All ships are constructed with single or double bottom structures. Smaller vessels such as tugs, ferries, and cargo ships of less than 500 gross tonnage have single bottom structures and larger ocean going vessels, other than old tankers fitted with double bottom
Single bottom structure
In this type of structure, the plate floors (see the previous article to understand the concept of ‘Floors’ in shipbuilding) themselves act as the stiffening members of the bottom shell plating. Vertical plate floors are flanged at their upper edge, which increase the bending strength of the plate floor.
Double bottom
The space in between the two bottoms is called a double bottom space and is often used as storage tanks for storing ballast water, bilge water, sewage etc. There will be inner and outer layers of the hull on the bottom as well as the sides of the tanker ships. It saves the ship from water ingress or flooding if the outer layer fails. The double-layer construction helps in reducing the risks of marine pollution during a collision, grounding, and any other form of ship’s hull damage.
Bracket floors are mostly placed at each frame, and plate floors are generally placed at every three to four frame spaces (see “framing” section for Double bottom framing)