Build for 100 gallon capacity aluminum fuel tank

[flyingfrizzle]

Thought I would share if anyone wanted to tackle their own tank. I will include lessons I learned from my past experiences and good practices I took away from a post on the Glen L site. Link below:


https://www.glen-l.com/phpBB2/viewtopic.php?f=7&t=28367

[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.

[Vezo, Part II]

Very cool and informative read. Like the idea of notching the stringers. In my MA, the fuel coffin box was inexcellent shape, however I did not inspect underside to determine if it was glassed or not. I did not use any foam for structural integrity, because I believe my tank unnecessarily pitted and leaked prematurely where foam was in contact high on the sides of tank, moreso than top or bottom. I think the only thing you may have mentioned is your opinion on “sealing” the tank before install. Mine was coal tar epoxied. Thanks Frizzle.

Vezo, Part II

[flyingfrizzle]

Here are some ruff dims of the tank:

32" wide x 12" tall x 60" long


It was originally going to be narrower (28" wide) to fit between the stringers but once I realized the tops had to be cut off to remove the foam I made it a few inches wider to sit in notches cut into the sides of the stingers.

[flyingfrizzle]

Here is a few pictures of the tank coming together. I let it sit for a month prior to welding or assembling so there was a lot of oxides built up on the aluminum parts already. I Took a cordless grinder with scotch bright pads and cleaned the surface as good as I could over the entire inside of the tank sections. I polished the areas I would weld even better so they would weld decent. It is also very important to clean up the edges from the parts that were shear cut. The dross from the shear and any contaminates would make the aluminum weld like crap and likely would leave pepper looking oxides in the welds. It shows up as linear porosity if not cleaned off. That is one on the most important things about tig welding aluminum. Clean Clean Clean. After cleaning I scrub the surfaces to be welded with a clean stainless steel wire brush that has only been on aluminum then wipe with acetone. Below is the tank half that was prepped and a few other shots of it being put together.

[flyingfrizzle]

Here are the baffles being installed and welded in. Normally baffles get tabs bent on them around 1" at the edges to be welded vs welding them in direct. Since I used 3/16" thick material I was able to weld them in direct as tee joints vs lap welds on the bent tabs. I did have to add tabs to the center short baffle on one side though. I could weld 3 out of the four tabs both sides but the last one got covered when the longer baffle goes in so it had to be plug welded to the tab from the outside. You will noticed the short pieces with tabs then the larger baffle with holes drilled in it. The baffle gets plug welded to the smaller baffle threw the holes. And yes, it is tight crawling in a 12" wide box half way in it to weld these in. The outer two were not as bad but the center one was tough to access.

[Vezo, Part II]

Very cool Friz, I do not know how to weld but certainly do recognize a stack of dimes. Also, never being inside a tank I never would have guessed the baffles are so restricting. Like enough for air pressure and fluid pressure to naturally level. Signicant amount of less sloshing, and and like an integral amount of overall tank support? Pretty accurate?

Michael.

[DonV]

Damn Frizz that's too pretty to put in an old boat and put gas in it!!! Nice welds and very nice work. I will not have to worry about having you build my tank, too pretty and looks expensive!!!

[flyingfrizzle]

Originally Posted by Vezo, Part II

Very cool Friz, I do not know how to weld but certainly do recognize a stack of dimes. Also, never being inside a tank I never would have guessed the baffles are so restricting. Like enough for air pressure and fluid pressure to naturally level. Signicant amount of less sloshing, and and like an integral amount of overall tank support? Pretty accurate?

Michael.

The baffles do aid both ways, they keep the sloshing to a minimum and adds support to the tank. You figure 100 gallons of fuel at 6.5 lbs a gallon you could end up with 650-700 lbs to support. I was told a rule of thumb to keep the sections around 1-1.5 cubic feet per cavity depending on material thickness and overall size.

Originally Posted by DonV

Damn Frizz that's too pretty to put in an old boat and put gas in it!!! Nice welds and very nice work. I will not have to worry about having you build my tank, too pretty and looks expensive!!!

Thanks! I figured I could build it cheaper than buying one from somewhere like sunshine but now after seeing the amount of time in doing one this large I am rethinking that. If I would of just installed the baffles with 2" stitch welds on one side like most manufacture's do vs welding it out 100% I would only have a 1/4 of the time in it and it wouldn't of been so bad on the labor side but I tend to overdo everything I do. They say there's no kill like over-kill right?

[gofastsandman]

nice work.