Aluminum Gas Tank Installation Question

Discussion in 'Boat Design' started by 84EdH, Aug 24, 2012.

  1. baeckmo
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    baeckmo Hydrodynamics

    This thickness is below today's standard in EU for this size of tank. But the critical issue here is support. A metal tank must NEVER be "sleeping on its belly", i.e. supported by material that covers any part of its surface!

    The reason is that if you follow the idea of "Moreheadnc", you will have a perfect environment for corrosion. In the support area, there will always be a thin film of moisture with a low oxygen content, that leads to a pitting corrosion over the surface. During a period in the seventies, a number of tanks (al and "stainless steel") were installed this way, and they were all destroyed from corrosion over large parts of their surfaces, so DON'T wrap metal tanks in, or support them in a foam layer or similar!

    The reason your tank is still looking good is due to is support method and the possibility of air to circulate around it. If it should go back in, pressure test it first.
     
  2. tunnels

    tunnels Previous Member

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  3. CDK
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    CDK retired engineer

    If the tank survived 33 years and still looks shiny I would put it back exactly the way it was. The air around it will keep your tank healthy.
    But it does not harm to remove the tank element and look at the bottom with a Maglite or any other bright light source. That won't be as shiny as the outside and you may have to wash out some sediment before you can take a good look. If the bottom surface is pitted or shows trails as if snails have lived there, replace the bottom or the whole tank; otherwise put it back in.
     
  4. jonr
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    jonr Senior Member

    I agree, maintain an air gap and it won't corrode.
     
  5. 84EdH
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    84EdH Junior Member

    well thanks for all the good info. I did inspect the tank more thoroughly and I found one spot on the bottom of the tank that looks like it was thinned from corrosion. I took the tank to a good machine shop nearby to patch this small area. then we will pressure test.
     
  6. SamSam
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    SamSam Senior Member

    I'm not going to take the responsibility of telling you how to mount a tank of highly flammable/explosive liquids in your boat, as I don't know enough about it in general, much less the specifics, such as whether or not the angle flanges will flex and crack the sides of the tank, but I will tell you that fully loaded you will have about 216 lbs of gasoline flopping around in there and coming off a wave and smacking into the water puts some Hellacious loads on things. A bilge full of diesel is one thing, a bilge full of gasoline is an entirely different matter, so make sure you get it right.

    Myself, I would not feel comfortable having a 33 year old, 54" long tank of thin aluminum, (that wouldn't pass today's standards even if brand new) holding 216# of gasoline, only being supported by flanges welded on the ends. The flanges as I picture them are there to keep the tank from shifting around and not to hang the tank from. The critical issue here is support, as has been said. The #2 issue is not corrosion, but structural integrity. If the thing has corrosion but doesn't leak is much better than all nice and shiny but buckled in half and split open.

    Being that old and free of outside corrosion is pretty unusual though and does point towards no contact with anything. Then again, I also don't remember ever seeing one supported on flanges on the ends. Are there any marks on the bottom or towards the ends that suggest support? Like shiny wear marks or any sort of lines or straight edges or something?

    You could always contact a big custom manufacturer, explain your situation and ask how much a new one, made exactly how yours is made, would cost and how you would mount it. You don't have to buy it.
     
  7. 84EdH
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    84EdH Junior Member

    samsam, I think that is good reasoning. I plan to support the tank somehow from the bottom. I think using 1/4" thick plastic strips attached to the tank with 3m 5200, and then the strips supported by glassed over ribs will do the trick.

    thanks for your input!
     
  8. baeckmo
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    baeckmo Hydrodynamics

    NO, NO, and again NO!!!

    Don't you read/understand what is said about corrosion? If you want to use this tank with additional support, then take it to a welding shop that knows Al-welding, and add T- or L-profiles that counter the inner stiffeners.

    The "skin" of the/any tank is not suitable or designed for contact over extended areas, BASTA!
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    What he said :)
     
  10. jonr
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    jonr Senior Member

    This is speculative, but possibly aluminum would do OK tight against strips of zinc.
     
  11. 84EdH
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    84EdH Junior Member

    Baeckmo, I do read "what is said". I have read "expert" opinions from many sources, both on this site and from tank installers and manufacturers. As you know, there is not a consensus of opinion on the "best" or "right" way to install these tanks. My ability to comprehend what I read is fine, but thanks for asking.

    The neoprene strips I suggested, seem to keep the tank elevated from the floor, so air can circulate around the tank, and avoid moisture based corrosion. The 5200 is coated on the neoprene strip and to the tank, with no voids, and keeps moisture away from the aluminum. The use of strips spread out over the bottom keeps the weight of the tank from hanging on the end brackets, which was my concern to begin with. The existing end brackets would hold the tank from moving at all.
    How do the neoprene strips fail the "no tank sleeps on its belly" mandate?

    I could have brackets welded on, but they would have to hang the tank from the top also, leaving the tank structure itself to hold the weight through bouncing waves etc. As you suggested above, the tank is thin to begin with @.090".
     
  12. Submarine Tom

    Submarine Tom Previous Member

    84EdH,

    CDK, as usual, has provided reliable, sensible, time proven advise.

    I would humbly recommend you reinstall it the way it was.

    If it lasted as many years as you say then there must be something correct about it.

    "If it works don't fix it."
     
  13. 84EdH
    Joined: Aug 2012
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    84EdH Junior Member

    Sub-Tom, thanks for your input. I wish it were that easy. I am a firm believer in the idea that if it was good enough for thirty years, it aint all bad.

    The problem, as I stated early in this thread, is that I dont know if the tank rested on the floor or not. I pulled the tank and there was a mess of rotted plywood and wet foam under the tank. I didnt think about whether the tank actually rested on the floor or above it.

    The front half of the tank may have sat on the floor, as my floor under the tank steps down about three inches, leaving the rear half of the tank at leas three inches above the floor).

    I mentioned that I found a small area of corrosion on the bottom of the tank, and this area is on the part of the tank that MAY have been sitting on the floor.
     

  14. Submarine Tom

    Submarine Tom Previous Member

    Right, sorry, I'm (we're) victims of thread drift here.

    I'm confident you are on the right track and have the knowledge (or research abilities) to do good.

    When one posts here, it can get really complicated, confusing, misdirected, misspelled, and misinterpreted (what I'd give for spellcheck on this gizmo).

    Good luck.
     
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