Girder
A Girder is a primary strength member of the hull, larger than an ordinary stiffener, that carries major loads and supports lighter structure. The centre girder (centre keelson) runs fore-and-aft over the keel as the backbone of the double bottom; side girders run parallel to it, and deck and bottom girders support the plating and the transverse frames.
Girders are part of the primary structure that resists the overall bending of the hull as it hogs and sags in a seaway. Their dimensions are set by classification rules and, like all main members, are checked for corrosion wastage at survey.
Girders carry the ship global bending loads, so their strength and condition matter to the safety of the whole hull, not just a local panel. Wastage or cracking in a main girder is a serious structural finding.
What is the centre girder?
The main longitudinal girder running over the keel along the centreline of the double bottom — also called the centre keelson — forming the backbone of the bottom structure.