Permissible length refers to the maximum length of a ship's compartment that can be flooded while ensuring that the sinkage, heel, or trim remains within acceptable limits without exceeding the floodable length. It ensures the ship remains afloat and stable after flooding.

Permissible Length Formula:

$$Permissible\:lenght\:=\:Floodable\:length\:\times Factor\:of\:subdivision$$

The Factor of Subdivision depends on the ship's length and the nature of its service:

• For passenger ships, the factor of subdivision is smaller compared to cargo ships.
• Smaller compartments ensure enhanced safety in case of flooding.

The position of bulkheads is determined based on the need to comply with the "reasonable amount of damage" criterion in case of flooding:

• Transverse Watertight Bulkheads should vertically extend up to the margin line.
• At least one watertight longitudinal bulkhead must be located 20% of the ship's breadth inward from each side.
• Bulkheads are positioned along the ship's length at intervals equal to the permissible length, but not exceeding the floodable length.
• The maximum permissible compartment length is limited to 10.7 meters.
• Collision Bulkhead must be located forward at a distance equal to the permissible length from the forward perpendicular.
• The ER must be enclosed by two transverse watertight bulkheads, with an aft peak watertight bulkhead enclosing the forward part.

The factor of subdivision introduces a safety measure by reducing the size of the compartments to limit the effects of flooding. It ensures that the ship's draft or trim has less chance of touching the margin line during flooding or heeling.

Permissible Length Formula:

$$Permissible\:length=\frac{Floodable\:length}{Factor\:of\:Subdivision}$$

A smaller factor of subdivision leads to a smaller permissible length, requiring more numerous and smaller compartments. This reduces the potential for catastrophic flooding, as a smaller flooded area is less likely to exceed the ship's reserve buoyancy and cause it to sink. The factor of subdivision is determined by the ship's length and its intended service. The nature of service is quantified by a "criterion of service" (Cs) number, which considers the proportion of passenger and machinery spaces to the total volume of the ship. A higher Cs number (indicating more passenger space) typically results in a lower factor of subdivision and therefore smaller compartments.