Bending moment (longitudinal)

The longitudinal bending moment is the internal moment along the ship treated as a hull girder, arising from the mismatch between the buoyancy distribution (from the sectional-area curve) and the weight distribution. Hogging puts the deck in tension when buoyancy concentrates amidships; sagging reverses it. The still-water moment from the loading condition is combined with the wave-induced moment (IACS UR S11) to size the deck and bottom section modulus against allowable stress.