Double Bottom
A Double Bottom is a structural arrangement in which a second, inner watertight bottom is built above the outer bottom shell, with a space divided into tanks between them. It runs along the length of the ship and is a required feature of most cargo ships under SOLAS.
The double bottom does two jobs: it protects the ship if the outer bottom is holed in a grounding, keeping the sea out of the cargo and machinery spaces, and it provides the tanks used for ballast water, fuel oil and fresh water. Its structure — floors, girders and the inner bottom — is a core part of the hull strength.
The double bottom is a fundamental safety feature: a grounding that would flood a single-bottom ship is contained in the double-bottom space, protecting cargo, machinery and the environment. It also holds the ballast that keeps the ship stable and trimmed, making it central to both safety and operations.
What is the tank top?
The upper plating of the double bottom — the inner bottom — which forms the floor of the cargo holds or machinery space above the double-bottom tanks.