# Water tank vent



## TSOJOURNER (Dec 16, 1999)

This may be a silly question but why should a water tank be vented out through the hull? Is there a chance of picking up salt water when healed. Can I avoid replacing a clogged vent in the hull and vent inside?


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## Faster (Sep 13, 2005)

Properly designed, there should be little risk of salt water intrusion of your fresh water tanks from the vent.

Having said that, our last two boats had water tank vents inside the hull, high up under the hull deck joint in one case, and high up in the head compartment in our current boat. Neither set up caused us any trouble either. (and it is one less fitting piecing the hull skin) 

One downside though; many people like to start to fill their tanks and walk away, shutting the water off when the fill line overflows - this is not recommeded practice in our case as the vents are slightly lower that the fill port and inside the boat.

I imagine that's the reason most are installed overboard. or in the cockpit area.


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## sailingdog (Mar 19, 2006)

As Faster has said, the main reason for venting a fresh water tank overboard is to prevent the vent from filling the boat with water if you start filling the tank and walk away.


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## Boasun (Feb 10, 2007)

you could have the vent coming up through the deck next to a stanchion or the house. With a loop at the top and screened to keep out the vermin. Then it is in sight as you fill your water tank. This is the way it is done on work boats. Then when we have an overflow we know that the tank is pressed up.
But what ever you do, DO NOT combine the fuel vent with the Water vent... A definite NO NO!


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## sailingdog (Mar 19, 2006)

Boasun said:


> you could have the vent coming up through the deck next to a stanchion or the house. With a loop at the top and screened to keep out the vermin. Then it is in sight as you fill your water tank. This is the way it is done on work boats. Then when we have an overflow we know that the tank is pressed up.
> But what ever you do, *DO NOT combine the fuel vent with the Water vent.*.. A definite NO NO!


But, it would simplify the installation... really it would...


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## TSOJOURNER (Dec 16, 1999)

Mine runs up between the liner in the head and the hull - doesn't vent outside. Don't like this as the head is always more humid than the rest of the boat. Going to run it somewhere...maybe into the anchor locker...


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## TrueBlue (Oct 11, 2004)

The water tank vent is necessary to release air within, as it is displaced by water from the deck fill. Conversely, as water is emptied from the tank, air displaces the water. 

If a vent is not provided, resulting negative or possitive pressure could rupture either the tank, hoses, or fittings.


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## morganmike (Oct 31, 2006)

TrueBlue said:


> The water tank vent is necessary to release air within, as it is displaced by water from the deck fill. Conversely, as water is emptied from the tank, air displaces the water.
> 
> If a vent is not provided, resulting negative or possitive pressure could rupture either the tank, hoses, or fittings.


Wow, who would have guessed?


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## labatt (Jul 31, 2006)

We'll have to track ours down. Is it possible that some boats don't have a vent? When we fill our tanks we get big "thunk" sounds as they fill. When you pull the hose out of the deck fitting, you usually get a big geyser shooting out of it. I'm assuming the sounds are just due to the weight of the water pushing on the tank, but I'm not sure about the geyser...


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## TrueBlue (Oct 11, 2004)

> Is it possible that some boats don't have a vent?


Contrary to morganmike's sarcastic post, it seems the vent's function is not that obvious to some.

All tanks need to be vented labatt. Yours may just be blocked.


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## Faster (Sep 13, 2005)

labatt said:


> We'll have to track ours down. Is it possible that some boats don't have a vent? When we fill our tanks we get big "thunk" sounds as they fill. When you pull the hose out of the deck fitting, you usually get a big geyser shooting out of it. I'm assuming the sounds are just due to the weight of the water pushing on the tank, but I'm not sure about the geyser...


The geyser you experience could be from feeding water at a rate that exceeds the vent's ability to handle it.

Upsize your vent lines/fittings and that should help. (or fill your tank more slowly)


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## labatt (Jul 31, 2006)

TrueBlue said:


> All tanks need to be vented labatt. Yours may just be blocked.


That's actually what I was figuring... the geyser just seemed strange and made me question whether it was vented. Being blocked makes more sense.


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## Valiente (Jun 16, 2006)

Mine vent into the bilges. Bloody annoying. I am going to group my fuel, water and exhaust vents and send them vertically, inside a big brass pole I'll get from a strip club (plenty of Nevr-Dull on it first, shine or not!) to multiple goosenecks atop the pilothouse. If I overfill something, the pitter-patter of little raindrops will let me know.

Vents at the hull/deck join are idiotic. I can't tell you how many stories I've read about fuel or water contamination once the rail is buried and the salt water goes (inevitably) downhill.


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## sailingdog (Mar 19, 2006)

Labatt- 

Sounds like you've got either a blocked vent—insects and such do this a lot—or no vent at all.


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## Boasun (Feb 10, 2007)

Thinking about this problem... Maybe if you routed your vent into the cockpit. That way while filling the water tank you can nap in the cockpit and when your feet get wet you know the tank is full. Also when you are sailing with beam ends in the water you won't worry about your tanks being contaminated.


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## Valiente (Jun 16, 2006)

Boasun said:


> Thinking about this problem... Maybe if you routed your vent into the cockpit. That way while filling the water tank you can nap in the cockpit and when your feet get wet you know the tank is full. Also when you are sailing with beam ends in the water you won't worry about your tanks being contaminated.


Amusing, but the idea of having all vents leading up and out well above the waterline is a good practice. When diagnosing an exhaust problem, I cable-tied a hose from the siphon break right out of the engine compartment and into the cockpit in order to achieve the recommended "16 inches above the waterline" requirement. The cheerful little ejaculations of H20 during engine operations were proof that I had both good flow and no water siphoning back into the manifold, as had been the case.

Now, running those vents to where I can see them is SOP.


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## SimonV (Jul 6, 2006)

The tank has to have a vent or the resulting vacuum would be such that no liquid could be extracted. Vacuum is stronger than pressure.


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## TSOJOURNER (Dec 16, 1999)

> a big brass pole I'll get from a strip club (plenty of Nevr-Dull on it first, shine or not!)


Or you could leave it dirty and have a unique air freshener "Memories of Amber" (and Tiffany, and Kristal, etc., etc.....)


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## morganmike (Oct 31, 2006)

I'm not arguing that there are some clueless people out there, I'm arguing that your post did nothing to answer the OP's question. The OP asked about best location for a tank vent, discussion ensued around that topic. You come in, master of the obvious, and post an off-topic know-it-all response about WHY we need a tank vent, which nobody was debating. The fact that someone came in afterward and asked if their tank might not have a vent does nothing to vindicate you.


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## labatt (Jul 31, 2006)

Found one of my vents - it's inside one of my interior cabinets (where I store all my emergency equipment such as flares). The geyser must be due to filling the tank too rapidly for the vent to keep up.


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## sailingdog (Mar 19, 2006)

Labatt-

The hose could be kinked, or the vent could be still blocked in some way... Spiders and other critters sometimes do that.


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## camaraderie (May 22, 2002)

Labatt...in addition to what dawg said...sometimes water finds a low point in the vent line and pools there...effectively blocking the vent. Blow out the line with an air compressor if you don't find an alternate solution.


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## labatt (Jul 31, 2006)

I'll look at the bug route, but it's literally a 3 foot run, almost straight up, from the tank to the vent. It's a 1/2" tube with a clear hose (potable water hose) with a white vent cap plugging the top. Not too many places for problems... We usually leave the hose on full open when filling, so maybe it's too fast... We only have the problem with the one tank though. It's on my list now, but under fixing my head issue... I love owning a boat!!!  I can't complain though...


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## TrueBlue (Oct 11, 2004)

morganmike said:


> I'm not arguing that there are some clueless people out there, I'm arguing that your post did nothing to answer the OP's question. The OP asked about best location for a tank vent, discussion ensued around that topic. You come in, master of the obvious, and post an off-topic know-it-all response about WHY we need a tank vent, which nobody was debating. The fact that someone came in afterward and asked if their tank might not have a vent does nothing to vindicate you.


It seems you're the one with a venting problem mike . . . lighten up, or try a little more fiber in your diet.


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## TSOJOURNER (Dec 16, 1999)

Thanks all! I think i will skip replacing the vents on the side of the hull which are almost impossible to reach without dismantling parts of the interior and put new vent hoses on the water tanks which I can lead out a port when refilling. I'll also stand by while filling rather than going to the store or taking a nap.


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## monroe (Jun 22, 2001)

Our Freedom 39 did not have vents, and we got the geyser frequently, especially when the Mexican water guys glug in the 20l jugs. For right now we have drilled a small hole in the top of each. We are concerned about bugs, dirt, mold, and bacteria. Is there some kind of check valve we should install?

Yes I know this thread is 8 years old ;-)


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## albrazzi (Oct 15, 2014)

No vent huh, how do you prevent the Tank from being collapsed while using water. Are you sure there is no vent anywhere ? if you fill fast through jugs it will backwash, the vent is not big enough to take a fill that fast especially if its tight in the filler as I suspect. No check valve here they are probably deflecting the tank while filling too fast and its just relaxing and spewing.
Vent could still be clogged find it and clean it.


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## capt jgwinks (Sep 24, 2013)

Routing of the vent hose is just as important. If there's a low spot, it's effectively blocked. Has to be down hill all the way from vent to tank so liquid is never held in the hose. More critical on waste and fuel tanks. Waste held in the hose will stink, fuel will rot the hose.


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## pdqaltair (Nov 14, 2008)

monroe said:


> Our Freedom 39 did not have vents, and we got the geyser frequently, especially when the Mexican water guys glug in the 20l jugs. For right now we have drilled a small hole in the top of each. We are concerned about bugs, dirt, mold, and bacteria. Is there some kind of check valve we should install?
> 
> Yes I know this thread is 8 years old ;-)


I assume "check valve" was either a misunderstanding or misuse of nomenclature; the vent must pass air both directions.

The plumbing code and AYBC require a 20 mesh screen to keep mosquitos and spiders out; I can't imagine why most boats do not take this simple precaution. An simple in-line water strainer do.

As for microorganisms, the ultra pure water people use very fine hydrophobic filters on vents, but unless you are sub-micron filtering all of the fill water, and so long as you chlorinate the water, this is overkill. Additionally, if the vent has a good high loop, that will reduce contamination too.


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## capt jgwinks (Sep 24, 2013)

Boasun said:


> you could have the vent coming up through the deck next to a stanchion or the house. With a loop at the top and screened to keep out the vermin. Then it is in sight as you fill your water tank. This is the way it is done on work boats. Then when we have an overflow we know that the tank is pressed up.
> But what ever you do, DO NOT combine the fuel vent with the Water vent... A definite NO NO!


A friend of mine was having trouble with water getting into his fuel tank, couldn't figure out how it was happening. Turns out the PO tee'd the propane locker vent into the fuel tank vent. Did I mention that most PO's are morons?


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## capt jgwinks (Sep 24, 2013)

Routing the hose is important. If there are any low spots that can hold fluid, the line is blocked. Needs a straight down hill run to the tank. It's more critical on waste and fuel tanks, waste held in the hose will stink, fuel will rot the hose.


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