It is also the maximum length of the ship that can be flooded without submerging the margin line. Margin line is a line just below the top of the bulkhead deck.The floodable length of a vessel varies from point to point throughout her length and is usually the greatest amidships and the smallest near the quarter length.
The floodable length of each point along the ship’s length is multiplied by the permeability to obtain the Permissible Length. The permissible length curves for 85 percent and 60 percent permeability are now incorporated into the floodable subdivision diagram. Here, for machinery compartments (e.g. engine room compartment BC) the vertices for the triangle needs to be checked against the 85% curve, instead of the floodable length (100%) curve
The general values of permeability used for different types of compartments are listed below, followed by a logical understanding of the variation in the values:
Watertight Compartment – 95 to 97%
Accommodation spaces – 95%
Machinery compartments – 85%
Cargo holds – 60%
Stores – 60 %