Double-bottom

A double bottom is a structural arrangement with an inner bottom plating set above the keel plating, creating a void or tank space between the two for ballast, fuel oil, or fresh water. It protects against bottom damage from grounding by giving a second watertight boundary, and the tanks balance trim, heel, and draft. SOLAS Ch II-1 Reg 9 mandates a double bottom over the cargo length on passenger and most cargo ships, with the inner bottom height tied to ship breadth. Floors, girders, and the inner bottom together carry hull-girder bending and local cargo loads. MARPOL Annex I separately requires double bottoms under oil tanks.