I've never felt comfortable about hanging the tank. I'd use poly, but I can
get more volume with a custom aluminum tank. I'll support the tank, either
by shelves that run along the sides of the tank (extending beneath the tank
to carry it from below of course) or will put an on-edge stringer under the
tank. Either way, will use neoprene between the support and the tank. The
tank will be etched, primered, and epoxy painted to prevent external
corrosion and I'll lift the tank out every 5 years or so for an inspection.
I'll maintain the "use the top as a deck" concept and will place enough
baffles in the tank, welded to the top, and use a 3/16" top too, to make
sure it is a stiff deck. Might be a gas fumes sensor in the tank area since
the drain plug from that chamber will lead into a fish box, which means
it'll be closed until the boat's back on a trailer.

Thanks,
Brian

--
My boat project: http://www.advantagecomposites.com/tongass


"Glenn Ashmore" wrote in message
news:%79kb.79853$sp2.33451@lakeread04...
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


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com