What is the space comprised of compartments between the inner and outer bottoms of a ship called?

The space comprised of compartments between the inner and outer bottoms of a ship is referred to as the double bottom. This design feature serves multiple purposes, including providing additional buoyancy, facilitating the storage of ballast water, and offering protection against structural damage to the hull. The double bottom also creates a space that can be used for fuel oil storage, which is particularly important for the operational efficiency and safety of the vessel.

The design allows for better stability by keeping the center of gravity lower and helps maintain the vessel's structural integrity by providing a barrier against potential leaks or breaches. Having this additional compartment can also aid in dividing potential spills, thereby enhancing safety on board.

The other choices refer to different aspects of ship design: the swash bulkhead is a partition that helps manage water that may slosh back and forth within a tank, transverse frames contribute to the overall strength and rigidity of the ship's hull, and longitudinal frames run parallel to the keel, providing structural support. Each has its own function, but none describe the specific compartmentalized space between the inner and outer bottoms as accurately as the term "double bottom."