Fuel Tank Vent Lines / Are they Necessary?

Discussion in 'Boatbuilding' started by snowbirder, Jul 13, 2015.

  1. snowbirder

    snowbirder Previous Member

    I have a new type of fuel fill for my tanks.

    The fuel fill incorporates the vent right into the fill, so there is less chance of a spill.

    They look like this:


    With this fuel fill, I do not see the point in running a separate vent line up from my tanks, other than the fill hose.

    Any expanding pressure would come up the fill line to the same point the vent line terminates.

    Wouldn't it be more safe to have less items potentially leaking gasoline below decks by eliminating the vent line?
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    This does not look like a vent line that vents the fuel tank to atmosphere but merely reduces the chance of a blow back when filling
    So any pressure caused by expansion of the fuel vapor in the fuel tank will still be present and cause issues
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You still need to have a vent. That only prevents fuel spilling overboard or backing into the fill. Also, the new regulations require that the vent has a carbon canister to absorb fumes. They can no longer be vented to the atmosphere.
  4. snowbirder

    snowbirder Previous Member


    My tanks don't have all this crap.

    I have a fill, a vent and a fuel pickup.

    Now what?
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It depends on the type of filler neck. As suggested, the one pictured doesn't appear to be atmosphere vented, just an overflow drain, which also double as a "hiccup" vent. There are filler necks that incorporate vents. My small block powered Chris Craft, has a 3/16" vent on the filler neck, with no obstruction to atmosphere.
  6. snowbirder

    snowbirder Previous Member

    Hmmm.. I'm going to have to look more closely at this particular filler neck. It looked like it had the vent incorporated to me on a glance.

    Boat launches in less than a week, everything is already fully installed except vent line.

    No time to f around here.

    If this one doesn't work, I'll go non compliant and put the standard vent line in via those typical thru hull vents and cap off these little extra vents at the filler neck.
  7. Oleboynow

    Oleboynow Previous Member

    there are vents and vents
    with an oceqn going vessel; the vent comes well above the fills and well above deck on a loop, even then we sometimes fit a flap to stop green water ingressing
    the vent must be large enough to prevent blow back when filling from a high flow bowser, typically ona one and one half fill, one uses a Minimum one inch ID vent
  8. snowbirder

    snowbirder Previous Member

    The tank itself doesn't have a 1" ID vent.

    It's brand new, ABYC, fully compliant, made by Moeller.

    It's about a 1/2" vent.

    I've never had a boat where the vent was above deck level. They are typically located topside, just below deck level.

    I like your ideas, though. Quite an improvement over production boats.
  9. snowbirder

    snowbirder Previous Member

    Called Perko.

    You guys have it wrong.

    This is an integrated vent, as I had first described. It bleeds off tank pressure while keeping water outn through a novel cap design.


    original question is valid:

    What the heck do I need 2 lines coming up from the tank to the same point for?

    Only thing i could think of is for fast fueling...
  10. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    It doesn't matter where the second line goes, same point or not just it's open for air to vent and not too exposed to let water in..
  11. snowbirder

    snowbirder Previous Member

    Right, but the fill line could do the venting as well on this type of deck fill.

    I an not seeing the need for 2 hoses terminating in the same place, in the same fitting, from the same place in the tank.

    The only thing it could be for is shoving like 40gpm (80 lpm) down the fill line.

    I suppose that is the only purpose.
  12. Barry
    Joined: Mar 2002
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    Barry Senior Member

    If you read the Perko info that Gonzo presented, it appears that the fill in question is not vented to atmosphere when the cap is closed. Ie they say that this is a sealed fill and when the cap is closed, and sealed, this forces the fuel vapor through the carbon canister that is a new requirement.
    It appears that this vent is to assist in filling, ie fuel volume in, vapor out WHEN filling. Then the cap seals the filler cap.

    When you look at the Perkos drawing with the integrated FUEL FILLING VENT, this is also what it shows.

    If this is a filler that meets the requirements, with the canister as shown in the drawing, this vent cannot be open to atmosphere when the filler cap is closed

    I would screw the cap on the filler, cover the 1 1/2 inch hole with your hand and blow on the vent. If air is getting to atmosphere, then it is vented to atmosphere. Non compliant

    Another reason that the tank must be vented is a potential vacuum that might occur from the fuel pump drawing the fuel into the fuel pump

    Further in the tech info, they discuss the air moving in and out of the canister to keep the canister carbon "clean" With the filling vent open to atmosphere, this would not happen.

    Nevertheless, the info provided by Gonzo as to the new regulations will be valuable to new boat builders and DIYers as well
  13. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Sorry, it appears that this cap if it is part number 0541 is vented to atmosphere and in the tech specs states that it does not meet the emission requirements specification but did meet the specs in 1977 USCG

    I wonder how they keep the water out when you are doing a washdown if there is a flame arrestor over the vent. ie would not water hitting the edge of the cap lip find its way into the vent??
    1 person likes this.
  14. Oleboynow

    Oleboynow Previous Member

    thanks I was really just talking ocean stuff, I am sure all the pts made are applicaple and valid
    often side decks are awash and so an open vent does need be above them, of course you could close the vent except when fuelling and have a relief valve in the tank top so ,s not to suck a vacuum into any tank
    I know this does not answzr the original question, others have done that

  15. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Snowbirder didn't say how old his boat is but if the fuel system was installed before 2010 then he can have a fuel tank vent that vents directly to the atmosphere and does not have to have the flowback feature

    The Perko vent shown is however, designed to comply with the current EPA standard for fuel systems. The new fuel systems must have a carbon canister and may allow up to one psi in the vent system but if the pressure increases beyond 1 PSI they open and reduce the pressure. They also must have a flow back feature that allows a fuel overflow to flow back to the tank.

    I realize some people want to comply with new EPA standards on old boats but in this case the entire system is designed to work together. Replacing parts a piece at a time will not achieve this. You need to redesign the fuel system if you want to upgrade older boats to meet the standard.

    Vent systems typically had a loop in them that went above the level of the vent on the boat's side that kept water from getting into the tank.
    Last edited: Jul 15, 2015
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