46 C.F.R. § 171.017 – One and two compartment standards of flooding.

(a) One compartment standard of flooding. A vessel is designed to a one compartment standard of flooding if the margin line is not submerged when the total buoyancy between each set of two adjacent main transverse watertight bulkheads is lost.
(b) Two compartment standard of flooding. A vessel is designed to a two compartment standard of flooding if the margin line is not submerged when the total buoyancy between each set of three adjacent main transverse watertight bulkheads is lost.