Stern Seal failure: Separating facts from hearsay

[charles_reed]

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.

Agree, the thread is long on conviction but short on facts.

Halyard withdrew their lip seal a long time ago. I had one of those and a Bukh seal, both leaked, either oil or water in time.

The Deep Sea seal has a questionable reputation amongst those in the marine stern-gear market.

The drawback of the packed seal is that it absorbs power - quite a lot on a small engine - and a number of FVs will complain of how they leak.

But glory be - most stern tubes are close enough to the surface that any leak is little more than a dribble.

One could always do a Pardey, and do away with prop-shafts entirely!!

 

[Chris_Robb]

Earlier PSS seals did not have the vent that current ones do, and therefore required 'burping' as Volvos do to this day. I understand that unburped Volvos make a good deal of noise that would alert the user if he failed to do it. PSS seals will not do this, so total failure could occur unnoticed.

Current PSS seals have no burping requirement and if installed correctly will run unattended for very long periods.

Photos of all types under 'stern glands' on my website.

Vyv, I can confirm that the Volvo seals make a noise like the drone of a distant prop aircraft. This is either because there is air in the system - they actually need weekly or daily burping, or that you should regrease them several times a year if you are doing big mileage,

 

[Leonidas]

I have an old fashioned stuffing box type so have had no catastrophic failures, but a friend of mine with a similar boat set off for France from Gosport and his patent seal failed after a few miles and he had to race back for an emergency crane out.

Another, often overlooked, advantage of the stuffing box is that your on-board spares kit only consists of a foot or so of packing. Very cheap and available anywhere in the world.

I'll go along with that and fully endorse it. Very reliable and tight for as long as the shaft is not grooved. The Achilles heal however is the rubber hose that connects the stuffing box to the hull.

 

[maxi]

Tides Marine Seal (lip seal), two years old, and approx 190 hours of engine time.

No known damage or trauma but the seal began letting large volumes of water into the bilge, but not consistently.

Short term action, daily vists to mooring to pump bilges.

Longer term solution, visit scrubbing posts and replace extant seal with a competent shaft seal of another make. Satisfactory outcome.

Examination of the Tides Marine Seal after removal showed an almost total absensce of the rubber seal - the shaft by way of the seal was clean and smooth with no burrs.

 

[CelebrityScandel]

Agree, the thread is long on conviction but short on facts...

...But glory be - most stern tubes are close enough to the surface that any leak is little more than a dribble.

Yes, the systems approach gets forgotten. I think many seem to forget that it is more than the seal that protects the boat; one should look at the whole seal, shaft log and bearings as a system and consider the environment they are in.

In our own cases we have always specified close tolerance plastic or metal bearings at the outer end of the log and for the seal assembly, and this means that should the seal itself fail (it never has) the inflow of water is limited by the low head (as you say) and by the close tolerance between the shaft and the bearings. We have always used radial lip seals as well which do not fail catastrophically (but the mounting of the housing may if on a flexible tube to the shaft log).

Furthermore, if the inner bearing is metal and greased then it will likely not leak at all when the shaft is stationary so gives some protection from slow leaks when the boat is left unattended in its berth/mooring/anchored.

There seems to be a fascination among pleasure boat owners in some countries (at least in the UK and USA) for Cutless bearings (I think because of a lack of knowledge of the existance of better alternatives and the preponderance of cheaper production boats in those countries) whereas apart from cheapness they are the low end choice for propeller shafts.

They do not add any protection to the boat in the case of a seal failure because they are not close toleranced to the shaft. The bearing itself is likely to be of short life, especially in an environment where there is marine life or other fouling that gets it into its mind to take up residence in the bearing crevices and so damage the bearing when the shaft rotates. Whereas a proper metal or plastic (of correct specialised water lubricated plastic such as Thordon) bearing with sufficient length to give adequate bearing surface will likely last as long as the engine.

 

[vyv_cox]

The leak detectors shown on this site www.blewsmarine.co.uk look interesting and not too expensive. Does anyone have one? It would be interesting to read some feedback.