The keel runs along the
centerline of the bottom plating of the ship and for the majority of merchant ships
is of a flat plate construction. At right angles to the flat plate keel,
running along the ship's centerline from the fore peak to the aft peak
bulkhead, is a watertight longitudinal division known as the center girder or
vertical keel . This provides considerable strength to the structure an resistance to bending.
Some double bottoms have a
duct keel fitted along the centerline. No duct keel is necessary in the machinery
space or aft of it. The construction of the duct keel uses two longitudinal
girders spaced not more than 2.0 m apart. Stiffeners are fitted to shell and
bottom plating at alternate frame spaces and are bracketed to the longitudinal
girders. The keel plate and the tank top above the duct keel must have their scantlings
increased to compensate for the reduced strength of the transverse floors.
