Double Bottom is a construction method used in the contruction of a ship. The double bottom space is formed by fitting of additional plating above the bottom plating, extending from side to side and over most of the length of the vessel. The inner bottom plating is called the Tank Top is constructed to provide a watertight space below it. This watertight space is called a double bottom tank.Most dry cargo ships and bulk carriers are fitted with double bottoms. Many tankers are also being constructed with these structural arrangements.

Typically, double bottom depths vary from 1-1.5m. the following are some of the advantages of double bottomed ships:

1. Provide double protection to the hull in case of grounding or bottom damage. Helps to prevent pollution in case of tankers.

2. Provides great longitudinal strength to the bottom of the ship where there is most required. As the mass of the cargo acts on the bottom, this area needs to be the strongest especially when carrying high-density cargoes.

3. Can be used for carriage of fuel oil, ballast or fresh water.

4. Because of their location, Double Bottoms help to maintain safe levels of stability (GM) during ballast passages while providing the required draughts and trim.

5. Since most of the Double Bottoms are longitudinally divided, from surface effects due to slack tanks are reduced to negligible levels.

Construction within the Double Bottom: the compartment may be transversely or longitudinally framed.

Transversely framed double bottom:

1. A water tight centre girder divides the compartment into two separate tanks, port and star board. Side girders will also be fitted on either side of the center girder. These are not usually watertight and are provided with lightening holes for air, liquid flow and access. The girders extend the full depth of the double bottom tank.

2. Transversely, three types of floors are fitted in Double Bottom’s, namely watertight floor- these are solid plates extending the full depth of the Double Bottom and the entire breadth of the tank. They are fitted underneath the watertight bulkhead and provided water tightness between tanks in the longitudinal direction. These are the strongest floor.

Plate Floor

These are plates very similar to the water tight floors, but having lightening holes cut in them, so that they are not water tight. Air holes, drain holes are also provided.

Bracket Floor

Fitted between plate floors to provide support for the tank top. The main part of the plate is omitted and only a bar is fitted. On each side of the center girder and at the side, flanged brackets are found.

Longitudinally framed double bottom :

1. Bottom longitudinals are fitted both on the bottom plating as well as under the tank top. Centre and side girders as in the transversely framed Double Bottom above.

2. Floor arrangement is similar to the transversely framed Double Bottom, except that in the case of the bracket, the bar is omitted.