Seacock on holding tank vent hose?

[Rich Stidger]

I have a fellow boater who has a 2005(?) Hunter 44 and he has a seacock on the holding tank vent hose. He never closes it. He is replacing the vent hose with a larger size and he asked me about the seacock.
I have never seen or heard of a seacock on a holding tank vent.
Does anyone else have this seacock? Opinions on keeping it?

 

[JBP-PA]

Is the vent low enough that it might be below water level when heeled over?

 

[thinwater]

Is the vent low enough that it might be below water level when heeled over?

The obvious problem is that pressure build up could blow the tank. Eww.

• Temperature change.
• Flushing the toilet. Worse if the tank is full, but this creates pressure even if it is not.
• Biological generation of gases (primarily CO2 and H2S). This is the one that would scare me silly if it were closed for more than an hour. I guess he never heard of landfill gas. I've operated digestors, and they bubble. I'm pretty sure that could blow the tank in a few days.

I would move the vent to where I could remove the valve. If not possible, then he should lock the valve open with a cable tie to avoid a catastrophe. The worst that will happen if it goes under a wave is that he will fill his tank. No biggy.

 

[Rich Stidger]

Is the vent low enough that it might be below water level when heeled over?

No. The vent is over half-way up the topsides. And the valve is never closed. We just can't figure out why the valve is present, or if it was installed by Hunter or the PO.

 

[Rick D]

Never seen one in my prior three boats. Sounds like a PO add. I'd leave it off on the re-do.

 

[MFD]

Different boat, Hunter 41DS.
In mine, there is a seacock on the hot water tank TPV discharge line that goes out up near the gunnel.

Which is a curiosity if it’s own. My understanding is that hot water blow out needs to go overboard, not the bilge, if the boat has plumbing for a shore water connection. Meanwhile the seacock in both cases makes me think of explosions…

Meanwhile is your friend sure it’s the holding tank and not the hot water tank?

 

[Peggie Hall HeadMistress]

First I've ever heard of a seacock on a holding tank vent line--leaving me wondering if that's what it is. My guess: It could be a PO's hair brained idea of the best way to prevent filling up the holding tank with seawater when heeled far enough to put the vent thru-hull under water and staying on that tack long enough for that to happen. I wonder if he confused a line coming off an overboard discharge pump with the vent line and thought he was putting the "missing" seacock on it.

--Peggie

 

[Foswick]

Like others, I am confused about the purpose. I can sort of see if the vent was on the deck and you wanted to prevent deck runoff (rain, seawater, etc.) from adding to the holding tank. I could see this as being (marginally) beneficial if the boat is left unattended for a long time and the vent was positioned where it readily swallowed this runoff ... I guess. Not how I would solve that problem, if I had it, but just speculating ... I can sort of see why a person might do that.

But, more to the point - if that's what it actually is and there is no confusion, I cannot advise keeping it. But, not knowing exactly why it is there, I can't advise getting rid of it. I guess I would just say that, if they keep it, they should know why, and not just keep it because it was already there.

 

[LLoyd B]

just off hand i wouldn't expect to see that on a boat unless it was a submarine

 

[JBP-PA]

Here is what Practical Sailor had to say. I don't know what they mean by "add security". I couldn't find the 1994 article mentioned.

"Holding tank vents should be plastic, not metal. There are no metals that deal well with the corrosive gases being vented; all metals will either discolor or pit in this application.

A simple through-hull design is best as it allows you to backflush the vent using a garden water hose. Follow the American Boat and Yacht Council (ABYC) standards for installing a through-hull above the waterline (section H-27), and be sure to include a beefy backing plate. Adding a seacock (PS test, Sept. 1, 1994) to the setup isn’t necessary, but it would add security."