thru hull head vent...?

I have read all the threads about heads, holding tanks and Peggy hall but NO one says about replacement of Vent and who makes it. I have found at West Marine a fuel line vent # 138792 that I think might work. Is that what it is?
One of my Sea Scouts took vise grips to it and twisted it off, (old and very bad shape carroided) in to the water it went. Is the one next to it the fuel vent?

Should a person replace both at the same time... knowing they are for two unrelated uses? They other (fuel one) looks much better.

Thanks folks,
Ralph in Nicaragua

My holding tank vent is technically a fuel vent. Gotta keep the gas out of the cabin no matter where it comes from.

Ralph,

Per Peggy Hall, the holding tank vent should be as large and unrestricted as possible in order to get as much oxygen into the tank to avoid stinky anaerobic organisms. She suggests using a standard thru hull fitting for the least restriction and max air flow. I used a std stainless thru hull fitting when I rebuilt my system. It's much larger (1.5 inches) than the original 5/8 vent, located up near the toe rail and wide open to outside. I have not had any issues with water getting in or sewage getting out. I spent a day standing next to the vent, with a full holding tank, while buffing the hull and there is absolutely no odor coming from the tank. Inside is totally odor free as well.

While I strongly prefer Marelon, non- metallic thru hulls, below the waterline if they are protected from potential damage inside the hull, I would use stainless or bronze ABOVE the waterline (where electrolysis is not an issue) in order to avoid UV damage which the Marelons will suffer more than metal.

For the fuel tank vent, I would change it now if you have any suspicions that it may be ready to fail. You absolutely do not want any water or moisture to get in your fuel tank! My fuel vent fell off and I did not notice for some time. I stupidly replaced it during the winter without considering how long the tank was open to the elements. Several months later, in early summer, the water in the fuel tank grew lots of gunk and I had to empty and clean the entire fuel system.

They make all types of fuel vents now, some have traps, moisture dryers and fuel surge baffles. I got an Attwood vent with a water trap because my vent is located directly below my main deck drain scupper - not the best choice by the worker at Ericson.

Mark

Ralph,

I second what Mark said. I would go with a standard Marelon through hull, probably with a clamshell cover which faces aft.

See this article;

http://www.practical-sailor.com/issues/37_25/psadvisory/Holding-Tank-Vent-Fittings_10949-1.html

Here is a Marelon vent that would do the trick;

http://www.forespar.com/products/boat-marine-plumbing-tank-vent-deck-plug.shtml

http://www.fisheriessupply.com/forespar-marelon-tank-vent

Keith,

Thanks for the article! All the points in the article match what Peggy Hall states in her book, except I had not seen the point on not using metal for the thru hull vent before. I'll see if the tank gases destroy the stainless thru hull sooner than the UV would have killed the Marelon. I think it will take a long time either way. I was on the fence over which type of thru hull to install when I rebuilt the system. The good news is that my vent is pretty easy to get at and the stainless is (for now) better looking than the Marelon.

I would add that my tank venting experience has confirmed to me that maximum venting is a huge part of an odor free holding system. I had a poorly run 5/8 inch vent line with the screen mesh type fitting when I started. I have a direct, but vertical, run from the tank to the vent location at the toe rail. I had the room to add any size vent line so I super-sized it to 1.5 inches and skipped the clam shell cover. It does look a little scary to have a big, open hose running directly to the holding tank but I have ~ 2.5 vertical feet from tank to vent so nothing gets out. I also used premium sanitary hose for the vent line, just in case. When re-installing the holding tank and lines, I could feel a steady flow of air coming down the vent into the tank all the time, confirming the big vent line and open vent thru hull were working well.

Mark

One bit of plastics trivia: marelon is a reinforced plastic material and is very UV resistant. When I replaced the lesser cast plastic plastic transom thru hulls on our boat, there was one marelon fitting and it was the only one in good condition that did not break or crack when being wrenched off.
All "plastics" are not the same, at all.

Loren