Multiple fuel tanks, single feed line

Discussion in 'Powerboats' started by romeomikehotel, Mar 18, 2019.

  1. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    Hi guys,

    I’m sure some of you know the answer to this so I’m hoping you can help me. Is it possible, or reasonable to run have multiple fuel tanks but only draw the fuel from one tank while the others feed the main tank as it is drawn down?

    Specifically, I’ll have an above deck tank and a below deck tank and I’d like to draw fuel from the below deck tank and have the above deck feed the below deck as fuel is burned instead of using a fuel tank selector valve. Is this possible or reasonable?

    Thanks
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I believe it is called a Day-Tank.
    But that's all I know.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Should be OK, so long as the above deck tank fuel level is not higher than the breather outlet of the underfloor tank.
     
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  4. romeomikehotel
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    romeomikehotel Junior Member

    So to be sure there’s no mistakes, I could in theory run a vacuum line from the breather outlet up to the gunnels or somewhere high for security?
     
  5. Barry
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    Barry Senior Member

    diesel or gas
     
  6. romeomikehotel
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    romeomikehotel Junior Member

    Gas
     
  7. Barry
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    Barry Senior Member

    No
    Not to meet the requirements for ABYC

    In order for you plan to work, you would need a bottom fitting that would drain into the lower tank. Bottom fittings are not permitted in gasoline tanks.

    In order to draw from a gasoline fuel tank you need a draw tube mounted to the top of the fuel tank. The requirements also state an anti-siphon valve. These are small inline
    spring loaded valves that will automatically close unless there is a vacuum put on the draw line. With the lower tank vented as per the rules, you would not be able to create a
    pressure lower than 0 psig, to permit the ant siphon valve to open. Hence requiring
    a lower gravity fitting to drain into the lower tank.
     
    Last edited: Mar 19, 2019
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  8. romeomikehotel
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    romeomikehotel Junior Member

    Ok thanks for the help.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    An outboard motor "tote tank" does not comply with that. The draw is from the top of the tank, but there is no anti-siphon valve on any such tank I have used.
     
  10. Barry
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    Barry Senior Member

    You are quite right about portable gasoline fuel systems. I was assuming built in tanks but should have asked the OP I referenced ABYC H24 which pertains to built in tanks. Portable tanks, covered under H25 restrict portable tanks to 26.5 litres. Still top draw, no mention of an anti siphon valve

    Hard to see the difference between a regular brass fitting to hose and an anti siphon valve.
    We discovered this item fitting with low fuel levels in rather large seas we had one engine stall, needed the fuel to get home, did a trouble shoot, no fuel coming into the line, took the upper draw apart and discovered a plugged orifice, due to the spring and ball restricting the flow. Cleaned it up and we had no more issues. Back at the marina, took the other one apart and found it was almost plugged. Something to keep in mind on old boats to check up on.




    [​IMG]
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, he could do it without a tank drawing fuel from the bottom, but if he wants to be in strict compliance, likely not without that valve.
     
  12. Barry
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    Barry Senior Member

    I cant see how this could occur unless the lower tank is not vented to atmosphere, an ABYC requirement. The fuel pump produces a vacuum, the flooded draw tube provides the fuel from the lower tank . The pressure in the lower tank, ie the gas fume/air mixture is at zero gauge, how would you get the fuel in the higher tank to move to the lower tank without a lower installed pickup.
    Just curious, perhaps I am missing an alternative, without a valve to isolate the lower tank
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The higher tank would siphon into the lower once it was primed. But that sounds like it would not comply with the rules. But maybe it would, if the connection between the two tanks was permanent. In practical terms, it seems easily doable, but the letter of the law I don't know.
     
    Last edited: Mar 20, 2019
  14. fallguy
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    fallguy Senior Member

    I have a rather nice setup for running a small Honda generator continuously. I use a 6 gallon marine outboard tank. The entire operation works by using the vent inlet of the generator as the fuel inlet from the tank being siphoned. Or basically moving the vent to the portable or auxiliary tank. This would never comply with boat rules.

    The entire affair works and meets regs with a tank selector valve. So simple.

    In the case of my generator, noone is there to fuel it, thus the need.
     

  15. Ike
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    Ike Senior Member

    Actually to answer this correctly, we first need to know is this an inboard or permanently installed engine, Or an outboard. The rules are slightly different depending on the type of engine. Obviously you are talking about permanently installed fuel tanks. An anti-siphon valve is not required in all cases. In particular if the top of the tank you draw from feeding the engine is below the fuel inlet fitting on the engine (usually the fuel pump) then an anti-siphon valve is not required. However, You may have to have one between the on deck tank and the below deck tank to prevent siphoning from the upper to the lower tank. But there are alternatives

    Here's what ABYC H-24 says

    24.15.10
    Fuel distribution systems shall be provided with anti-siphon protection by keeping all parts of fuel distribution and return lines above the level of the tank top from the tank to the carburetor inlet or its equivalent, (e.g.,throttle body, port fuel injection, or a location where fuel leakage cannot enter the boat when the boat is in its static floating position); or
    NOTE: In pressure relief system consideration should be given to the effects of tank pressure and the anti-siphon requirements
    .
    24.15.10.1 by installing an anti-siphon device at the tank withdrawal fitting, or along the fuel distribution line, with a rated siphon protection head and flow rate greater than required for the installation and a screen complying with H-24. 19.2.10.2 installed on the tank side of the anti-siphon device; or

    24.15.10.2 by installing an electrically operated valve at the tank withdrawal fitting, or along the fuel distribution line, connected to be energized open only when the engine ignition switch is on and the engine is running; NOTE: A momentary type override is acceptable for starting. or,

    24.15.10.3 if the fuel tank top is located below the level of the carburetor inlet or its equivalent, and the fuel line is rigid metal or USCG Type A1-15 hose, by installing a manual shut
    -off valve directly at the fuel tank connection, arranged to be readily accessible for operation from outside the compartment

    The whole point of this is to keep fuel from siphoning down through the system and flooding the engine compartment with gasoline. In a system where the fuel pump is on the engine and has to suck the fuel to the engine, this is unlikely to happen if the flow is all uphill and the pump is off. It's required if the flow is downhill but not necessarily if the flow is uphill.
     
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