The dangers of mechanical shaft seals?

rwoofer

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I'm just about to invest in a PSS shaft seal to replace my standard stuffing box with greaser when I read Nigel Calder;s book on boat maintenance. He seems to advocate the standard stuffing box because it cannot undergo a catastrophic failure.

Mechanical shaft seals such as the PSS he implies can reach a point in their life where they can catastrophically fail due to the mechanical seal breaking.

Is he being overdramatic?

Is there actually a recommended lifetime for modern shaft seal before they are in danger of catastrophically failing?
 

Porthandbuoy

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"Invest" may not be the right word; try "risk". I fitted two PSS shaft seals on a Catalac I once owned, but what I never considered (or was never advised on), was the type of sailing I did. If you sail in muddy waters (Morecambe Bay in my case) the soft carbon seal pressing against the hard tungsten carbide seal face will erode away frighteningly fast. It won't drip until the water starts pi--- sorry, squirting in.
If you dry out you have to bleed the seal of trapped air every time the tide comes in. If you don't the seal will run dry; at least until sufficient carbon has worn away and the air has bled off naturally.
The idea is sound, after all these type of seals have been used in industry for years, but in my boat? Never again. Mind you, I did discover that a Yamaha can run totally submerged, just point the air intake upwards.
 

mireland

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I have a mechanical seal and I feel it offers the advantage of a very dry bilge! I cannot see that catastrophic failure is likely unless, as with anything, you do not look after it properly. It has the advantage too of an emergency sealing mechanism should water ingress become a problem. Just tighten the sealing ring and although your shaft is then fixed and unusable at least you won't sink!
 

Porthandbuoy

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I'm glad you're happy with your mechanical seal and hope it never fails you. The point I was trying to make is that they are not the "cure-all" the manufacturers claim. I put them down at the bottom of my wish-list alongside those stern drive engines where you cut a bloody great hole in your hull, stuff a glorified outboard down it, and seal it with a bit of inflatable dinghy fabric.
And what about those log impellers you can clean while afloat! "Unscrew plastic retaining nut - withdraw impeller - ignore water gushing in through 30cm hole - frantically clean impeller of stubborn crustacean - stuff impeller back in hole"
Now where did I put that plastic retaining nut?
 

catmandoo

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There is an unwritten rule in engineering gained by years of experience that the more complicated a system is the more you have to look after it . So all is well when this happens .

Unfortunately we are human beings and prone to error . So when we fail the organisation fails and when it fails the system fails and then disaster .

You have only to look at the Russian system and the contamination dangers of their nuclear programme .
 

webcraft

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They work great until they go wrong . . . of course, if you don't expect the unexpected you will be fine.

I know of a charter boat towed into Rothesay sinking rapidly after the engine broke a mounting and twisted the shaft, ripping the seal and allowing a frighteningly rapid ingress of water.

- Nick
 

LORDNELSON

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I do not like the idea of a catastrophic failure of the stern gland; as a result I have kept my traditional stuffing box but packed it with "drip free packing", its expensive but, so far, has worked well.
 

rwoofer

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Seems like Mr Calders fears are shared by many.

Prior to posting this question the general consensus of this forum seemed to be very pro the mechanical type seals. I guess that everyone with opposing views must have kept quiet about their concerns..
 

Robin

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I personally don't have a concern about catestrophic failure, that is what preventive maintenance is about! You need to adjust a stuffing box periodically (and most are adjusted wrongly probably) and refill the greaser, remember to grease it etc, so how hard is it to check a PSS once or twice a year? I don't have Calder's latest edition, in mine he illustrates only the Deap Sea Seal which I personally think is poorly engineered, I had one and replaced it with a PSS. The 'clamp it up' if it fails feature of the Deep Sea Seal is simply a jubilee clip with a wing nut - whoopee! I think if a Deep Sea Seal fails it will more likely be a split of the very soft bellows used which needs to be soft so that the safety jubilee clip thing can work, but this will not protect from a split bellows. The PSS seal has a very robust corrugated bellows, similar to types fitted to standard stuffing boxes.

BTW stuffing box seals can also fail. Shafts have been ripped out of couplings and back through the hull when a rope has been caught in the prop (I had it happen on a delivery trip of a friends boat), the answer is a couple of jubilee clips clamped on the shaft forward of the seal to stop the shaft sliding out - this feature is present anyway in the PSS in the form of the S/S collar on which the graphite seal runs. Standard seals can also fail if overtightened, they run hot and 'grab' the shaft, then the flexible hose twists off the stern tube and bingo, oodles of oggin.

Also BTW the need to 'burp' a seal after the boat has been ashore can be removed by using the water injected version which is the same except for a small hose spigot. You can then run a small hose from this to above the waterline as a permanent air bleed or you can connect it to the cooling water system and feed it with pumped water, only really needed with high shaft speeds. We have a PSS with the water injection used as an air bleed as described.
 

LORDNELSON

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Re: Dripless Packing

Drip-less Moldable Packing is marketed by West Marine (USA) (Westmarine.com) and costs about $65 per kit plus transport plus import duty. It is "a clay like material that molds to the inside of the stuffing box, creating a permanently lubricated seal. Internally lubricated with low friction compounds it requires no water for lubication". Nigel Calder discusses what seems to be much the same material as "Drip Free" packing in his "Boatowner's Mechanical and Electrical Manual" (Second Edition 1996, reprinted 2000, pages 315 - 316). Mine has worked well after initial problems because I had not tightend the stuffing box up enough (but do not over-tighten!). Whilst expensive the attraction to me was that it is not, so far as I can ascertain, subject to sudden catastrophic failure
 

Birdseye

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[ QUOTE ]
I'm just about to invest in a PSS shaft seal to replace my standard stuffing box with greaser when I read Nigel Calder;s book on boat maintenance. He seems to advocate the standard stuffing box because it cannot undergo a catastrophic failure.

Mechanical shaft seals such as the PSS he implies can reach a point in their life where they can catastrophically fail due to the mechanical seal breaking.

Is he being overdramatic?

Is there actually a recommended lifetime for modern shaft seal before they are in danger of catastrophically failing?

[/ QUOTE ]

Sailing is a very conservative business. All seals are mechanical but the newer styles you are referring to is what is used in industry, in shipping and in much more arduous applications is your car and boat diesel engine. I took a trip on a big container ship recently and was fascinated to see that it too used a modern seal on its enormous shafts.

Personally, I wouldnt consider an old type stuffing box and more than I would consider hemp ropes or cotton sails. Mind you, I wasnt impressed by the very early deep sea seal I once had - that certainly wasnt fail safe. The one I repaced it with had three lip seals and an oil reservoir to feed it. Never a single drop of water in the five remaining years I had the boat

Specified life? Is there one on the seals on your crankshaft?
 

fireball

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[ QUOTE ]
ignore water gushing in through 30cm hole

[/ QUOTE ] - what log have you got? Big wooden one sawn off an oak tree??! We are talking normal yachts - not the superyachts!!
Anyway - sensible thru-hull fittings have a little flap that closes off the hole reducing the inflow to a small bubbling.
 

Porthandbuoy

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It looked like a 30cm hole! Must have been the magnifying effect of the column of water. Wasn't on my current boat anyway; she's just a little one sawn of an oak tree & miscellaneous tropical harwoods.
 
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