Margin line

The margin line is a reference line drawn at least 76 mm (3 inches) below the upper surface of the bulkhead deck at side, defining the highest the damaged waterplane may reach in the final condition of flooding under the subdivision rules. It is the limit the floodable-length and permissible-length calculations work to: with any compartment or set of compartments flooded, the ship must come to rest, in trim and heel, with the waterline nowhere above the margin line, leaving the 76 mm as a safety buffer below the bulkhead deck. The concept is the basis of deterministic SOLAS subdivision; the probabilistic SOLAS 2009 (Chapter II-1) rules supersede it for most ships but the term remains in use.