Sorry if this is off topic.
A few NOTs from someone with 15 years hands-on experience building
welded tanks--aluminum, steel and stainless:

First it {6061) does not weld anything like as well as 5052 or 5086

NOT. Welders don't even notice when they're moving from 5000 to 6000
unless one or the other is extrusion instead of plate. They're apt to
wire brush the extrusion a little more whichever alloy it is. They
don't change gas or rod/wire and don't even adjust their current if the
thickness and position are similar.

second it is much more susceptible to corrosion than the 5000 series.

NOT. 5083 and 5054 create a slight electrical potential and with 6061
you have slightly more. Don't use it in direct contact with salt water.
Bill Gates could afford to specify 5083 throughout a boat, but even he
could not guarantee that every piece of alloy on his boat is exactly
what he specified and probably paid for.

Only about 20% of those shiny tankers are aluminum. Most are
stainless.

NOT. ALL the shiny tankers you see dropping off fuel at the gas station
are aluminum.

They have a longitudinal structure, either chassis rails and cross
members or internal webbing to prevent excess flexing of the tank.

NOT. Truck tanks like semis and pull trailers are supported front and
back on bearing pads or short subframes. There is no longitudinal
stiffening other than the shell, rings, baffles and bulkheads that form
a structural tube. By excess flexing, you probably mean 'failure'. Any
aluminum semi tanker driver will tell you his 5000 gallon balloon flexes
with a period equal to the period of his suspension...And the first 50
years he drove the damn thing it scared the hell out of him.

The design is fairly standardized and carefully refined

NOT. There are off-the-shelf tanks and off-the-shelf boats. Every time
the customer buys a new tanker he recalculates the proportions of
regular, midrange and premium or diesel--always moving the baffles,
bulkheads and piping and often demaning a custom tube shape. Kind of
sounds like boaters. Everyone wants a 40' boat, but there are a lot of
interpretations of what goes in a 40' boat.

kand that kid that welded it has 2 or 3 years of training,

NOT. He completed a 60-day welding course at the county ROP training
site and the rest of his training was on-the-job training working with a
lead welder, definitely doing his share of weld-offs on the girth seams
of the semi-tankers we're talking about.

uses a $5K TIG welder and lays the same bead 8 hours a day 5
days a week.

NOT. All welded aluminum production tankers are mig welded. Period.
And nobody in a tank fabrication shop welds off every day. Your
aluminum hull might be a 90-day building project followed by 5 days of
weldoff. Tanker fabrication has a similar ratio of building : welding.

The guy I am working with can hardly write his name but he
lays a beautiful full penetration bead. :-)

NOT. One pass full penetration in butt welds is a myth. Your guy does
it the same way everybody else does. 1st he tacks it all together. 2nd
he runs a solid weld around the inside of the seam. 3rd he runs a
portable skill saw around the outside of the seam to remove
contamination and increase penetration. 4th the welds off the outside.

It is very hard to compare automotive tanks to marine tanks. While
automotive tanks have to deal with vibration they don't get the constant
cyclical loading of marine tanks. They also don't have to contend with
a constant exposure to a highly corrosive environment.

NOT. The Principle of Uniformitarianism assures us that the
mathematics, physics and chemistry on the 3rd rock from the Sun is the
same as on the 3rd rock from Polaris or anywhere else. Granted, the
period of waves is going to be different from the period of any given
set of trailer springs. The bending moments will differ. The number
and placement of anodes will differ. But we're talking about different
numbers, not different sciences.

Cheers,
Boat Dreams