The space between two bulkheads set close together, especially between fuel tanks (two walls separated to use for drainage or safety).
The narrow, empty space between two adjacent watertight or oiltight compartments. This space is designed to isolate the two compartments from each other and/or provide additional buoyancy. It prevents any liquid contents of one compartment from entering the other in the event of a bulkhead failure. In oil tankers, cargo spaces are usually isolated.