Fuel tank vent question

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Hog Guts
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Fuel tank vent question

Post by Hog Guts » Tue Feb 09, 2016 11:46 pm

Since I've been dealing with a fuel tank that went bad apparently from water getting inside through the air vent due to expansion and contraction and the moist air cooling I'm trying to figure out how to prevent that from happening. It's for a larger offshore boat but same really applies to any tank that's going to be sitting for a while. What are the methods to keep the water from getting in the tank? Or how to deal with it if it gets in there? I'm sure if it were being filled and used regularly it would not be an issue the water separators would deal with it over time. I know they make filters for air compressors that remove the moister from the air in the line, would that or something similar work on the vent line? Is there a way to put a... Hmm. I'm not a plumber but - the doohickey they put on the plumbing that handles pressure surges - Linda like the same idea as the tank attached to your water heater to handle pressure. If something like that could handle the expansion of the fuel tank when it got hot then the air vent could be closed with a small fuel valve and would prevent moist are (read water) from getting into the tank since it would be a closed system.

Do they make them and what would they be called? If they don't maybe I should... Or maybe there is a reason not to??? Any feedback? Thanks


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fladan
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Re: Fuel tank vent question

Post by fladan » Wed Feb 10, 2016 12:24 am

I'm not sure about the pressure tanks and surges, but my Dad was a pilot and he always said that if you keep the tank tip-top full, there isn't any air in there to get hot and cold to condense and add water to the gas. That is the theory I follow and always fill my tank up on the way home after going out. That way there is no air AND it is full and ready to go when you are.

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Re: Fuel tank vent question

Post by Gary S » Wed Feb 10, 2016 7:18 am

You sure it's condensation? I built a boat that the vent thru the side of the boat would catch water when I hit a wave. Had to change out the fitting from a nice chrome on brass fitting to a plastic fitting that had a check valve in it. Stopped the water though. Is this boat new to you or have you had for a long time. Also had a boat hear that leaked water in thru the fill fitting. Bad design, change the fitting and cured the problem.

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Southern Style
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Re: Fuel tank vent question

Post by Southern Style » Wed Feb 10, 2016 8:06 pm

I agree with the full tank theory, but there are no places to get Avgas at 1am when I am headed home... So, I just fill it back up at the first possible chance.. Might be a day or 2, but it beats getting condensation in the tank. Is your boat in the sun? I ask that because if the side of the tank is getting that hot and then that cool, if its covered, that might cut it down a lot.. I keep mine under a 10 x 20 canopy and never really have an issue..
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John Fenner
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Re: Fuel tank vent question

Post by John Fenner » Wed Feb 10, 2016 8:35 pm

X2 on full tank upon return from trip. Biggest thing on venting airboat tanks is that in Florida, we have weather swings very hard, summer with hot days and rainy evenings, if vent is a cap alone, or a small tube/hose, the warm tank cooling fast will draw in rain drops, same as slower cooling with dew and condensation.
Offshore boats get salt water and air/vapor/mist the same way.
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glades cat
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Re: Fuel tank vent question

Post by glades cat » Thu Feb 11, 2016 7:37 am

On my airboat I have 2 pvc caps, one is vented, the other isn't. It is stored with the non-vented cap and run with the other. The vent hole is drilled at an angle to prevent water from entering. The solid cap will hold the vapor pressure and prevent tank from breathing.
On my ocean boat the vent hose goes up first after the breather, then dips and creates a trap so any water will pool and hopefully evaporate instead of going into tank. It also has plastic tanks.
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CactusJack
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Re: Fuel tank vent question

Post by CactusJack » Thu Feb 11, 2016 10:44 am

The full tank theory is fine, as long as no rrr's wipe steals your gas...

I would be more inclined to have a vent tube with a filter in it (dry paper like a gas filter) and change the filter regularly.... and when winter comes, or you know you wont be using the boat for a short period of time, empty the tank completely and let it air out to get rid of any moisture in the tank. Also, some forms of alcohol, over here it is 'metholated spirits', can be added to the gas and encourages water to mix with the gas, rather than sitting separately in the tank..... in other words, the water becomes part of the gas and will either burn or get caught in the water separator.... :thumbleft:

Also, on the full tank method... the sh!t they pass off as gas over here has a shelf life of 2 weeks (in a can or gas tank) before it starts to deteriorate... maybe your gas is better? But I would rather have fresh gas each time I go out :) depends if there is gas thieves near where you park your boat :dontknow:
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Re: Fuel tank vent question

Post by mojoe » Thu Feb 11, 2016 12:58 pm

That is the million dollar question. I know sooo many guys with built in tanks on their saltwater boats that cannot keep the water and subsequent corrosion out. Lots give up and add external tanks. I don't know the answer, but you aren't alone.

I would say running it often would solve it, but I have a buddy who is on the water constantly. He fishes as a guide when he isn't catching fish to sell... And he has given up after removing the tank and cleaning and replacing everything 2times. He now has portable tanks in both rear hatches!
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Re: Fuel tank vent question

Post by Skeeter-Gee » Thu Feb 11, 2016 2:08 pm

ok first question , is your vent built into the cap of the fill? The perko all in one caps are horrible they make a spacer to shim the fill up so water doesnt puddle . If its a standard fill make sure the oring is correct . and the vent points aft. Expanding and contracting isnt your issue a good fuel water seperator will catch the residual moisture .Most water issues arent from ethanol , they are from rotted fuel senders or poly tanks they are warped.There are plenty of other scenarios but that is the most prevelant .

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Re: Fuel tank vent question

Post by AirHog » Thu Feb 11, 2016 4:44 pm

Google Atwood 90 degree p trap fuel tank vent. They work and are cheap.

As said, ensure running does not splash your vent.

Good Luck

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Hog Guts
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Re: Fuel tank vent question

Post by Hog Guts » Thu Feb 11, 2016 8:11 pm

I'm most interested in closing the system when not in use. Makes the most sense to me and is most practical for 200 gals of fuel. Filling it up at 9pm in the evening after waking up at 4:30am and fishing all day isn't going to happen. Dragging it out after that to fill up isn't gonna happen either it's too big to deal with for just a fill up. It gets filled on the way out next time. Sealing the system while handling the pressure changes just seems like it solves all the issues. Have fresh fuel for new trip, less to steel, no moist air, etc I'm just trying to figure out how to accomplish that the best way. All the other stuff is standard. I'm outside the box right now, that's my favorite place.


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Re: Fuel tank vent question

Post by mike99705 » Thu Feb 11, 2016 10:40 pm

Condensation in a tank is very difficult to stop especially in humid enviroments, try using HEET in the red bottle it may come in larger containers for your situation of 200 gallon fillups, the alchohol will mix with the water and suspend it in the gasoline or diesel fuel and it will pass through and burn. You will not have water laying in the bottom of your tank. The main ingredient in HEET is isopropyl alchohol. Have used it for years and do not have water in tanks even after several years of storage. But we do not have ethanol in our fuel either, that may change the picture.
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Gary S
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Re: Fuel tank vent question

Post by Gary S » Fri Feb 12, 2016 6:59 am

AirHog wrote:Google Atwood 90 degree p trap fuel tank vent. They work and are cheap.

As said, ensure running does not splash your vent.

Good Luck

AirHog
This is the one that cured my problem.

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