water and fuel tank materials

Just wondering what thicknesses of sheet i need to use when fabricating diesel and water tanks. Boat is a 42ft steel hull.
looking at either steel or stainless diesel tank approx 400l and stainless water tanks.
Also is 400l enough for cruising offshore ? what are recommended volumes for fuel and water??? any help would be much appreciated
cheers

 

From years ago...
Some of the Boats I worked on, repaired, etc had steel tanks. In fact I think they all did. In those days the tanks were at least 1/4" thick, or 3/8" thick.
Some of the military boats, like Sub-chasers had 1/2" steel tanks.

There are better alloy choices for today.
If you make your tanks in such a way that they can be inverted you might get double life out of them.
The Steel tanks rust at the bottom and top. We hardly ever saw the sides or ends rust.
All the fittings rust.
Be sure to make some pretty large inspection holes so you can check on rust inside the tank once in awhile.

I had a wooden boat. It came with and old Galvanized, riveted, 40 gal. Hot water tank for a fuel tank. That tank had been in the boat for 20 odd years before we got it, and we had it for another 25 years. It was still good, although a little rusty around some of the fixtures on the outside when we Cut it up and burned the boat.

 

aluminium is best for fuel, by far, and IF you have room for inspection hatches great for water too
we use International paints tank liner for alloy, but of course on the diesel no paint, and they will last forever
you should aim for 1000 l water if you can and as much fuel as you can, for a true cruiser

 

ABYC recommends a minimum steel thickness of 0.0747 inch (1.9 mm) but thats for a small tank, less than 100 gallons (380 L).