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Old 10-21-2014, 10:35 PM   #1
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Holding Tank vent thru-hull location

I am increasing the size of the holding tank vent on my 38' power boat. This will require a new 1 1/4" thru-hull on the side. The easiest and shortest run for the vent hose will put the thru-hull about 16" above the water line and about 4" above 2 bilge pump outlets.
Just wondering if there is any reason that this location would not be a good idea?
Thanks\
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Old 10-21-2014, 11:27 PM   #2
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"Just wondering if there is any reason that this location would not be a good idea?"

Sounds good to me. That's a nice big vent. Your aerobic bacteria will be happy. That's a good thing. Have you read Peggie Hall's book on boat odors? She's a big proponent of large vents.

I see that is your first post, Welcome Aboard! Please avoid asking anchor or gun questions. The following is all you need to know.

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You should go to the Welcome Mat forum and introduce yourself.
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Old 10-21-2014, 11:35 PM   #3
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I am increasing the size of the holding tank vent on my 38' power boat. This will require a new 1 1/4" thru-hull on the side. The easiest and shortest run for the vent hose will put the thru-hull about 16" above the water line and about 4" above 2 bilge pump outlets.
Just wondering if there is any reason that this location would not be a good idea?
Thanks\
That's fine as long as the vent hose is as short as possible and up hill all the way (no dips).
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Old 10-21-2014, 11:40 PM   #4
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"That's fine as long as the vent hose is as short as possible and up hill all the way (no dips)."
Good point Ron.
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Old 10-22-2014, 08:05 AM   #5
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Just when I thought I knew it all about the subject... ok, I'll bite... why does the vent line need to be up hill all the way? Someone forgot to tell Hatteras this, and 33 years later the dang thing still ignores this advice.
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Old 10-22-2014, 08:17 AM   #6
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Just when I thought I knew it all about the subject... ok, I'll bite... why does the vent line need to be up hill all the way? Someone forgot to tell Hatteras this, and 33 years later the dang thing still ignores this advice.
So if you get liquid in it it doesn't form a trap and block the air flow. Like the trap under your kitchen sink (at home).

The same applies to a fuel tank vent hose.
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Old 10-22-2014, 08:20 AM   #7
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Just when I thought I knew it all about the subject... ok, I'll bite... why does the vent line need to be up hill all the way? Someone forgot to tell Hatteras this, and 33 years later the dang thing still ignores this advice.
George

Some feel marine heads and holding tanks must be designed and operated like National Park outdoor privy. Others like Hatteras didn't read that memo and assumed a vent line was, well a vent line.
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Old 10-22-2014, 08:23 AM   #8
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Fitting supplier

Quote:
Originally Posted by Tolly Roger View Post
I am increasing the size of the holding tank vent on my 38' power boat. This will require a new 1 1/4" thru-hull on the side. The easiest and shortest run for the vent hose will put the thru-hull about 16" above the water line and about 4" above 2 bilge pump outlets.
Just wondering if there is any reason that this location would not be a good idea?
Thanks\
TR, where are you getting a vent fitting that size? Who is you supplier?
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Old 10-22-2014, 09:19 AM   #9
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So if you get liquid in it it doesn't form a trap and block the air flow. Like the trap under your kitchen sink (at home).
Um, How?

Just to clarify, on the Hatt, the holding tank is molded into the bottom of the boat/keel. The vent line goes up to the starboard hull side, well above the waterline, then loops down to the through hull nearer the waterline, which has a backward facing L fitting on the outside. Really no way for liquid to get in. I believe it is designed this way to keep any potential smells down. 3/4" hose vented a 210 gallon tank just fine and we never have had any smell issues, even when I removed the vent and pump out lines from the tank. No chemicals used either. Fresh water heads may play a role in that.

While the fresh water and fuel tanks vent out lets are well above the water line, they have some slight dips. They also have the same L fittings outside.
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Old 10-22-2014, 09:30 AM   #10
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George, that set up is fine. What you don't want is a vent line that goes up, then down, then back up again. You don't want to form a trap like you see under a sink.
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Old 10-22-2014, 10:13 AM   #11
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Thanks Everyone,
My current vent set up is only 1/2" clear tubing. Looks like that was a factory thing back in 83. The tubing goes straight up and across the engine space and then up just under the rub rail to a very small chrome vent with a screen. It must be a 12' run. It's everything your not suppose to do!
The tank has a 1-1/2" NPT female port on top that is reduced to a 1/2" barb fitting. My plan is to replace that reducer to give me a 1" ID hose, shorten the run to about 7' and install a standard 1-1/4" Poly thru-hull. The goal here is to increase the air flow to the tank, reduce the smell, reduce the amount of chemical and get rid of that long run of clear tubing that most likely is permeated. Any advice on what kind of hose/tubing to use??
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Old 10-22-2014, 11:16 AM   #12
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Um, How?

Just to clarify, on the Hatt, the holding tank is molded into the bottom of the boat/keel.
George

Are all the tanks integral? Was this common in Hatts of that vintage? Wonderful sea worthy live aboard and hard to beat for that purpose.
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Old 10-22-2014, 11:17 AM   #13
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Since it is just air, anything will work. Bilge pump hose comes to mind. Be mindful of places it passes through bulkheads.
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Old 10-22-2014, 05:10 PM   #14
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On Bay Pelican there were originally two heads. When we converted one head into an additional locker, I used the then unused 1-1/2" inlet at the top of the holding tank to connect to a 1-1/2" thru-hull 4 feet above the water line. When running the 1-1/2" hose I made sure that there was not a dip which would form a trap.
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