Dripless Shaft Seals

[greysailor]

Any statistics on failure rate? Any numbers on failures leading to sinking?

Might be difficult to amass reliable data regarding failure rates.....We've all heard a few stories of boats scrambling to haulout yards because of a leaking seal system, but who knows how many of these stories are from direct observation or "echo chambers".
I personally only know first-hand of one boat having a bellows failure on a PSS seal (the only design I'm familiar with). That boat had longer shafts than our DeFever 44, and had a history of shaft vibration necessitating multiple shaft removals (which requires disturbing the compression of the shaft bellows). I believe that boat's original seals were a little over 15 years old at the time one failed.
I've owned two sailboats with PSS seals, sold both at age aprox 10-15 years old, with no signs of cracking on the original shaft bellows.
Who knows what environmental conditions exist on boats that have failures.....Overcharged/out-gassing batteries will degrade rubber, so will ozone generators that are used to remove boat smells on brokerage boats.
Salty is 15 years old now, with the original PSS seal bellows. They are still soft and pliable with no cracking evident. However, we are hauling out next month for a much needed bottom job, and I'll be replacing the shaft bellows as a precaution. Easy job in a DeFever engine room.....the wildcard will be busting the tapered shaft coupling loose from the shaft![emoji51] I fabricated a puller from 3/4" plate and grade 8 threaded rod, and have torch ready to party![emoji28]

 

[mvweebles]

Any statistics on failure rate? Any numbers on failures leading to sinking?

I know of no empirical evidence, only anecdotal. I have one personal experience with a 'failure.' I use quotes because I'm 95% sure it was improper install. PSS system - the annular ring has a double/stacked set screw. The ring had backed-off allowing an alarming amount of water intrusion. Took me a minute or so to figure out what happened - the second set screw was nowhere to be found, and the primary set screw backed-out. Was fixed a minute or two later with a hose clamp backing the pressure ring.

If access to the stuffing box is decent and maintenance is easy, I prefer traditional stuffing boxes with modern packing. If access sucks (many v-drives), dripless starts to make sense.

Peter

 

[psneeld]

A common theme on TF is that weak links should be eliminated and potential problems with significant dangers should be carefully thought through and redundancy is golden.

Electrical, plumbing, ground tackle...heck all different possible weak links get the thumbs down by many here.

Well...even a brand new PSS bellows can have a factory defect and let go at any moment.

So..... worry about a dry bilge 100% of the time or worry about a ticking bomb in your bilge?

PS...if worried about a dry bilge...read up on the dry bilge thread.

 

[Hippocampus]

Best system I ever saw was on a steel Puffin sailboat. Shaft log was fairly long and filled with grease. There was a nipple to allow its replacement. Shaft was held by bearing at start, end and I think halfway. Thing did leak a minute amount of grease I could see. Owner said he refilled it with fresh grease at every haul but that was it for servicing. Don’t know anything further about it.

 

[Hippocampus]

I had PSS on the last boat. I had it serviced with replacement of the bellows and had a more lubricating water added at 5 years. Prior given it was a sailboat it had no pump pressurized source. View this part as as mission critical as I do long ocean passages.
Today put an offer in on a nordhavn. Of course have been looking at sisterships. None have a traditional stuffing box. All dripless. Find it unusual that this family of vessels which can’t run to a travel lift if failure occurs have made that decision. The majority are PSS with a few Tides. Wonder if that’s marketing or is the PYI device better?

 

[mvweebles]

I had PSS on the last boat. I had it serviced with replacement of the bellows and had a more lubricating water added at 5 years. Prior given it was a sailboat it had no pump pressurized source. View this part as as mission critical as I do long ocean passages.
Today put an offer in on a nordhavn. Of course have been looking at sisterships. None have a traditional stuffing box. All dripless. Find it unusual that this family of vessels which can’t run to a travel lift if failure occurs have made that decision. The majority are PSS with a few Tides. Wonder if that’s marketing or is the PYI device better?

I'm surprised at the dripless on nordhavn. I probably delivered 4 dozen new ones. They all had traditional stuffing boxes with two smaller bolts on the pressure plate (vs a giant gland nut). Perhaps the dripless is an after thought?

Stuffing box access is excellent on nordhavn. I personally strongly prefer the traditional with modern packing.

Congrats on finding a potential dance partner. Hope the transaction goes smoothly. We'll miss you here at kiddy table if you join the NOG

Peter

 

[Hippocampus]

Also looked at David Gerr boat and as backup now looking at some KKs. But it seems regardless of original spec what’s in use now is a dripless. Would note if you google this subject it seems virtually all commercial ships use dripless as well. Apparently there’s less concern if your not dealing with skinny water. Similarly only the foolish wouldn’t do a a test run (or several) after swapping out parts before going on passage.
Just had shaft alignment adjusted prior to sale of the last boat as you could feel something. It was off 3/1000” of an inch. Detectable by feelers but not visible watching while running. Just at the edge of it worth adjusting. Think in a trawler you would get use to how it should feel pretty darn soon so misalignment as a cause for failure is a stalking horse.
I don’t think this is about a dry bilge. People put check valves in their bilge pump runs. I don’t. Would rather put up with a wee bit of water than have anything that could impede flow when they’re truly needed. From what I can gather catastrophic failure is much talked about but little seen. That’s why I continue to be most interested in the real numbers of failures.
I think regardless of brand if your shaft alignment is reasonable, you change out what’s necessary every 5 years , your use doesn’t exceed 10,000m/y and it’s not subjected to a difficult environment (excessive heat, chemicals etc.) dripless isn’t the failure point that’s going to sink your boat. View it similar to hitting moose in Maine or. bison in Sturgus. Yes it does happen but it’s very unlikely with a modicum of good behavior on the part of the operator.

 

[psneeld]

Maybe Nordhavns with their watertight bulkheads think the potential problem, with the bulkheads and rarity of a total bellows failure they can live with the new tech.

Unlikely but possible...roll the dice.....

 

[mvweebles]

Also looked at David Gerr boat and as backup now looking at some KKs. But it seems regardless of original spec what’s in use now is a dripless. Would note if you google this subject it seems virtually all commercial ships use dripless as well. Apparently there’s less concern if your not dealing with skinny water. Similarly only the foolish wouldn’t do a a test run (or several) after swapping out parts before going on passage.
Just had shaft alignment adjusted prior to sale of the last boat as you could feel something. It was off 3/1000” of an inch. Detectable by feelers but not visible watching while running. Just at the edge of it worth adjusting. Think in a trawler you would get use to how it should feel pretty darn soon so misalignment as a cause for failure is a stalking horse.
I don’t think this is about a dry bilge. People put check valves in their bilge pump runs. I don’t. Would rather put up with a wee bit of water than have anything that could impede flow when they’re truly needed. From what I can gather catastrophic failure is much talked about but little seen. That’s why I continue to be most interested in the real numbers of failures.
I think regardless of brand if your shaft alignment is reasonable, you change out what’s necessary every 5 years , your use doesn’t exceed 10,000m/y and it’s not subjected to a difficult environment (excessive heat, chemicals etc.) dripless isn’t the failure point that’s going to sink your boat. View it similar to hitting moose in Maine or. bison in Sturgus. Yes it does happen but it’s very unlikely with a modicum of good behavior on the part of the operator.

I mostly agree with this. Catastrophic failure is often cited, but actual first hand experience is rare - as mentioned, I did have the collar back away in a PYI and it was dramatic. I believe it was faulty install, bit a failure. Besides, easily fixed albeit a wet ordeal. Contrast with frequency of scored shaft or wet bilge masking other issues, and the balance tips to dripless for many.

But I still prefer traditional gland with modern packing. Gland itself has a 40+ year service life vs 5-7 for dripless. Traditional needs fairly frequent tweaking, but for me it's just part of my routine. Hard to break after all these years.

As many have said, keeping a dry bilge - as in bone dry, is not that difficult. That and a very clean engine will express potential errors very early, long before failure.

Peter

 

[psneeld]

Maybe we only hear of catostophic failures occasionally...but we DO hear about the failures because of the results being newsworthy.

Like total loss fires and sinkings (newsworthy) as opposed to all the other problems boaters find and fix ( not newsworthy).

 

[mvweebles]

PSN - I know they are out there, but in the dozens of threads I've read - and I read them closely as the topic interests me, all but one "catastrophe" post was a variation of I-know-a-guy-who-knows-a-guy. The bellows on the PSS is extremely robust and it's not really possible for anything to seize and cause a massive failure. As long as they are installed properly, they are reliable. I'm less thrilled with the lip seal version.

Risk of scored shaft and wet bilge have to be balanced along with access and owners proclivity to do maintenance. For me, I like traditional. I actually purchased a PSS but couldn't bring myself to install it even though I had the shaft out. But I certainly understand going dripless for many people. Just not me. Don't really know why, but they just aren't for me.

Peter

 

[psneeld]

2nd hit on google and similar to my experience as a assistance tower. (not my quote). My tow boat has several different issues over 14 years, but they would be abnormal for a rec boat, but not impossible.



https://www.passagemaker.com/technical/dripless-shaft-seals



"As an operator of a TowBoatU.S. rescue and towing boat, I have responded over the last four or five years to a surprising number of assistance calls from boats that were sinking because a dripless shaft seal had failed and caused flooding."


third on google...


http://www.themaritimesite.com/dripless-mechanical-shaft-seal-inspections-failures/



This damaged shaft seal (above) was detected by an eagle eyed marine mechanic in Downeast Maine. It’s hard to see in the pictures, but if you look VERY closely you can see a hairline crack running through the whole length of the hose connection threads and out around the stator body. Had this crack gone undiscovered it could have resulted in a severe flooding situation jeopardizing the safety of the boat and its passengers."


Again rare occurances, but they happen....roll the dice....

 

[mvweebles]

2nd hit on google and similar to my experience as a assistance tower. (not my quote). My tow boat has several different issues over 14 years, but they would be abnormal for a rec boat, but not impossible.



https://www.passagemaker.com/technical/dripless-shaft-seals



"As an operator of a TowBoatU.S. rescue and towing boat, I have responded over the last four or five years to a surprising number of assistance calls from boats that were sinking because a dripless shaft seal had failed and caused flooding."


third on google...


http://www.themaritimesite.com/dripless-mechanical-shaft-seal-inspections-failures/



This damaged shaft seal (above) was detected by an eagle eyed marine mechanic in Downeast Maine. It’s hard to see in the pictures, but if you look VERY closely you can see a hairline crack running through the whole length of the hose connection threads and out around the stator body. Had this crack gone undiscovered it could have resulted in a severe flooding situation jeopardizing the safety of the boat and its passengers."


Again rare occurances, but they happen....roll the dice....

This is helpful - fact based with specific recommendations on what to check. I hadn't heard of cracked rotor failure. Thanks PSN.

 

[Northern Spy]

I see mechanical seals fail on pumps often. Even in a high maintenance environment.

Been installing, maintaining, specifying and troubleshooting rotary mechanical seals and packing systems for 35 years on naval ships, chemical plants, and heavy industry.

When mechanical seals work, they work fine. When they fail, well, they don't work fine at all. They have very short P to F interval; almost digital. It is fairly easy to define "failure."

Packing is more analog, gives a indication of when it needs service, but is harder to define failure, because leakage is normal.

So I'll have packing on my boats, unless it is in an inaccessible location, or I suddenly feel the need for recreational periodic maintenance.

Mind you, shaft sealing on a recreational boat is an easy task. Trawler shaft mechanical seals have a low fpm, relatively benign fluid, and extremely low pressure differential, without immediate consequences of failure. But I really see no need for the added complexity and cost.

 

[Frosty]

I have seen one boat brought into the boatyard for work after it sank due to failed dripless seal. I wouldn't swear to it now (this was ten years ago), but I think it was a bellows failure. It was something like a 37-40' Tartan IIRC.

Not sure if it sank at the dock or while cruising, but it did sink. Shallow area in general, so since it had a deep draft I'm going to guess it didn't go "out of sight to the bottom" and was thus able to be raised fairly easily. Came in for a full rebuild of things (mud in the interior, the whole nine yards).

So in my (limited experience) they may not fail often, but can fail catastrophically. Perhaps due to a lack of maintenance? OTOH, I don't know of any sinkings due to a traditional stuffing box (maybe someone else does though).

The bilge was dry.... until it wasn't.

Not saying no-one should get a dripless seal. Odds are my next boat will come to me with one. Just responding to the question of if anyone knows of a failure.

 

[rslifkin]

On the idea of regular stuffing box failures, how often do those thick rubber hoses that attach them fail? I never seem to hear of that as an issue, even when they're really old.

 

[Frosty]

On the idea of regular stuffing box failures, how often do those thick rubber hoses that attach them fail? I never seem to hear of that as an issue, even when they're really old.

I haven't seen that personally, although I did buy a boat once that had "regular" exhaust type hose instead of bona fide stuffing box hose (eg Buck Algonquin 5-ply). When I removed the lesser hose to upgrade it wasn't looking any too healthy (cracks, etc.). The clamps were also run of the mill and not looking too good. Oh, and the stuffing box and stern tube weren't the same size, so the hose fit the larger diameter and was "pouched down" to the smaller size. Boat hadn't sunk though. Felt good to replace it all (and correct the size discrepancy).

 

[Ski in NC]

I do failure investigations in the marine world. I have investigated several flooding incidents due to dripless seals. Some of them have a plastic bushing that requires cooling water. Lose that and bushing can get hot and seize to shaft. Then seal spins with shaft and rips the bellows. That's a big damn leak!! Not easy to determine if cooling water flow is adequate, until it is not.

One of these incidents occurred when a boat was being towed due to a grounding and there was zero cooling as props windmilled. Tow crew tied it to a dock and next morning it was flooded.

For those reasons my personal boat uses an old skool packing gland with teflon packing. Drips a bit now and then, but drips go to AC sump, not the bilge. No cooling water line needed at all, and runs cool. 2700hrs on it and never needed a re-pack. Have snugged it a couple of times.

 

[motion30]

I have a question, how can you install a dripless seal if your current stuffing box is rigidly mounted and there is no hose log involved in the system ?