NormanS
Well-Known Member
On a "proper" installation, the stuffing box is screwed or flange bolted to the sterntube.
Stern Seal failure: Separating facts from hearsay
- Thread starter jfkal
- Start date
On a "proper" installation, the stuffing box is screwed or flange bolted to the sterntube.
Bosun Higgs
N/A
So you have an engine on modern flexible mounts to minimise vibration fastened to a rigid shaft?
Tranona
Well-Known Member
So you have an engine on modern flexible mounts to minimise vibration fastened to a rigid shaft?
Have a look at the photo of my boat's stern gear on Vyv's site to see how that problem can be solved. Stuffing boxes on hoses are new fangled ideas, brought in when flexibly mounted engines arrived and stern tubes became (mostly) glass tubes bonded in - although many bronze tubes were adapted to take hose mounted boxes. Generally reasonably trouble free because the hose is very heavy duty and is not in compression at the same time as allowing some movement of the engine and shaft relative to the stern tube.
NormanS
Well-Known Member
So you have an engine on modern flexible mounts to minimise vibration fastened to a rigid shaft?
No, but I do have a flexible shaft coupling, and a thrust bearing so that the forward thrust is not taken by the engine mounts.
CelebrityScandel
Well-Known Member
Not quite what you have asked but I have tried to create what would be a catastrophic failure with a mechanical shaft seal and failed.
All our boats have always been specified with lip seal based shaft seals. On one of the early boats (1-1/2 inch shaft from memory) the assembly was on the propeller side a bronze water/grease lubricated bearing and on the engine side of that a single lip seal held in place by "lantern" ring.
In an experimental mood I one day took the "lantern" ring and the lip seal out while the boat was still in the water. With NO SEAL AT ALL no water came into the boat the grease and fit of the bearing stopped it. I then started the engine and put it in gear and at most I could just get an easily controllable dribble of water coming into the boat much of which may have just been the water injection from the engine raw water pump. Furthermore it was obvious that if one took the seal out and cut it from the shaft one could replace it with a twist or so of conventional packing - I quizzed the manufacturer on this and they confirmed that they too had that in mind as a possibility when they designed the seal.
I have seen a number of differing lip seal based shaft seals in the market, including the oil bath ones as well, that are incapable of failing catastrophically and I would suggest that people should extend themeselves and go looking for them instead of following the forum dribble about PSS and Volvo ones as if they were all that existed. Perhaps the original question should have been aimed at a specific mechanical seal type (if one was interested in a specific type) or else asked what seals are incapable of catastrophic failure. The latter would be more informative - and I have answered that question already.
All our boats have always been specified with lip seal based shaft seals. On one of the early boats (1-1/2 inch shaft from memory) the assembly was on the propeller side a bronze water/grease lubricated bearing and on the engine side of that a single lip seal held in place by "lantern" ring.
In an experimental mood I one day took the "lantern" ring and the lip seal out while the boat was still in the water. With NO SEAL AT ALL no water came into the boat the grease and fit of the bearing stopped it. I then started the engine and put it in gear and at most I could just get an easily controllable dribble of water coming into the boat much of which may have just been the water injection from the engine raw water pump. Furthermore it was obvious that if one took the seal out and cut it from the shaft one could replace it with a twist or so of conventional packing - I quizzed the manufacturer on this and they confirmed that they too had that in mind as a possibility when they designed the seal.
I have seen a number of differing lip seal based shaft seals in the market, including the oil bath ones as well, that are incapable of failing catastrophically and I would suggest that people should extend themeselves and go looking for them instead of following the forum dribble about PSS and Volvo ones as if they were all that existed. Perhaps the original question should have been aimed at a specific mechanical seal type (if one was interested in a specific type) or else asked what seals are incapable of catastrophic failure. The latter would be more informative - and I have answered that question already.
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Tranona
Well-Known Member
Many different types of shaft seals have come and gone over the years. However, there are very few with replaceable lip seals - I think the only ones readily available in smaller sizes are Tides Marine and Vetus (the latter only in metric sizes). Both of these are expensive - roughly twice the price of a Volvo seal and physically bulky - particularly length.
As this thread shows, failure seems to occur more commonly on face seals - either the faces not sealing properly because something gets between them, or more commonly failure of the bellows. Of course this sample is very small and some of the examples not well evidenced, but a survey in the USA a few years ago showed similar results - although if I recall correctly the survey itself was poorly constructed and lacked detail.
CelebrityScandel
Well-Known Member
Hi Tranona
That may indeed be true for what may be available in the UK but I am not in a position to comment on that. There is a wider range available if you look further afield on the international market but I cannot give names as we have been into bigger boats for quite a while now but Google throws some up. Possibly more productive to look for suppliers oriented to commercial boats than pleasure. And I see a few different ones (some oil bath types too, which are expensive) from time to time.
The one that I referred to on an earlier small boat of ours was, if I recall correctly, imported from Australia or New Zealand and I don't think it would have been any or much dearer than a packed one as was very simple constructon.
Sorry but I don't recall it's product name but among others Google turned this up http://www.chatfieldmarine.com/shop...s/Blue+Water+Shaft+Seal+-+Standard+Model.html which I am pretty sure was the one. If not it, it was much the same. Nothing fancy like the Sureseal (which I had to look up to see what it looked like) and just used a standard nitrile lip seal in a normal type recess in the cast bronze body, so no special housing like the Sureseal seems to have, with a ss garter spring (any reputable seal supplier will fit a ss spring to whatever lip seal one needs for whatever purpose in front of ones eyes and supply the seal itself for few US dollars - whatever that is in pounds).
The raw water lubrication from the engine was not considered necessary normally in slower boats such as sail boats as induced water back up the log was claimed to be enough but we did the raw water thing.
We carried a spare seal but never needed it throughout our ownership of the boat with many thousands of miles of motoring in total. There was never any shaft wear from the seal but the shaft was a high performance ss marine shafting which may have been more resistant to wear as it is, I believe, harder. I don't recall the actual shaft material now but was similar to the US Aqualoy products, for example, if interested. I haven't compared the hardness to 316. Another factor in life may have been the shaft was carried in a hard plastic bearing at both ends of the shaft log, rather than a Johnson Cutless type (dunno if US Johnson get into the UK, maybe different manufacturers there?), so all ran true.
Have wandered a bit but may be useful to someone.
Tranona
Well-Known Member
That looks like an interesting design. Don't think it has ever been promoted in the UK.
The dominant player in the OE yacht market for small sizes is Volvo as it is simple, cheap and reliable. It is, of course a declining market because of the steady growth in market share of saildrives. The replacement market for older boats is very diverse and small volume. It is well served by well established products so it is quite difficult to get entry into the market. Many designs and suppliers have come and gone over the years and nowadays there are really only 4 or 5 established products on the (UK) market.
The dominant player in the OE yacht market for small sizes is Volvo as it is simple, cheap and reliable. It is, of course a declining market because of the steady growth in market share of saildrives. The replacement market for older boats is very diverse and small volume. It is well served by well established products so it is quite difficult to get entry into the market. Many designs and suppliers have come and gone over the years and nowadays there are really only 4 or 5 established products on the (UK) market.
Bosun Higgs
N/A
That's cheating by doing the job properly!
Except that you would still be better off with a modern oil reservoir type seal.doug748
Well-Known Member
I have followed this debate for ten years here, and have seen most types in the dock - so to speak.
jfkal
Well-Known Member
Hello Guys. Is that all? Come on and keep the horror stories coming.......
Thanks
Thanks
Delfin
Well-Known Member
Mine is a current PSS seal, with a vent. The stern tube is around 4' long. After haul out and on the hard for a period of time, burping is mandatory as indicated in the installation instructions from PSS - If the boat sits idle for a long period of time (generally 3 months or more),it is necessary to move the carbon face back to allow a small amount of water to enter the boat. I wouldn't focus on the 3 months in this instruction, as the issue is whether the water drains from the stern tube during the period out of the water. Once it is gone, it seems to only slowly re-enter the tube unless you burp the seal and allow someplace for the water to go.
I made the mistake of not burping mine and while my PSS seal didn't fail after a long haul out, but the forward cutlass bearing did.
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CaptainBob
Well-Known Member
Mine's a Volvo shaft seal (burpable type).
Hasn't failed catastrophically on me, but thought it important to mention that for some reason there's always air in it when I burp it after every couple of hours of motoring (possibly also after a couple of hours of sailing). Often there's 3+ seconds of air hiss before water drips in. I've made it part of the routine for preparation - and something I do periodically on long trips.
Also important to note that despite it having enough air in to necessitate re-burping, I've never heard a single sound from it.
Hasn't failed catastrophically on me, but thought it important to mention that for some reason there's always air in it when I burp it after every couple of hours of motoring (possibly also after a couple of hours of sailing). Often there's 3+ seconds of air hiss before water drips in. I've made it part of the routine for preparation - and something I do periodically on long trips.
Also important to note that despite it having enough air in to necessitate re-burping, I've never heard a single sound from it.
vyv_cox
Well-Known Member
You are correct, it does say that. I understand 'burping' to mean removal of air from the tube, as anyone who has had baby vomit down their backs will know!
I take the PSS instruction to mean wetting the seal faces so they don't run dry. I always check the riser tube from mine on launching, to ensure that water has passed the seal.
Chris_Robb
Well-Known Member
Volvo shaft seal as yours - I also burp it daily as part of the engine checks. I have had odd noises from it in the past (a drone) and now grease it every 30 hours or so as well (we did 500 hours last years!)
Tranona
Well-Known Member
Think you have probably got a fairly representative sample, although small. Years of watching threads on the subject together with having a trade involvement in the past in this area suggests.
1 Failures are most commonly due to neglect
2 Traditional stuffing boxes can be troublesome in terms of leaking but rarely fail.
3 Face seals have been troublesome with faces sticking and bellows failing (sometimes connected). Latest designs generally overcome these issues.
4 Volvo (and other lip seals) are largely, but not completely trouble free.
Personally I think the oil bath lip seal type which were briefly popular 10-15 years ago are the best, but bulky, heavy, over engineered and expensive. Probably why they are no longer generally available.
jfkal
Well-Known Member
Yeah, you sum it up quite nicely. I will wait couple of days and than "publish" the summary. A real pity that the oil based seals have all vanished. I like their dual line of defense and the fact that the seal does not run on the shaft itself. Also they give nice early warnings before anything major (I do not think that the word catastrophic is appropriate ;-) ) happens.
Tranona
Well-Known Member
Catastrophic is technically the correct word, indicating a sudden and complete failure as when a bellows splits. Major is only a question of degree - so you can have a minor leak and a major leak and anything in between. Seals around the shaft tend to fail progressively. Face seals can also fail progressively if the faces wear or get damaged, but as you have seen many failures of this type are in the mounting arrangements or because of inadequate bellows pressure. This is their major weakness, acknowledged in the latest version of the Deep Sea seal design which has an emergency clamp to lock it solid when it fails!
Robin
Well-Known Member
Failure of the bellows is similar to failure of the hose in a stuffing box design. I was on a friends nearly new Westerly with a standard stuffing box once when we were leaving Cherbourg on our way home and picked up a large polythene sheet on the prop that stopped the engine with a bang. Muggins drew the short straw to go in and cut the poly away (in April!) but a subsequent check of the shaft inside revealed a badly split stuffing box hose that we had to patch with self amalgamating tape to get us home.
When we bought our last boat it had a DeepSea seal and the bellows was starting to split. I replaced it with another the same but that too was splitting after just one season, the bellows IMO was not fit for purpose. I replaced the seal again but with a vented PSS which has an altogether more robust bellows, 18,000 miles or more later and no problems at all.
When we bought our last boat it had a DeepSea seal and the bellows was starting to split. I replaced it with another the same but that too was splitting after just one season, the bellows IMO was not fit for purpose. I replaced the seal again but with a vented PSS which has an altogether more robust bellows, 18,000 miles or more later and no problems at all.
doug748
Well-Known Member
We have had tales of the Volvo seal splitting in the way of the rear clamp. One chap said it fell into pieces with radial splits and flooded his boat. There is a report of this type of split nearby at the moment. Perhaps the lip seals jam and put unfair stress on the rear of the unit? They should be changed every 500 hours.