Double Bottom Structure

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An inner bottom (or tank top) may be provided at a minimum height above the
bottom shell, and maintained watertight to the bilges. This provides a
considerable margin of safety, since in the event of bottom shell damage only
the double bottom space may be flooded. The space is not wasted but utilised to
carry oil fuel and fresh water required for the ship, as well as providing
ballast capacity.

The minimum depth of the double bottom in a ship will depend on the
classification society’s requirement for the depth of centre girder. It may be
deeper to give the required capacities of oil fuel, fresh water, and water
ballast to be carried in the bottom. Water ballast bottom tanks are commonly
provided right forward and aft for trimming purposes and if necessary the depth
of the double bottom may be increased in these regions. In way of the machinery
spaces the double bottom depth is also increased to provide appreciable
capacities of lubricating oil and fuel oil. The increase in height of the inner
bottom is always by a gradual taper in the longitudinal direction, no sudden
discontinuities in the structure being tolerated.

Double bottoms may be framed longitudinally or transversely , but where the
ship’s length exceeds 120m it is considered desirable to adopt longitudinal
framing. The explanation of this is that on longer ship tests and experience
have shown that there is a tendency for the inner bottom and bottom shell to
buckle if welded transverse framing is adopted.

This buckling occurs as a result of the longitudinal bending of the hull,
and may be avoided by having the plating longitudinally stiffened.

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