Double bottoms are significantly safer than single bottoms. In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment, and the main occupied areas of the ship remain intact.
Double bottom: general term for all watertight spaces contained between the outside bottom plating, the tank top and the margin plate.
BALLAST WATER Sea water, confined to double bottom tanks, peak tanks, and other designated compartments, for use in obtaining satisfactory draft, trim, or stability.
Ballast tanks: Double bottoms for carrying water ballast and capable of being flooded or pumped out at will.
Ballast: Heavy weights packed in the bottom of a boat or ship to give her stability.
Batten down: To make watertight.
To provide for a return journey without a cargo, in addition to the double bottom and peak tanks, large water ballast tanks are provided abreast of and above the cargo spaces, and arranged so that when ballasted down the metacentric height, ...
Double bottoms not sealed to the hull or which are not completely filled with flotation material;
Closed living spaces;
Closed stowage compartments in which combustible or flammable materials are stowed; or
Permanently installed fuel tanks.
Where grounding is possible, this type of keel is suitable with its massive scantlings, but there is always a problem of the increased draft with no additional cargo capacity. If a double bottom is fitted the keel is almost inevitably of the flat ...
 See also: Ship, Hull, Serve, Draft, Right
  
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