[flyingfrizzle]

To start off with most marine fuel tanks are built from 5052 grade aluminum plate that is much more corrosion resistant than common grades of aluminum. There are other 5000 series grades that will work also but by far the 5052 is the most commonly one used in the marine tank industry. It has a higher amount of magnesium than standard 3003 grade your common diamond plate tool box is made from for added strength and corrosion resistance. It is welded with 5356 filler normally which is one of the best matches for this grade and is much stronger than the more common 4043 filler used for most Tig welding. The cost of 5052 is a little more than 3003 and pricing will wildly change biased on metal prices. I got a quote for 3 sheets (4x8) and noticed the price was only good to "the end of the business day". I ended up purchasing the plate a few months latter after the market had seen an increase and ended up paying $220 a sheet vs the $180 quoted months before if I remember correctly.
I ended up going with .190 (3/16" thick) which was slightly over kill but for good reasons. Most Tanks are built with .125-.140 (1/8" up to 100 gallons) then .140-.190 for stuff over 100 gallons. I could of got by with something thinner but had several reasons to go with thicker material.

First off this tank is going in a 23 foot seacraft that will have a flat floor with no tank access cover / coffin and needed to last 30+ years. Secondly, its slightly over 100 gallons and will be heavy when filled. Third, the thin stuff warps easily when welded and has to be cooled frequently or pulse welded. Fourth, overall strength. Fifth, doing 1/8" would require me to bend lips or tabs on all the baffles for welding to prevent hard spots. Sixth, the largest radius die my machine shop had was still fairly tight for bending the corners and I didn't want to thin out the material too much or crack the bends in the sheet metal press brake.
I almost wish I went with 5x10 sheets as I could of bent the whole tank from one sheet minus the top cap and end caps. That would of made for less fitting and better access for welding in the baffles. The 4x8s were about half the cost and made for less waste or scrap though. I ended up bending two "U" shaped pieces that lapped into one another for two welds down the sides. One benefit of doing it this way us I will be able to weld the inside of the tank also. This creates two seals or welds to keep the tank from leaking. It has to pass the inside weld somehow threw a pin hole then would also have to pass the outside weld before it could leak.
The tank will get three baffles the short way and four short baffles the long way in between the first set. This will create eight separate compartments for fuel division. This will keep the sloshing down and strength of the tank up. A full tank this size may end up with close to 800lbs in it so you want it braced up internally when jumping 4 footers out 50 miles offshore.
A few sketches of the tanks dims. and layouts below. Note this tank is wider than the spacing between the stock stringers on a 23 seacraft. It would have to be narrower to drop in a factory fuel cavity. Plans on this hull are to open stringers up to remove wet foam and notch them slightly to create spacing for the tank so it can rest on top of the notched stringers. The center factory shelf was only glassed top side and water rotted the ply out as the bottom did not have any lay up on it. There will be cross braces but we are trying to keep air circulation around the tank to prevent future corrosion.