PYI shaft seal vent question

Delfin said:

If water is injected through the vent tube, it would lubricate both the cutlass bearings and the sealing plates of the PYI. If water isn't injected, at slow speeds the stern tube just fills up with water and lubricates both of these again, so I don't think injection is required; just perhaps desirable if the stern tube is to be a little cleaner.

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Delfin,
Was the crud sea growth or corrosion? What about connecting the vent to plumbed water and giving it a rinse after the boat is run. Also how since the shaft had been out and the stern tube had actually been inspected? If the build up was five years or soin the making it may not be an issue.

I do wonder is through friction,cavitation,negative pressure due to the hull moving forward etc. if it is possible to build up a air bubble at the water side of the PYI and the vent is also there to address this?

I cannot see this happening but I am no hydro engineer..

HOLLYWOOD

I have Duramax packing on my sporty. Put in 5 years ago. With NO drip it never gets even warm, even at 25 knots. It is simple to install and requires ZERO maintenance. No water in the bilge below the shaft. So,,,, if you've never used it you cant really call it a "step backwards" can you. On the other hand I dropped a pipe wrench on a carbon seal, cracking/chipping it, and had to get the boat hauled to fix it, at my expense. I have read a few accounts of "dripless" seals seizeing and sinking boats, lip type and carbon face. You could say I'm not a fan of them.

hollywood wrote "why would he go backwards and go with a conventional stuffing box.."

There's a reason why they are called "conventional". They are standard fare for small boat drivelines and certainly not "a step backwards".

Being considered "high tech" does not make them better.

I almost changed back from a "PYI seal to a conventional stuffing box on my recent refit. There are advantages to both .. and disadvantages.

hollywood8118 said:

Delfin,
Was the crud sea growth or corrosion? What about connecting the vent to plumbed water and giving it a rinse after the boat is run. Also how since the shaft had been out and the stern tube had actually been inspected? If the build up was five years or soin the making it may not be an issue.

I do wonder is through friction,cavitation,negative pressure due to the hull moving forward etc. if it is possible to build up a air bubble at the water side of the PYI and the vent is also there to address this?

I cannot see this happening but I am no hydro engineer..

HOLLYWOOD

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I wondered the same thing about water bubbles, and yes, that is what the tube is supposed to do if cavitation bubbles enter through the stern cutlass.

Unfortunately I wasn't there when the yard pulled the shaft and they just described it as a "bunch of crud". Not growth, just debris, which I suspect may contaminant the stern tube when we are running in very low water. This happens in the marina, has happened going up the Fraser, etc. I called PYI and they confirmed that no, injecting water wasn't necessary for slow moving craft, but yes, if you were getting rubbish in the shaft it would be a good idea to provide some flow. I specifically asked about simply plumbing it into the through hull and they confirmed that a number of vessels did this, that the flow was adequate to flush the tube and would do no harm from a corrosion standpoint as TwistedTree indicated. So that seems to be the simple solution - a 3/8" line from the through hull to the vent providing continuous flow.

I also did think about the flushing idea using even fresh water and that might be just as good, but this sounds simple, has no moving parts and apparently has been used successfully by others before.

Many thanks to all for helping me think this through!

>If you have a stern gland with packing, as opposed to a mechanical seal, you should ALWAYS have a slight leak inboard. If you tighten the packing so as to entirely stop the leak you will wear your shaft. The "leak" lubricates the packing<

Perhaps a few decades ago this was true , today NOPE!

I realise that rhetoric trumps technical knowledge on this site but packed glands ARE decades old technology, all thats changed is the packing.
You can stuff as much teflon or graphite or whatever you like into the packing but the coefficient of friction 'wet' is much less than 'dry', the friction gives you heat and losses and the dirt and wear products that get stuck in there will eventually give you wear, but you carry on with your over tightened dry packing...happy new shaft

>the friction gives you heat and losses<

Perhaps with flax or teflon , decades past this was true , but the folks that USE the Duramax and other similar packing find this to be false.

Sand grit and other garbage coming in with the cooling water can imbed in flax and wear the shaft .

The modern packing material does not have this problem as it requires NO water to have a low coefficient of friction , so no slurry intrudes.

The folks that USE the system seem to be happy, I guess they dont know as much as you.

so lets get this right....you are saying
1.that the coefficient of friction for this packing material "dry" is the same as when it is in a lubricated condition ?
2.that friction does not lead to heat and losses ?
3.that sand and garbage come in through the cooling water supply ?
I give up

stone beach said:

so lets get this right....you are saying
1.that the coefficient of friction for this packing material "dry" is the same as when it is in a lubricated condition ?
2.that friction does not lead to heat and losses ?
3.that sand and garbage come in through the cooling water supply ?
I give up

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If you read closely..I never said dry...just not actively dripping.

It's possible to get moisture to the inside without a steady drip...even a very long term drip.

There is obviously moisture there as it can be seen, the shaft runs cool and there is no abnormal wear after thousands of hours.

I can't say all vessels can be set up this way...but until you try and monitor...you will never know. It's worth the try as little or no water regularly coming in isn't a bad thing and eventually it will...and a slight tightening will put you back into the ooze situation rather than the drip.

I really couldn't care what the coefficient of friction is if both ooze and dripping are the same...with no wear or heat...must be good enough and without the drip is a plus.

psn, i didnt take any issue with what you posted....but there are some serious misunderstandings I would like to help overcome on this topic, better understanding is helpful to all isnt it ???

there is undoubtedly friction at play within the packing / shaft interface, whether it is a conventional gland or a mechanical face seal type

friction is undoubtedly different wet or dry...thats why cars skid on wet roads....thats why dry rubber us good for car tyres (high friction condition) and wet rubber is good for propellor shaft bearings, (low friction condition when water lubricated)

the op is concerning water feed to a mechanical seal, it is a good idea in all cases but not strictly neccessary in lower speed applications, the water supply is to lubricate the seal, cooling is not its primary purpose, although of course if lubricated, and therefore in low friction condition it doesnt get hot....but not because it has been cooled, it is because it has been lubricated

in a conventional packed gland the same is the case, ensuring the packing is lubricated ensures it is operating with lower friction and this gives rise to less heat

the point about dry operation is more to do with not overtightening the gland. this is the primary reason for running with a small leakage, oozing, dripping etc is splitting hairs, all conditions where some amount of the liquid is passing through the gland are correct, they will all ensure the packing is wet, therefore lubricated and the friction is minimised for the packing in use, whether it is modern magic stuff or old fashioned stuff

unequal tightening and overtightening of a conventional gland are the most serious issues...once you have tightened enough to squeeze the packing into all the voids within the stuffing box and it is just tight enough on the shaft to stop the leak you should back the nuts off a tad, one flat or so and lock them, in that condition the gland packing will be lubricated, friction will be as low as possible and there will be some amount of leak but you are in control of the pressure you are applying onto the shaft, axial compression of the packing is converted into radial compression by the gland and you want this to be as low as possible. tightening beyond the "just dry" point you are overtightening and applying a brake to the shaft ....and this is the issue here...at all conditions of tightening beyond "just enough" the sahft is dry...there is no way of knowing if it is mildly overtightened, not much problem .....or seriously overtightened, the road to failure...dry is dry . therefore some leak or ooze and even a drip when it is running is much better than dry. yes the perfect technician can get it to run dry, the avergae guy will overtighten if trying to achieve the same condition

As far as I know for some of the new style packing mechanical seals such as the PSS... the carbon rides the stainless and it is a water free/dry seal...

When we were discussing old style compression packing...with at least the sorta new teflon packing...not even the really friction free stuff...even a drip can be eliminated by an amateur which is what I was with my old boat.

Lets not proliferate a myth that doesn't apply all the time.

I am not the one proliferating myths...I have given you a technically based logical argument, read any text book on the topic, you will find they largely agree with what I have written, i didnt invent this stuff.....it is standard engineering practice

friction free however is a myth and friction when dry being the same as friction when lubricated (for any given material) is another

if you want to tighten your stern gland until it is fully dry thats completely up to you, however, proliferating that as "advice" is all i am cautioning against with a supporting explanation

finally, a pss seal will not perform satisfactorily if run dry.....why do you think they want the water supply (which started the post) ??? primary reason is to ensure the seal is operating in a lubricated condition

stone beach said:

If you have a stern gland with packing, as opposed to a mechanical seal, you should ALWAYS have a slight leak inboard. If you tighten the packing so as to entirely stop the leak you will wear your shaft. The "leak" lubricates the packing

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Not really. With GFO packing you can run it dry. I've done it on countless boats and found the box will run nice and cool and there is no scoring of the shafts.

I worked oilfield for 30 years or so and must have repacked a thousand centrifugal pumps, most used for drilling mud with a high viscosity and solids content. The "drip" used to be a spray within a few days requiring constant monitoring and tightening. About every 5 - 10 repacks, you would change the impellor, shaft and both wear plates as they were totally eroded. Then came the EPA with drip pans under each pump, expensive bromides etc and I guess someone decided to reengineer the decades old packing. Now they no longer leak although you still have the wear and tear on the inside of the pumps. So I think everyone is right, just perhaps not current. I sure don't believe they reengineered the packing for us sailors but we reap the benefit. I have it on my boat, monitor with a temp gun and have had no issues. I believe if I had a constant drip into my bilge I would change to the "dripless" system but as I am already "dripless", don't see the need.

>in a conventional packed gland the same is the case, ensuring the packing is lubricated ensures it is operating with lower friction and this gives rise to less heat.

.I have given you a technically based logical argument, read any text book on the topic, you will find they largely agree with what I have written, i didnt invent this stuff.....it is standard engineering practice<

The hassle is the new material has so little friction DRY that no heat rise is created

Folks with heat guns see the shaft temps Drop after the change from flax or teflon.

With no need for a cooling flow the grit in the lube water does not get between the packing and shaft , where it gets imbedded in packing and scores the shaft.

I have no idea of the Duramax would run cooler , if it was set up to drip underway ,

but since most operate at local water temp, who would care?

Mate here's my PYI PSS. it came with the hose nipple. The "vent" is so you don't need to burp it. Going to or around 12knots doesn't require water injection.

Don't over complicate it.

IMHO, I would be more concerned about the collar and the clamps. When i bought the seal I immediately removed the worm screw clamps and fitted t bar clamps and fitted the shaft retention collar.

They are more critical than a breather line that is redundant at your speeds.

Im still building the boat so can't comment on the performance of the product, just what I've learnt

Hope it helps.






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dimer2 said:

I worked oilfield for 30 years or so and must have repacked a thousand centrifugal pumps, most used for drilling mud with a high viscosity and solids content. .

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Same for me but in mining related processing applications, with pumps as big as 5000 hp. The amount of slurry released by a "standard" packing system was so large that dripless systems with filtered "fresh" water for lubricating the gland seal became common if not mandatory decades ago. When dealing with corrosive slurries a drip at the gland is not acceptable so dripless systems utilizing all manners or seals came to be.

A properly installed dripless system is not a new design and like anything on a boat requires watching and acceptable access. Ditto duramax or hemp!

Delfin, did your shaft show any problems on your recent haulout? If not it seems your system is not suffering from stagnant water related ills. But when parked at the dock several useful approaches have been noted. Salt water intrusion into a very nice steel hulled boat though is not something I would do on a routine untended basis, maybe just once every few days and done manually into a shower sump??

psneeld said:

As far as I know for some of the new style packing mechanical seals such as the PSS... the carbon rides the stainless and it is a water free/dry seal...

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The PYI seal depends on a water interface between the carbon ring and the sealing plate for lubrication. It is only a few molecules thick, but it is there. There is no drip because this water gasket has no flow. The vent, if used as a point of water injection pushes water into the stern tube, not between the carbon and the seal. The system is burped when you first launch to get the air out and to get the water gasket established. After that the main reason to inject water, according to PYI, is if the boat is moving fast enough to create negative pressure at the stern cutlass bearing that would cause evacuation of the water in the stern tube. The secondary reason that we've discussed is to have a slow flow of water to purge contaminants out of the tube to reduce wear on the bearings, which is why I've decided to plumb a line from the through hull to the vent - a practice PYI said was not uncommon for the reasons stated.

Hendo78 said:

Mate here's my PYI PSS. it came with the hose nipple. The "vent" is so you don't need to burp it. Going to or around 12knots doesn't require water injection.

Don't over complicate it.

IMHO, I would be more concerned about the collar and the clamps. When i bought the seal I immediately removed the worm screw clamps and fitted t bar clamps and fitted the shaft retention collar.

They are more critical than a breather line that is redundant at your speeds.

Im still building the boat so can't comment on the performance of the product, just what I've learnt

Hope it helps.

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Hendo, if you read your instructions you will see that you definitely need to burp it on splash. The yard didn't the first time I launched and after around 2 minutes of run time the squeal from the shaft on the dry cutlass bearings was pretty horrendous. The vent was clear but for whatever reason, the stern tube had insufficient water in it.

Burp the baby.

sunchaser said:

Same for me but in mining related processing applications, with pumps as big as 5000 hp. The amount of slurry released by a "standard" packing system was so large that dripless systems with filtered "fresh" water for lubricating the gland seal became common if not mandatory decades ago. When dealing with corrosive slurries a drip at the gland is not acceptable so dripless systems utilizing all manners or seals came to be.

A properly installed dripless system is not a new design and like anything on a boat requires watching and acceptable access. Ditto duramax or hemp!

Delfin, did your shaft show any problems on your recent haulout? If not it seems your system is not suffering from stagnant water related ills. But when parked at the dock several useful approaches have been noted. Salt water intrusion into a very nice steel hulled boat though is not something I would do on a routine untended basis, maybe just once every few days and done manually into a shower sump??

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Thanks Tom. The shaft was fine. Run out was less than .002 as I recall - not enough to worry about according to Townsend Bay Marine. It was actually Paul Z, manager of the yard, who made the recommendation that I "think" about water injection. The concern was not the shaft but the cutlass bearings, where detritus would increase wear. They only lasted 5 years, so I think there is reason to be concerned.

I asked about corrosion from introducing oxygenated water and was told by PYI that it wouldn't be an issue, as did the yard. The capper for me is the vent tube, which I consider the weakest part of the entire system. If it failed, you would have a 1/8" hole in your boat. Not something the bilge pumps wouldn't handle, but still. Given that, plumbing it into the through hull, which I always close on leaving the vessel, seems like a no change situation, flooding wise, but an improvement in longevity of the cutlass bearings. I have read about systems for coating the inside of the stern tube, but every time I have asked a yard they give me blank stares.

Paul Z at the yard also told me something I really wasn't aware of, but seems to be true. He said that sea water a couple feet down has so little oxygen in it anyway that the corrosion rate is minimal. Didn't seem right to me, but we had one area where I had to move the through hull to accommodate the stabilizers and it caused a failure of the epoxy barrier coat around 4 feet under the water line. Scratch at it with your finger, and you can reveal essentially uncorroded, but unprotected steel, so I think he is right, at least in our cold PNW waters.

Delfin said:

The PYI seal depends on a water interface between the carbon ring and the sealing plate for lubrication. It is only a few molecules thick, but it is there. There is no drip because this water gasket has no flow. The vent, if used as a point of water injection pushes water into the stern tube, not between the carbon and the seal. The system is burped when you first launch to get the air out and to get the water gasket established. After that the main reason to inject water, according to PYI, is if the boat is moving fast enough to create negative pressure at the stern cutlass bearing that would cause evacuation of the water in the stern tube. The secondary reason that we've discussed is to have a slow flow of water to purge contaminants out of the tube to reduce wear on the bearings, which is why I've decided to plumb a line from the through hull to the vent - a practice PYI said was not uncommon for the reasons stated.

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I have one on the assistance towing boat and have been using it and repairing it for 12 years.

I couldn't locate whether there was a water layer or not, I thought if it were a decent carbon seal it wouldn't need it as some mechanical seals don't.

But the point is whether carbon against stainless or Teflon against a stainless shaft...the water layer needs only be enough to keep friction at bay....not necessarily a drip and that's all I was pointing out from the beginning.

Delfin said:

Hendo, if you read your instructions you will see that you definitely need to burp it on splash. The yard didn't the first time I launched and after around 2 minutes of run time the squeal from the shaft on the dry cutlass bearings was pretty horrendous. The vent was clear but for whatever reason, the stern tube had insufficient water in it.

Burp the baby.

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Ok I'll burp it but only cause you said so mate ?


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