Double Bottom

The double bottom is a fundamental structural feature of most large ships: instead of a single layer of bottom plating, the ship has two, an outer bottom that is the actual skin of the hull in contact with the sea, and an inner bottom, or tank top, some distance above it, with a framework of girders and floors joining the two. The space between them, divided into tanks, is the double bottom. This arrangement gives the ship a second, watertight barrier along its whole bottom, which is why it is required for safety in most cargo ships.

The double bottom serves several important purposes at once. First and foremost it is a safety feature: if the outer bottom is holed in a grounding or contact, the inner bottom keeps the sea out of the cargo holds and machinery spaces, so a minor bottom damage does not flood the ship. Second, the tanks formed within the double bottom are used to carry liquids low down in the ship (water ballast, fuel oil, and fresh water) and because these weights are carried right at the bottom they lower the ship's centre of gravity and help its stability. By shifting ballast between double-bottom tanks the crew can also adjust the ship's trim, list and hull stresses. Third, the deep framework of the double bottom adds greatly to the longitudinal and transverse strength of the hull.

Double-bottom tanks, being enclosed spaces, require full enclosed-space precautions for entry, and they must be maintained against corrosion, since they hold water and are hard to inspect. On tankers the concept is extended to the sides as well, giving a double hull that provides protection against pollution from side damage. The double bottom is a clear example of how a ship's structure is designed to combine strength, useful tank space and, above all, protection against flooding.