A general term used for all watertight spaces contained between the outside bottom plating, the tank top, and the margin plate. Double bottoms are usually sub-divided into a number of separate tanks and can be used to hold clean ballast, portable or boiler feed water, or fuel. They also provide a measure of protection for cargo tanks if the bottom plating is damaged in the event of grounding. Chances of pollution may be diminished due to this protection.
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