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Old 11-04-2014, 05:08 PM   #21
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Thanks Jeffrey. Their instructions are pretty clear with respect to the vent being sufficient for speeds < 12 knots. My issue was that when the shaft was pulled, the stern tube had a lot of what the yard described as "crud", and suggested that this crud may have diminished the life of the cutlass bearings. As a result, I'm thinking that if I have some positive pressure moving water through the tube, it may solve that problem. Since this is all somewhat theoretical, I'm not sure PYI can say more than they have about positive flow not being needed to prevent overheating at my speeds. I am trying to solve a slightly different problem.
I think the challenge is that you need water for two devices; the PYI seal and whatever cutlass bearings you have. The initial answer would seem to be to provide for whichever device needs more water. Hopefully that isn't in conflict with the other device.
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Old 11-04-2014, 06:44 PM   #22
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I think the challenge is that you need water for two devices; the PYI seal and whatever cutlass bearings you have. The initial answer would seem to be to provide for whichever device needs more water. Hopefully that isn't in conflict with the other device.
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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Old 11-05-2014, 06:06 AM   #23
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>I can't see much difference between the Duramax seal and the PYI design<

Sorry my suggestion was if the boat is old enough and still has the setup for flax packing , and the bellows and ceramic seal were an add on,going to Duramax PACKING solves the problem.

After 1st adj they go hundreds of hours with narry a drop.
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Old 11-05-2014, 06:15 AM   #24
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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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Old 11-05-2014, 06:30 AM   #25
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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
You can adjust it so that it has as FF said..."nary a drop" if you have relatively slow turning shafts.

Yet even on my sportfish I had it so that it was more ooze than drips. The Shafts are just fine after thousands of hours.

Having constant drips may be necessary for some to stay cool, but not all in my experience.
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Old 11-05-2014, 06:34 AM   #26
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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.
My understanding is that much depends on the exact arrangement of the boat's drive line. By way of example, when Nordhavn extended the running gear on the 60, the shaft log/tunnel became longer. I gather there is a half cutlass inboard and with the extra tunnel length there was not enough natural water circulation, so a thruhull pickup was added to feed water into the PYI and also to provide more positive flow to the cutlass. I had asked about a forward scoop on the thruhull and was told PYI recommended against that since it would create too much water pressure.

This is just one example, but I think it illustrates that there are a lot of factors at play. Shaft RPM, boat speed thru water, number of cutlass bearings, bearing's distance to free water, etc. There are probably more.

I like Jeff's advice about calling PYI, but I'd make a little sketch of the running gear setup with dimensions so be sure I have all this info. Then maybe call the cutlass bearing manufacturer with the same info and see what they say.
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Old 11-05-2014, 09:52 AM   #27
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My understanding is that much depends on the exact arrangement of the boat's drive line. By way of example, when Nordhavn extended the running gear on the 60, the shaft log/tunnel became longer. I gather there is a half cutlass inboard and with the extra tunnel length there was not enough natural water circulation, so a thruhull pickup was added to feed water into the PYI and also to provide more positive flow to the cutlass. I had asked about a forward scoop on the thruhull and was told PYI recommended against that since it would create too much water pressure.

This is just one example, but I think it illustrates that there are a lot of factors at play. Shaft RPM, boat speed thru water, number of cutlass bearings, bearing's distance to free water, etc. There are probably more.

I like Jeff's advice about calling PYI, but I'd make a little sketch of the running gear setup with dimensions so be sure I have all this info. Then maybe call the cutlass bearing manufacturer with the same info and see what they say.
Good advice, thank you. Delfin's stern tube is around 6' underwater so filling it isn't a problem. When you burp the PYI it is like you just took a torpedo given the amount of water released. Once I talk to PYI, I'll let everyone know what they say.
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Old 11-05-2014, 10:03 AM   #28
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Delfin's stern tube is around 6' underwater so filling it isn't a problem. When you burp the PYI it is like you just took a torpedo given the amount of water released. Once I talk to PYI, I'll let everyone know what they say.
My understanding is that it's not just about getting water in there, but also about the degree to which fresh water circulates through, either naturally or via some sort of forced flow. Stainless will corrode in the presence of stagnant sea water, where it holds up well with freshly oxygenated water. It's counter intuitive that well oxygenated water reduces corrosion, but that's what I understand is the case with stainless. And water circulation is key to cooling as well, both for the seal and the cutlass. My understanding is that on the N60 getting water in there was not the problem, but that there was insufficient circulation to properly cool and/or prevent corrosion.

I'll be real interested to hear what you learn from the manufacturers.
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Old 11-05-2014, 10:05 AM   #29
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Originally Posted by Delfin View Post
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.
Maybe an occasional rinsing is all that's needed to keep the stern tube clean. What about using raw water from the genny attached to the vent line with a 3-way valve. You can make valve position #1 vent only as you currently have and position #2 is rinsing, where water from the genny is injected and the vent is closed.
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Old 11-05-2014, 10:16 AM   #30
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>I can't see much difference between the Duramax seal and the PYI design<

Sorry my suggestion was if the boat is old enough and still has the setup for flax packing , and the bellows and ceramic seal were an add on,going to Duramax PACKING solves the problem.

After 1st adj they go hundreds of hours with narry a drop.

He is not having a issue with the PYI itself.. just the vent.. why would he go backwards and go with a conventional stuffing box.. duramax or not?? what next.. get rid of the electric windows and door locks in his car?.

Some here seem to have the chicken little syndrome regarding shaft seals.. I do not think boats are sinking all over due to the "new" type of shaft seals.. and they have been in use for over 20 years.

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Old 11-05-2014, 10:25 AM   #31
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Originally Posted by Delfin View Post
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.
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..

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Old 11-05-2014, 01:47 PM   #32
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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.
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Old 11-05-2014, 02:00 PM   #33
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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.
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Old 11-05-2014, 03:16 PM   #34
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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 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!
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Old 11-06-2014, 06:55 AM   #35
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>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!
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Old 11-06-2014, 08:01 AM   #36
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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
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Old 11-10-2014, 07:08 AM   #37
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>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.
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Old 11-10-2014, 07:39 AM   #38
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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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Old 11-10-2014, 07:54 AM   #39
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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
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.
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Old 11-10-2014, 08:44 PM   #40
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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
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