Hull girder section modulus (Z)

The hull-girder section modulus $Z = I / y$ is the moment of inertia of the midship section about its neutral axis divided by the distance to deck or keel, setting the bending stress $\sigma = M / Z$ for a given longitudinal moment $M$. IACS UR S11 fixes a minimum rule section modulus from ship length, breadth, and block coefficient, and the design must satisfy it at both deck and bottom. It is the headline strength number of the structural design.