Fuel vent for an outboard.

Discussion in 'Powerboats' started by LP, Apr 1, 2009.

  1. LP
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    LP Flying Boatman

    I'm in the process of prefitting systems and hardware on an 18' runabout that I'm building. I have a question regarding location of the fuel tank vent.

    In theory, it makes sense to locate the fuel vent in the splash well as this area, I believe, is fairly protected from spray and I would consider it separate from any passenger areas. Would this be an acceptable location or am I pushing the limits of good build practices?

    Reading some literature on fuel vent design, I came across the legalities of venting fuel to the environment. To me, this would dictate that the fuel vent be located at least as high as the filler neck to prevent fuel discharge out the vent line when topping the tank. This line of reasoning rules out the splash well and has me putting the vent adjacent to, but somewhat behind the filler neck on the boat deck.

    Comments?
     
  2. pistnbroke
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    pistnbroke I try

    usually goes in the transom and has a splash mesh built in
     
  3. AroMarine
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    AroMarine Junior Member

    Your vent line should have a loop which will be the highest point in the venting system. The vent fitting will be below that so in most instances the fill will be a little higher than the vent loop.
     
  4. Ike
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    Ike Senior Member

    So far good advice. But here's the rest of the story. Fuel vent lines should be routed so that they vent outside the boat, that is, any fuel that burps out the vent won't go into the boat. The obvious question then is what if it goes in the water? Yeah, then you are in violation of spilling fuel into the environment, but you have to weigh the safety (into the boat is an explosion hazard) against the environmental hazard.

    One: USCG rules for fuel vent systems do not apply to outboard powered boats.
    Two: ABYC standards for Fuel systems do.

    So how do you reconcile all this. Several manufacturers of fuel fills make fittings that have openings for both the fill and the vent. If the vent burps, it just goes back down the fill.

    Also there are devices on the market you can put under the vent to catch any burping.

    To avoid burping, the tank vent should be at the high end of the tank. This also allows you to completely fill the tank. When it's at the low end you can't fill the tank all the way. as tom said their should be a loop at the top. This prevents burping and water from entering the tank through the vent. The mesh on the vent is a flame arrestor. It helps keep bugs and other stuff out too but it's main function is to preveent fumes that are exiting the tank from igniting if a spark or flame is present.

    The splash well is used on some boats as the location of the vent, as long as any fuel that burps into the splash well won't go into the boat.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You also need to consider if you get a flash on the 'splash area' or 'save all', this could cross over to the vapour from the vent! Therefore not such a good idea to place the splash and vent in same proximity.
     
  6. Landlubber
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    Landlubber Senior Member

    The vent has a screen inside it to stop flash, buy one of the fill caps that has the vent incorporated, they work well, otherwise, the transom well is just as good.

    pistnbroke, ....not a splash gauze at all mate, it is an anti flash gauze, made of fine mesh, it cools the "fire", to burn, you need fuel, heat and oxygen...the gauze removes the heat, so there is no fire....same as in filler pipes on board ship, the vents have flash gauze in them to do the same job.
     
  7. LP
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    LP Flying Boatman

    Thanks.

    Thanks to everyone for their replies. I was really favouring the splash well location, but was concerned with whether it would be considered "external" to the boat (or any other unforeseen reasons). I can add a "catch bin" in the well area to catch any burps with little effort.

    I had forgotten about looping the vent line. Good catch! My vent should be a good 8-10" above the tank. I'd have to fill the tank most of the way up the filler tube to get flow out the vent without the loop. It's amazing how something as simple as a loop in the vent line can serve such an important role.

    Thanks, again.
     
  8. El Sea
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    El Sea Junior Member

    FWIW:
    I have been entertaining another approach to fuel tanks. My thought is to eliminate the vent and utilize a flexable type tank. While this sounds contrary to the normal approach, let me explain.

    With the push for ethanol and the growing problems that rise from this fuel when exposed to the atmosphere. When ethanol is exposed to air it starts to deteriate. Part of the process is phase separation, when ethanol absorbs water from the atmosphere and condensation. The tank/bladder bag/fuel cell would be filled the normal way and allowed to expand, but would collapse during usage, therefore no air to promote the problems.

    While my theory is simply, the actual application will pose more problems such as holding a fuel cell in place or a cell large enough for suffience time on the water.

    While this would hurt my business because gas fuel tanks would not be fouling I do think it would be advantages. Hope to hear some input from others...


    Luther Carrier
    Absolute Tank Cleaning
    St Petersburg, Fl
     
  9. Ike
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    Ike Senior Member

  10. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    I am rebuilding a 34' Silverton. I plan to relocate the tank vents . They are presently about 10 inches below the fill openings, in the side of the hull. I want to put them about 6 inches above the fuel fill openings, above the walkway. I think this is a better location for keeping water from splashing thru the vents. Any fuel coming out of the vents would spill onto the walkway (I plan to use a catch can when filling to keep from spilling fuel). Is this ok to do, by ABYC and USCG standards?
     
  11. Ike
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    Ike Senior Member

    You didn't say whether this is gas or diesel.

    ABYC says the same thing for gas or diesel boats

    "The fuel vent system shall be designed to prevent
    spilling liquid fuel into the boat, or the environment, when
    tested in accordance with H-33.17.4 while fueling the boat
    in accordance with the boat manufacturer's instructions."

    This is from H-33 the Diesel Fuel System standard but the Gasoline Fuel system standard is identical.

    Spilling diesel into the boat is messy, but spilling gasoline into the boat can be catastrophic. If spilling into the walkway is inside the hull, whether it is in the open air or not then it violates ABYC standards.

    USCG regulations say that you can't spill gasoline into the boat but makes no mention of diesel.

    There is no distance srequirent for fill and vent, in fact some fuel fills have the vent inside the fill. But any fuel fill or vent openings have to be at least 15 inches from any ventilation opening into the boat, including the ventilation intakes for the engine room.
     
  12. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    Thanks, Ike. The walkway does not drain inside the hull. There is a vertical wall several inches high that prevents anything spilled on the walkway from entering the boat - unless it leaks past one of the rod holders mounted in the walkway. I'd say that, since the fill opening is located in this walkway, that it will be ok to put the vents above this walkway. Any fuel spilled out the vents would end up on the same surface as spills from the filler opening.
     
  13. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    Oh, my boat is gas - outboards. Used to be in inboard boat - now will be powered by twin 225 Optimax outboards.
     
  14. Ike
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    Ike Senior Member

    Just an aside. ABYC committees have argued for years over the term "inside the boat" so don't think you are alone. Especially the motorwell for outboards. Is it inside the boat or outside the boat? Even the pros argue this one.
     

  15. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    Interesting...I suppose I could attach some 12" pieces of flexible tubing to extend the vents - point them straight out over the walkway...

    I sure don't want to give gasoline any way to get into my bilge, or even into the cockpit. My vents will be about three feet or more above the tanks, so fuel will probably have a hard time getting up and out the vents to spill onto the walkway. I wouldn't be opposed to attaching a "spill cup" to the vents, to catch anything that came out. They'd have to be guarded to prevent seawater or rain from filling the cup. If I could find something that worked, I'd put it over the vents.

    I plan to run the vent line up higher than the vent opening, then back down to the vent, in order to make it more difficult for any water that did enter the vent from making its way into the tanks. Would there be any problem with this design?
     
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