In some smaller vessels, a single-bottom construction is employed. The single bottom in smaller ships is similar to double-bottom construction but without the inner skin of plating. The upper edge of all plate floors must therefore be stiffened to improve their rigidity. The single bottoms is for vessels under 61 Meters (200 Feet) in Length." Where transversely framed, floors or bottom frames are spaced from about 457 to 610 mm (18 to 24 in.). They are required by the Rules to be provided with center and side keelsons of specified scantlings. Bottom transverse frames are not required to be provided with side keelsons, but a center keelson the same depth as the bottom frames is required for docking. A center girder or center keelson is required for docking.
The center keelson and side keelsons are intended primarily to keep the floors from buckling, or tripping, and to assist them in acting together. When the ship is in dry dock, the center keelson has the additional duty of transferring the pressure from the keel blocks into the floors and bulkheads, and thus into the hull as a whole. The scantlings of these members increase with the length of the ship because of their contribution to the longitudinal strength of the hull.
Reference :
Prof. Khaled Atua; Ship Construction Lectures
