Aluminum vs Polyethelene Gas Tanks

[Bullshipper]

I can not see a way to weld the inside and outside seams on a baffled aluminum tank and am thinking that it may be better to just buy 2 Moeller 58 gallon polyethelene tanks for my project.

Any input or tips?

[Calloy]

Bullshipper,

Aluminum tanks can be welded inside and outside except for the top where a beveled groove weld at the side to top junction will suffice. Usually the transverse tank baffles have a flange or angle clips such that the tank top can be plug welded via slots to the baffles/clips beneath.

Keep all tank penetrations on the top of the tank (pick-up tube, fuel level sender, vent, etc.) for safety reasons. Use alloy compatible fittings with a good anti-seize/sealant. Alloy tanks must be electrically bonded to a metal hull.

Use appropriate alloys such as 5052, 5086, and 6061 series aluminum, .250 inch would be preferred, .090 inch minimum thickness. Baffle spacing should be 30 inches or less. Tig welds would be best but mig is fine.

Alloy tanks would be tougher but plastic tanks would be lighter and a lot easier if the right size and shape is available.

[Jay Perrotta]

We used the poly tanks for a year or two.

The gas molecules actually make heir way through the plastic and you end up with a bilge full of fumes. They are completely legal but I didn't care for them and wouldn't have used them had I known that going in...

Definitely stick with aluminum. Don't foam it in place. Baffle the heck out of it.

[Bullshipper]

Thanks for the good tips.

I had planned to keep the bilge ventilated, but sure see your point.

I am still a little nervous about just welding on one side of the top or ends. Maybe not further down the line when I have more experience.

As I sit now the plastic fits, is approved, works out to be a lot cheaper to buy as opposed to building it out of aluminum, and is engineered.

[warthog5]

I wouldn't use a plastic tank for just the reasons that Jay mentioned + they have no baffles.

There are other way to "trap" the top edge of the baffles to the top of the tank. 2 pieces of angle alum creating a slot. The angle gets welded to the underside of the top and the baffle slips between them.

The other way is to have a 90deg bend in the top of the baffle and rosette weld it to the top.

The baffles get the corners clopped off. aprox 2in to3in across the bottom and up the sides. This creates a triangle cut off on each side of the baffle.

[welder]

Jeff, I would spend the $$$ on a tank made to your specks by a tank shop.
That is one item that I would not do my self as my friends a family might be on the boat .

The rest of the hull and other items that you Fab. can be inspected easily at any time.

Once that tank is under the deck its out of mind till you smell gas then it's to late. [ Major repair ]

How close are ya to starting your build ?
Are you going mono hull ?
What lenght 23-26-28 ?

[Ironwood Boats]

We stopped using plastic tanks a few year ago.

The tops would collapse and twist. Then the tank would not vent properly and the gasket around the sender would leak.

[kmorin]

Bullshipper,

you probably have the work done by now but there is a pretty simple way build baffled tanks welded inside and outside.

First, if possible press brake the tank as much as you can to avoid any welding seams that can be avoided. Remembering the minimum bend radius for the different alloys where '52 will bend without stress cracking in a 2T bend but 5086 will need 4T (!) meaning that the 5086 alloy bend is either A) a multi-shot bend in the typical press brake or B) someone has bending dies for that alloy. B is not common.

If you get a trough bent for a rectangular tank, then the top seams and butts at the two ends are all that is needed to outside corner weld.

There will always be some seam or another you can't get too unless you have a manway large enough for you, your hood, and the MIG gun.

If you have a long tank that is large enough to fit your shoulders then tack up -trough (bottom & sides) and the far end and top. Now..... with a breather on and the vacuum pulling out the welding soot and dust, lay in the tank sideways and weld away.

I usually clamp an angle (American Standard has a curved apex at the legs) to the outside of the seams to keep the drop-through minimal, and to act as a chill bar to increase the freeze rate of the MIG bead inside. Stitch and stitch and cuss and sweat and all the other tank welder's common practices until the first 1/3 of the tank is welded inside.

IF the tank is too small to fit into try reaching the MIG gun in with an arm extended through the open end. If you're using a push/pull gun Python or Cobra this is easy. If you're using a pistol grip gun Prince/Miller Spoolmatic - this is more challenging. If you have to weld out this way (?) then try to roll the tank so all the welds are downhand and you can drag the cup on the corners' two adjacent tank sides.

Now put in the first (farthest away) baffle and stitch the near side (all that is needed) around the four sides and continue to weld your way out of the tank.

The last end is TIGged [or MIGged] from the outside. The alternative is to do the same procedure as above only with the top off, and then keyhole the baffles when you put the top on.

A second alternative (related to the first one above) is to weld stiffeners onto the tank top using MIG when its off the tank. In this method the stiffeners don't touch the baffles which are cut short of the tank top. Then a bar, or plate is laid on top of the baffles below the top panel stiffeners' depth and this holds the top of the baffles rigid to the tank ends without keyhole welding.

One method of using MIG to build tanks that will stand up well is to put a liner on all the edges. This can be done with a sheet brake so the 'pans' are bent and the welds are 'back' from the actual corner edge of the tank- OR you can tack some heavier strips of bar to the edges and fit the welds with about 3/4" T gap.

MIG has a bit more problem getting gas out of the weld zone before the puddle freezes so anyone who can, uses TIG. But, if you put the "gapped and backed" joint to use, you can carry much more amperage in the weld, while using MIG, than you normally would on 1/8" sheet.

By increasing the heat the freeze time goes up a bit and allows the root to be more free of gas bubbles compared to the same weld with just to edges of 1/8" parent metal. The increased molten time for a given inch of weld makes for a little less porous tank weld.

I'd suggest you not only baffle the tanks but use X braces on the panels in between baffles to make sure the is no diaphragm flexure of the ends, tank sides, or bottom in the areas between the baffles. Don't use bar for this X bracing just use sheet cut-offs with sheered edges to get easy straight edges.

I prefer to use an engine mount style of mounting bracket where a flat padeye is horizontal to some part of the tank. At right angles to this plate, vertically at the edges of the flat plate, are a pair of triangular gussets back to the tank wall above the flat mounting plate. This entire mount works best on a doubler plate a inch or so larger than the footprint of the bar and gussets on the tank. I like to round the corners of these tank wall doublers then weld from the middle of one side continuously around the corner to the middle of the adjacent side. This means there is no start/stop at the corners and the corners' radius keeps from point loading the tank walls. Spacing the tank mounts to match the internal baffles isn't a bad idea either.

I'd air test to 30 - 60" of water (1-2 psi) and rework the leaks, hopefully and easiest with TIG. If you do have to repair with MIG on the outside then make sure to cut all the way into the weld to leave only clean parent metal so you don't have to bubble the first weld's impurities out of the MIG seam or butt.

I prefer inside edge fits with TIG outside and 1/8" material seems to hold up fine as long as there is no panel flexure. If you have pulsed TIG you can skip the inside weld as the drop through is as good as back-welding in a bend break test, so why go to the extra work to inside weld if you can make one pass (high quality) from the outside?

cheers,
kmorin

[warthog5]

I'd air test to 30 - 60" of water (1-2 psi)

Totally agree. :)

People think "Oh it's only 5lbs, what the big deal?"

Ha! Look at this tank modeled.
In fact it is the tank in my boat and what 1 1/2PSI does to it.
My brother is a Mechanical Engineer and did the modeling as well as building the tank.