Now here I agree with Steven that tanks should be supported from below.
While the static load per inch may be low the dynamic cycling from
pounding through the waves can cause stress corrosion and cracking in
the weld. A thicker top could actually make it worse. Think about what
happens when 500 pounds of diesel bounces. The bottom of the tank
flexes pulling the sides together. The flange therefore pulls against
the weld. The top goes into compression and because metals are stronger
in compression than tension it deflects less so the weld has to bend.
Being the weakest point it will bend the most. Also having lost any
resistance to stress corrosion during the welding it will corrode faster.

If you are hanging the tank a better solution would be to at least add
angle irons across the bottom every couple of feet to reduce flexing
and possibly up the sides to mounting points on the stringers to take
the load off the weld.

Brian D wrote:
So, the bending may be OK after all...but if the lips for hanging the tank
are bent, then a lid is welded in, the weld will run right along the bend
that makes the lip. Probably not so good again. There must be *some* way
to make an aluminum tank that can be suspended by stringers. I want the top
of the tank to be 3/16" so that together with the welded in baffles, it'll
make a stiff deck to walk on. I know lots of people who've built the boat
that I'm building have used tanks like these, but they let them 'hang by the
lid weldment'. The biggest tank that I know of like this is about 8 feet
long and contains 80 gallons of fuel, or about 60 pounds per foot ...30
pounds per weldment, pounding in the waves. Is that OK or are these guys
driving around with a failure waiting to happen?

Brian


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Glenn Ashmore

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