46 C.F.R. § 64.17
Minimum tank thickness
(a) Except as allowed in paragraph (b) of this section, a tank with a diameter of—
(1) 6 feet or less must have a shell and head of
(2) More than 6 feet must have a shell and head of
(b) If the tank has additional framework to guard against accidental puncturing of the tank, the shell and head thickness must be