What does Double bottom mean?
Meaning of "Double bottom"
A tank whose bottom is formed by the bottom plates of a ship, used to hold water for ballast, for the storage of oil, etc. Also a term applied to the space between the inner and outer bottom skins of a vessel. Also applied to indicate that a ship has a complete inner or extra envelopeof watertight bottom plating. A double bottom is usually fitted in large ships extending from bilge to bilge and nearly the whole length fore-and-aft.