Modulus of section (Z)

The section modulus Z is the ratio of the second moment of area I about the neutral axis to the distance y from that axis to the extreme fiber, Z = I / y. For hull-girder strength it sets the bending stress: sigma = M / Z, so the required midship Z follows directly from the design bending moment and the permissible stress. IACS UR S11 fixes a minimum midship section modulus for ships over 90 m, evaluated at deck and keel. Because deck and keel lie at different y, a ship has two section moduli; the smaller governs. Adding material far from the neutral axis raises Z efficiently.