and withraw sterntube bearing. We'll see what we see but intention is a new latter and whatever else it takes.

[smileypete]

Resurrecting thread since I am now back in drydock to endeavour to stop the leak I went in for before Xmas. At first drydock owner/worker thought a bodge with sylglas & Jubille clip would work then aquiesced that water was fair trickling in from betwixt boss and tail bearing. Shaft is now uncoupled and stuffing box off. Tomorrow prop off and withraw sterntube bearing. We'll see what we see but intention is a new latter and whatever else it takes.

 

For sealing a large coarse thread like that some low modulus (stiffness) neutral cure clear silicone should do, applied to the cleaned inside/outside male/female threads.

 

Don't use Marineflex or anything like that if you want to get it apart again

 

cheers,

Pete.

[WotEver]

For sealing a large coarse thread like that some low modulus (stiffness) neutral cure clear silicone should do, applied to the cleaned inside/outside male/female threads.

Or copious amounts of PTFE tape, no?

[dor]

Gas ptfe is thicker than normal plumbers ptfe tape which might be better on course threads. If using silicnoe, get the stuff in a small tube from a plumbers merchant can't remember the name at the moment but think it ends in -ox. Much better at sealing than the stuff in the gun tubes.

[alan_fincher]

If using silicnoe, get the stuff in a small tube from a plumbers merchant can't remember the name at the moment but think it ends in -ox. Much better at sealing than the stuff in the gun tubes.

Fernox LS-X ?

 

Brilliant stuff, much better on plumbing leaks that normal clear silicone from a big tube.

 

Quite pricey though for quite small tubes, and, like other silicones, don't cap it and assume it will be available for future use. Opened tubes don't survive long.

[smileypete]

Fernox LS-X ?

 

Brilliant stuff, much better on plumbing leaks that normal clear silicone from a big tube.

 

Quite pricey though for quite small tubes, and, like other silicones, don't cap it and assume it will be available for future use. Opened tubes don't survive long.

 

+1

 

 

(clicky)

 

Never let me down yet.

 

I think plumbers prefer PTFE tape as there's no setting time, so the job can be finished more quickly.

 

cheers,

Pete.

[Night Hawk]

+1

 

 

(clicky)

 

Never let me down yet.

 

I think plumbers prefer PTFE tape as there's no setting time, so the job can be finished more quickly.

 

cheers,

Pete.

 

http://www.screwfix.com/prods/23614/Plumbi...S-X-Leak-Sealer

 

I might buy a tube of this for the boat's CH system...not that expensive?

[blodger]

+1

 

 

(clicky)

 

Never let me down yet.

 

I think plumbers prefer PTFE tape as there's no setting time, so the job can be finished more quickly.

 

cheers,

Pete.

Thanks for advice; On the way to drydock this am picked up a clear silicon tube to be armed just in case.

However, withdrew the sterntube/bearing, which is a one piece, inside having uncoupled the R&D. There is no stop in the boss, it just threads stright in and relies on them.

Thinking is that there is an inherent fault (it's a TT boat) and no amount of packing/gunging will work long term so it looks like cutting the boss out and replacing with one with a stop, etc.

There is a fair bit of wear in the prop shaft in way of the gland packing so am replacing the shaft again after only four years use (CIRCA 2,000 ENGINE HOURS).

I suppose it will be worth it to run a dry bilge after all this time!

[Timleech]

There is a fair bit of wear in the prop shaft in way of the gland packing so am replacing the shaft again after only four years use (CIRCA 2,000 ENGINE HOURS).

 

Two common causes for this - bad alignment or overtightening the gland (or a combination of the two)

 

Tim

[blodger]

Two common causes for this - bad alignment or overtightening the gland (or a combination of the two)

 

Tim

It could well be that I am responsible for some gland overtightening since I have had a continual ingress of water that greaser turns did not help. I have tightened and then backed off for free shaft movement. Normally one should only tighten to reduce a leak to a drip. I have been unrelentlessly forcing grease in and trying to get it to the back instead of coming out of the front!

[BEngo]

How common is it for the stern tube to leak from where it fits into the boss welded into the end of the swim?

 

After two years of putting up with the leak, particularly bad in deep clear water like the Thames, I have been in dock for the usual blacking and to have the sterntube sorted. I went to the same dock that had fitted a new bearing, prop shaft and prop shortly after I had acquired the boat about six years ago. They welded up around the boss and the swim end weld suspecting weld failure there. Loads of K99 grease was forced through to make sure the bearing end was saturated with grease. On refloating and running the engine the same leak is persisting but slightly less because I am feeding a greaser full through at every opportunity. (Just as I did when it first started).

 

The drydock owner tells me that on a couple of boats recently with a similar problem he has had to back the sterntube out and clean out the threaded boss up to the stop and apply something akin to gasket cement before screwing back up, etc to cure the problem. He proposes to do this for me if more grease does not work which it will not.

 

Anybody had a similar experience?

 

I just want to be able to give the greaser a couple of turns after engine use and not have to be paranoid about the two auto bilge pumps working all the time!

 

 

I have seen this on a Les Allen hull- not sure it was the original stern tube bearing, but bowsed it up tight with a stilson and that cured it. It was surprising howe much water came in.

 

On my own boat I had a leak from the stern tube which I thought was like this but once I'd dug out the spray foam it turned out to be from the bearing, halfway along. It looks like the runner or riser where the bearing metal was poured into the tube. So far it's sealed up with a jubileee clip and some rubber sheet. When we come out I'll either replace it or fix a permanent patch on the tube.

 

N

[Timleech]

On my own boat I had a leak from the stern tube which I thought was like this but once I'd dug out the spray foam it turned out to be from the bearing, halfway along. It looks like the runner or riser where the bearing metal was poured into the tube. So far it's sealed up with a jubileee clip and some rubber sheet. When we come out I'll either replace it or fix a permanent patch on the tube.

 

N

 

A narrowboat stern tube with a cast-in-place bearing would be very unusual. What is much more common, though not often seen new now, is a slip-in bearing, bronze or, in the past, white metal. This bearing will have a slot in the inner end which engages with a very short peg (dowel) in the outer tube. Much more likely in your case (without the benefit of seeing it) is that you have one of these and this peg has worked loose or even fallen out.

 

Tim

Edited March 8, 2010 by Timleech

[BEngo]

A narrowboat stern tube with a cast-in-place bearing would be very unusual. What is much more common, though not often seen new now, is a slip-in bearing, bronze or, in the past, white metal. This bearing will have a slot in the inner end which engages with a very short peg (dowel) in the outer tube. Much more likely in your case (without the benefit of seeing it) is that you have one of these and this peg has worked loose or even fallen out.

 

Tim

 

Thanks Tim, I'd not thought of a slip in bearing. Odd, since that's how my Kelvin gearbox back bearing is constructed. The hole is definitely about halfway along the bearing tube. Helpfully it's on the underside of the tube so it's very hard to see and whatever was in it has fallen out. The bit that came out was whitemetal so that's what made me think it had been cast in. An earlier attemp to cure the problem was to peen the thing with a small ball-pein. It worked for a while!

 

I'm out in September so it will get dismantled and sorted then.

N

[blodger]

Thanks Tim, I'd not thought of a slip in bearing. Odd, since that's how my Kelvin gearbox back bearing is constructed. The hole is definitely about halfway along the bearing tube. Helpfully it's on the underside of the tube so it's very hard to see and whatever was in it has fallen out. The bit that came out was whitemetal so that's what made me think it had been cast in. An earlier attemp to cure the problem was to peen the thing with a small ball-pein. It worked for a while!

 

I'm out in September so it will get dismantled and sorted then.

N

Despite the fact they put the last sterntube in, a one piece up to the boss stop and into the stuffing box, drydock thought leak could be as of above. They do see a lot of boats, however. Problem with mine was stop was worn away, even when they fitted the new sterntube. The PTF tape lasted a year.

 

In drydock yesterday old boss was burned out and new boss welded in. A bit of a pain of connecting up for alignment, tack welding then removing, etc. Used PTF in boss threads and clear silicon in stuffing box threads. I was blacking the 'damage' at 1030 last night!

 

I'll see how it fairs and how much its cost me when I leave dock today.

 

Proper job should have been done six years ago and docking before Xmas where Boss & swim end weld were overwelded was a waste! Still got there in the end and starting off with an unworn bearing and propshaft again.