• Mobile App For Android Now Online!

    Download it here. The app is searchable in the Google Play Store under Sailboat Owners.

    Sorry iPhone/iPad users, we are still waiting on Apple. :(

    Click the X in the upper right corner to make this go away

Solved:The mystery of the PSS shaft seal overflowing vent tube

Dan_Y

.
Oct 13, 2008
491
Hunter 36 Hampton
We have a PSS dripless shaft seal installed in our h36 and I ran the vent tube up in the engine compartment into an overflow bottle per Mainsail's suggestion. The top of the vent hose is about 14" above the waterline. The overflow bottle would catch water on our h30 due to aggressive use of reverse gear, but this does not typically happen on our h36, maybe just a oz or two. So I was surprised to find the bottle full and overflowing a few times over the last 3years we owned the h36 and I was mystified.
Turns out that when changing the zincs in the water, air begins to collect in the scalloped area where the shaft log exits the hull and bubbles up around the shaft into the PSS bellows. The trapped air in the bellows then pushes water up thru the vent tube into the overflow bottle to escape. No kidding. It works like an old aquarium filter. The scuba air rises thru the vent tube, pushing lots of water ahead of it, easily filling the bottle in the time it take me to change the zincs. The Admiral watched it happening and emptied the bottle when I surfaced for the second zinc. So next time I'll either plug the vent line or let it overflow into a bucket.
 
Jun 11, 2011
1,216
Hunter 41 Lewes
Good catch Dan. I'll have to look for that the next time the diver is under.
 
Dec 19, 2006
5,645
Hunter 36 Punta Gorda
I have the bottle high on the venting tube but to be honest have not found much water in the bottle over the last few years and do check it often but will see if a difference next time change zincs.
My bottle is in the cabinet just above the engine in the rear berth next to the water lines going to the shower and check very easy often.
Nick
 

Rick D

.
Jun 14, 2008
6,864
Hunter Legend 40.5 Long Beach, Shoreline Marina, CA
Hmmm, sounds like an issue for those of us left-coasters who have monthly bottom cleaning.
 
Dec 19, 2006
5,645
Hunter 36 Punta Gorda
I think it depends how high the vent tubes are and proper air release and I hardly ever get much water in the bottle maybe 1 or 2 oz in bottle.
My tube goes straight up high.
Nick
 
Jul 1, 2010
728
Seaward 25, Catalina 350 Erie, Pa
Neat scuba trick. When our bottle overflows, it's because I've forgotten to check it in a while, and it makes a mess on the floor. I'm always happy when I find it though, because the boat isn't leaking.
 

Dan_Y

.
Oct 13, 2008
491
Hunter 36 Hampton
It's interesting Nick doesn't have the same problem because we have almost identical boats and our bottles are at about the same height...his is a few inches higher. Maybe I exhale too much when under the boat...:). The images below show the scalloped area in the hull that collects air where the 1" shaft exits. Note I had the engine mounts adjusted to get the shaft back in the middle of the log opening. In the photo showing the collection bottle, the water level in the PSS vent hose can be seen as the dark band just before the hose drops out of site behind the engine. I remeasured and it's 22" up from that point to the top of the bottle. So I suspect the small diameter hose (1/4 or 3/8') doesn't let the air by from a good exhale under the boat like a larger diameter hose would and pushes the water into the bottle. Next time I'll plug the line at the bottle when cleaning the bottom.
Dan
ShaftLogOpening.JPG
PSS Vent Line 1.JPG
PSS Vent Line 2.JPG
 
Last edited:

jviss

.
Feb 5, 2004
4,307
Tartan 3800 Westport, MA
I just got a boat that has a PSS dripless shaft seal, and the "vent" line is connected to a "T" on output side of the raw water pump. Does this make sense?
 
Feb 14, 2014
4,668
Hunter 430 Waveland, MS
is connected to a "T" on output side of the raw water pump. Does this make sense?
Yes. I did mine that way also.

Most higher RPM shafts are sealed that way.

I like it since I sometimes have to do a Reverse off a silty bottom.
The PSS can suck up silt and is later purged of silt with clear water.

PSSPruge.png


2 years this way. No issues.
Jim...
 

jviss

.
Feb 5, 2004
4,307
Tartan 3800 Westport, MA
Thank you! I'm glad to hear that.

Did you get the "T" from PYI? I would be concerned about how brass would hold up, I think bronze is preferred.

By the way, the "T" on mine is just after the raw water pump, if I recal correctly. There's a mess of raw water lines on mine, one of which looks original (yikes!), and I plan on sorting it out soon. I think yours, after the siphon break, is the proper way to go.
 
Last edited:
Feb 14, 2014
4,668
Hunter 430 Waveland, MS
Did you get the "T" from PYI?
Yes. I called them and talked at length about how to properly vent my new PSS.

I need to correct that picture caption. It was a BRONZE 1" x 1" x 3/8" hose barbed TEE for $45.

Tadapt.jpg

This is from PYI manual.

the "T" on mine is just after the raw water pump
Mine too! The round object in the picture with the "Rabbit Ear" hoses is the engine raw water pump.

Someone was worried about the flow loss from engine cooling. Minor! The tight clearance on the seal carbon face and stationary SS plate is so small, Seal cooling flow does nothing to engine cooling.

I think yours, after the siphon break, is the proper way to go.
No!:badbad::badbad:
Mine is where it should go, BEFORE the breaker. My water flow is left to right on my picture.
Down stream of Siphon break could by-pass the siphon break effects by sucking sea water around it.

However if your sea water pump had new impeller, it should not be a major issue.
Jim...
 
Feb 14, 2014
4,668
Hunter 430 Waveland, MS
about siphon breaks.
Water falling, creates a pull or vacuum, that will continue to flow from a higher elevation source to a lower one.

In this case...
When the engine stops and pumps stops, it is possible for flow from sea, to flow past the pump impeller and continue to flow into a open engine valve via the exhaust riser. Not likely , but possible.

The Siphon breaker has a small valve at top of "hump", that opens when under a slight vacuum for air to enter. Closed when under pump pressure. The input of air "breaks" the siphon and stops it.
______
Since the PSS seal can flow sea water backwards when the engine is down, putting it on the down stream side of the Breaker, would allow the PSS seal to flow[backwards] into the open engine valve, if the elevation of the seal purge line is higher than the engine top.

Putting it in front of the siphon breaker, would let the breaker do its job.;)
Jim...
 

jviss

.
Feb 5, 2004
4,307
Tartan 3800 Westport, MA
OK, thanks. I understand the concept, I just have to draw a picture I guess.
That said, I wouldn't be worried about water flowing past the impeller, but I would about water siphoning via the PSS. But, the destination would have to be a lower level that the waterline. I guess the muffler is, but I don't think the riser elbow is. Anyway, it's worth having the siphon break, and mine has it.
 
Feb 8, 2014
1,168
Columbia 36 Muskegon
And any boat that motors over ten knots has to have positive water pressure to the vent line. Without it the boats forward motion would suck the water out of the shaft log and burn the seal. My commercial boats are plumbed that way, with a direct line from the rw pump to the seal. In fact its the only reason there is an rw pump as the engines have keel coolers, no rw pump needed for engine cooling.
 
  • Like
Likes: Rick D