The bending moment is the amount of bending caused to the ship’s hull by external forces.
For example, The bending moment is the highest in the midship section when the ship’s ends are supported by crests of a wave ,known as `sagging’ or `positive bending’. When the ship is riding the crest of a wave at its midships, the bending moment is known as `hogging’ or `negative bending’. Bending moments are measured in tonne – metres.
The beam which we have been considering would also have a tendency to bend and the bending moment measures this tendency. Its size depends on the amount of the load as well as how
the load is placed together with the method of support. Bending moments are calculated in the same way as ordinary moments i.e. multiplying force by distance, and so they are expressed in weight-length units.
As with the calculation of shear force the bending moment at a station is obtained by considering moments either to the left or the right of the station.