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Old 11-02-2014, 08:43 PM   #1
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PYI shaft seal vent question

I would very much appreciate the experts' opinion on a PYI shaft seal question. My wheel only turns around 400 rpm through the Twin disc, and I have gone a year or more with the vent closed off after burping the seal. The question is whether there is any reason not to keep it closed, or whether it should have water injected into it, or whether to route the hose attached to it to a higher location. The latter question relates to the fact that when powering at hull speed, the stern will sink low enough that the overflow attached to the tube will begin to fill up and overflow itself because it is not mounted high enough. It isn't practical to mount the overflow higher than it is, so if I fixed that, I would have to drill a hole through a steel bulkhead so as to be able to route the vent line to a location well above the waterline under all power conditions.

But I'm asking myself, why bother? At such low speeds I have never detected any heating of the water in the stern tube using an IR gun, and the only reason to think about injecting water is that when we pulled the shaft recently, the stern tube was full of crud. Presumably, if I injected sea water into the vent, this would be somewhat reduced, but again, do I care? By injecting fresh sea water into the stern tube, won't I accelerate corrosion because the water injected would have higher oxygen than the stagnant water that probably sits in it now. Or, should I just extend the vent so it doesn't overflow? Or should I just keep the darn thing plugged and find something else to worry about? Like all those logs Marin says are out there......

One more question - do these jeans make my butt look big?
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Old 11-02-2014, 09:10 PM   #2
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I've never heard of it being referred to as a vent. Thought they were always supposed to be connected to the raw water system on the engine. Have had one on my charter boat for 12 years and am putting one on the trawler as we do the repower. Can't think of a reason not to connect it to the engine. Like the idea of a constant flow of water going through the stern tube and cutlass bearing.

Ted
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Old 11-02-2014, 09:17 PM   #3
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Mine isn't vented ... that I know about.

There is a tube/hose that comes off of the sea water injection into the exhaust riser. This hose brings sea water to the PYI seal where it enters the stern tube and returns to the sea through the flutes in the stern bearing (cutlass bearing).

You must have no water injected into your stern tube. Why would you want that over water injection? They said I didn't need it but I liked the idea of having fresh sea water flowing over/through the stern bearing. Being a friction bearing I thought the sea water cooling may help w bearing life and after 10 years and about 1000hrs it really didn't need replacing but w R&R'd the shaft and I decided it was time to service the friction face of the PYI seal.

Never had a problem w the system except for the SS collar moving fwd on the shaft and causing a bit of flooding. That was easily fixed.

Oh I forgot .. I've "burped" the shaft every now and then especially after hauling out, being on the grid or not being under way for a time. I just (w my hands) move the carbon fibre bearing that faces the SS collar aft a bit allowing a cup or so of water to spurt into the boat and fall into the bilge.
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Old 11-02-2014, 09:52 PM   #4
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I've never heard of it being referred to as a vent. Thought they were always supposed to be connected to the raw water system on the engine. Have had one on my charter boat for 12 years and am putting one on the trawler as we do the repower. Can't think of a reason not to connect it to the engine. Like the idea of a constant flow of water going through the stern tube and cutlass bearing.

Ted
PYI refers to it as a vent. From their instuctions:

"15A. Low speed boats: (Under 12 knots of boat speed under power and no bearing in the shaft log).

Using a ⅜” (8 or 9 mm) ID “underwater rated” hose (not provided with the PSS), connect the hose to the hose barb fitting installed on the carbon and secure the hose with two (2) hose clamps. Run the hose to a point in the boat at least two (2) feet above the waterline, making sure that the hose does not apply any load on the carbon part of the seal. Keep the hose as close as possible to the centerline of the vessel so the top of the vent hose is never below the waterline, even if the boat heels. Secure the hose in place with the necessary fittings that insure it will not pull free and drop. This hose is now a venting hose that will help ensure that no air is trapped in the seal."


The question is whether after there is no air in the stern tube, why not plug the thing altogether? I mean, how would air enter the tube in the first place once it is burped on launching?

Positive water flow seems to be recommended if the vessel is traveling fast, but not necessary at trawler speeds. We don't have raw water cooling, so a separate pump would be necessary, which basically means that I would be continuously pumping sea water into the shaft tube, depending on the integrity of the tube to keep from bringing a fair amount of water on board if a breech is not noticed. Doesn't sound like that smart a plan, but I may be missing something.
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Old 11-02-2014, 09:58 PM   #5
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Mine isn't vented ... that I know about.

There is a tube/hose that comes off of the sea water injection into the exhaust riser. This hose brings sea water to the PYI seal where it enters the stern tube and returns to the sea through the flutes in the stern bearing (cutlass bearing).

You must have no water injected into your stern tube. Why would you want that over water injection? They said I didn't need it but I liked the idea of having fresh sea water flowing over/through the stern bearing. Being a friction bearing I thought the sea water cooling may help w bearing life and after 10 years and about 1000hrs it really didn't need replacing but w R&R'd the shaft and I decided it was time to service the friction face of the PYI seal.

Never had a problem w the system except for the SS collar moving fwd on the shaft and causing a bit of flooding. That was easily fixed.

Oh I forgot .. I've "burped" the shaft every now and then especially after hauling out, being on the grid or not being under way for a time. I just (w my hands) move the carbon fibre bearing that faces the SS collar aft a bit allowing a cup or so of water to spurt into the boat and fall into the bilge.
Eric, possible preservation of the cutlass bearings is probably the only reason to inject water that I can think of. As you can see from their instructions, PYI doesn't recommend it for slow speed boats like yours and mine, but I do see the possible advantage of keeping the stern tube clear by forcing water through the it. My problem is I don't have raw water easily available because we are dry stack. So a separate pump would be needed. Ergo, my dilemma.
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Old 11-02-2014, 10:36 PM   #6
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Many slow trawlers have a water addition to the dripless system, our vessel does as do all other newer DeFevers I've seen. Either way works by the book though. Easy enough to do for those of us with wet exhaust as lots of RW water rushing around, dry stack can pose an issue if no easy RW take off point.

My take, call Port Townsend Marine - they build and repair lots of these systems.
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Old 11-02-2014, 10:37 PM   #7
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I just bought a new Las Drop- their instructions are the same for vessel speeds under 10 knots. I considered using a through hull lower than the end of the stern tube and running a hose up to the seal but I think I will just go with the vent line as per the instructions. The cutlass bearing is rubber. As long as it is wet it should be happy.
I hope.
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Old 11-03-2014, 12:56 AM   #8
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As per manufacturers' advice, we ran a 'burp' hose to a spare waterline skin fitting so trapped air doesn't prevent water rising in the shaft.

If it turned above 500rpm, I would plumb in some water flow.
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Old 11-03-2014, 06:54 AM   #9
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As per manufacturers' advice, we ran a 'burp' hose to a spare waterline skin fitting so trapped air doesn't prevent water rising in the shaft.

If it turned above 500rpm, I would plumb in some water flow.
Sounds like a great setup....

For those that never heard of it....yep it's either posi flow or a vent based on boat speed.

I too worry about stagnation of water near the seal but with the boat sitting for 8 months while I work, I would have to plumb it to a pump like some larger yachts are now. I think RickB was the one that gave me a lot of guidance there, plus an article or two...one larger yacht had it plumbed into the air conditioner system. People with water injected exhausts and mufflers would have to think that one through.
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Old 11-03-2014, 07:01 AM   #10
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Quote:
Originally Posted by Delfin View Post
PYI refers to it as a vent. From their instuctions:

The question is whether after there is no air in the stern tube, why not plug the thing altogether? I mean, how would air enter the tube in the first place once it is burped on launching?

Positive water flow seems to be recommended if the vessel is traveling fast, but not necessary at trawler speeds. We don't have raw water cooling, so a separate pump would be necessary, which basically means that I would be continuously pumping sea water into the shaft tube, depending on the integrity of the tube to keep from bringing a fair amount of water on board if a breech is not noticed. Doesn't sound like that smart a plan, but I may be missing something.
Hmm, guess I will have to read the instructions. On my trawler it will be pretty simple to run a line up to the engine room ceiling and make a 180 loop. Would recommend doing it in clear fiber reinforced tubing so that you can see the water level.

Here's a thought, connect it to the air conditioning condensate line and have the shaft log full of fresh water.

Ted
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Old 11-03-2014, 08:40 AM   #11
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response to op
If you have sw cooling on your engine (as opposed to keel cooling) then it should be simple to arrange a small hose to the shaft seal fitting from the exhaust elbow supply or similar
yes this may not be absolutely neccessary for a low speed boat but it is a better arrangement, it helps keeps the seal cool and flushes the stern tube (if you have one) and any bearing that may be in the stern tube. If you dont have a stern tube it is still worth doing for the seal cooling / lubrication
if you have keel cooling imo it is not worth adding a pump for this purpose and i would just stick with the vent line. If you have a stern tube with a bearing in it you should definitely keep an open vent line led to above deck or a higher skin fitting to ensure it is kept full of water
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Old 11-03-2014, 10:51 AM   #12
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Thanks everyone! The ideas have stimulated a possible solution and I would appreciate any comments on it. Because we are keel cooled on the main, I can get water either from the wet exhaust on the keel cooled genset, from the one through hull, from the condensate line of the a/c or from the discharge line of the water maker. We run the a/c about twice a year up here in the PNW, so that likely wouldn't work out. Putting a dedicated pump on is possible, but gives one more little piece of equipment to maintain and I'm afraid it's shooting a mosquito with a shotgun. Plumbing off the genset would flush only when the genset was running and I wonder how much flow I would get anyway when there is a free path for the water to take via the 3/4" line 18" long dumping unrestricted into the water lift muffler vs. shoving water down a 15' 3/8" line to the shaft seal vent. The same concerns applies to tapping into the water maker discharge line although the distances are less.

At 400 rpm, I'm not worried about the cutlass bearings heating up unless the tube is dry but I really don't know the physics of how a stern tube full of water after burping might nevertheless have a build up of air over time causing problems - which is why running around with the vent tube plugged was a pretty dumb idea. That said, if I had a regular packing gland, that would have no vent, so maybe it doesn't matter after all....

So that leaves tapping into the through hull for passive flow from the through hull back to the vent. I think this might work, as underway, the screen on the outside of the hull is the typical design, which creates a bit of hydraulic pressure on the inlet as you move through the water. Theoretically, this should move a small amount of sea water continuously through the vent tube for flushing. Make sense?

p.s. Tom - I asked Townsend Bay marine about this at haulout and Paul didn't really have an answer either. Because pumps are common on go fast boats, that was his suggestion IF we did anything, but he wasn't sure we needed to.
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Old 11-04-2014, 07:22 AM   #13
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Ever think about Plan B,???

Duramax for packing and loose the danger of a burst bladder and rapid sinking?

Going modern is less maint , cost , worry and danger.
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Old 11-04-2014, 09:55 AM   #14
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Ever think about Plan B,???

Duramax for packing and loose the danger of a burst bladder and rapid sinking?

Going modern is less maint , cost , worry and danger.
After 6 years, the accordion bellows I replaced didn't look any different than the new one. No cracks, nothing, so worrying about it bursting isn't high on my list. I can't see much difference between the Duramax seal and the PYI design. Both have bellows, both have mating surfaces for a wet seal. Looks like a good product but I can't quite see the dif, unless the claim is that their spring loaded, covered bellows never needs replacing. Maybe.
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Old 11-04-2014, 10:49 AM   #15
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Lots of stern bearings are equipped w an angled hole at the front of the bearing housing that scoops in water for the cutlass bearring. If your keel is configured so a hole could be cut near the head of the stern tube or at least ahead of the cutlass you'd get a constant flush while underway. But if you needed to plumbing from the inside of the hull to the outside of the stern tube probably would be a project breaker. Also instead of a pump in an ideal place (mostly so) you could put a small through hull w a screened scoop and a small hose to the PYI hose barb. You'd have another through hull and a small hose but far less trouble than an electric pump. You could vent to burp air at the high point of this hose.

Also you could use a clear hose for a vent tube and monitor the WL in the hose to insure the PYI seal was flooded. But burping would accomplish that however less convenient while underway.
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Old 11-04-2014, 10:55 AM   #16
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Lots of stern bearings are equipped w an angled hole at the front of the bearing housing that scoops in water for the cutlass bearring. If your keel is configured so a hole could be cut near the head of the stern tube or at least ahead of the cutlass you'd get a constant flush while underway. But if you needed to plumbing from the inside of the hull to the outside of the stern tube probably would be a project breaker. Also instead of a pump in an ideal place (mostly so) you could put a small through hull w a screened scoop and a small hose to the PYI hose barb. You'd have another through hull and a small hose but far less trouble than an electric pump. You could vent to burp air at the high point of this hose.

Also you could use a clear hose for a vent tube and monitor the WL in the hose to insure the PYI seal was flooded. But burping would accomplish that however less convenient while underway.
I thought about adding a small through hull right next to the stern tube and PYI vent. I don't like through hulls, so multiplying them isn't my first choice, but it may be the best option. However, it does seem like if I just plumb into the existing 2" through hull I should get some flow into the vent and out the back end. The only problem may be that the flow cold be reversed when the water maker is sucking on the same line, and this might cause the small port on the vent to clog. Probably free itself when the water maker is off, but I suppose it is possible.

Thx Eric.
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Old 11-04-2014, 11:01 AM   #17
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To insure the PYY bellows and seal remain in place, like many others I added a shaft zinc between the PYY and shaft coupling. It seems a universal recommendation.
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Old 11-04-2014, 11:12 AM   #18
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To insure the PYY bellows and seal remain in place, like many others I added a shaft zinc between the PYY and shaft coupling. It seems a universal recommendation.
Tom, I used a hose clamp. Same concept but it is belt and suspenders.
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Old 11-04-2014, 04:52 PM   #19
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Maybe I missed it but have you talked to PYI? I've called them numerous times and they're always willing to help. It just seems that they're in a pretty good position to know exactly what's needed for the vent.

They're on the west coast.
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Old 11-04-2014, 05:02 PM   #20
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Maybe I missed it but have you talked to PYI? I've called them numerous times and they're always willing to help. It just seems that they're in a pretty good position to know exactly what's needed for the vent.

They're on the west coast.
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.
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