Ship Structures and Strength
Longitudinal strength
The hull acts as a box girder. Still-water bending moment comes from the weight and buoyancy distribution; wave bending moment is added by the standard wave, giving hogging (deck in tension, keel in compression) or sagging (the reverse). Section modulus is Z = I/y about the neutral axis, and bending stress is σ = M/Z. Shear force and shear stress, buckling of deck and bottom panels, torsion in open-deck ships, and local loads from tanks and cargo are checked separately. Classification rules, the IACS common structural rules, load line assignment and fatigue assessment govern scantlings and details.
Traps
Keep hogging and sagging signs consistent; deck stress in sagging is compressive. Use the section modulus for the fibre in question, since deck and keel have different y values. Do not confuse still-water with total bending moment. Corrosion additions mean net scantlings are less than as-built thickness, and buckling strength depends on plate slenderness as well as yield stress.
Worked example
A midship section has I = 40 m⁴ about the neutral axis and deck y = 8 m, so Z_deck = 40/8 = 5.0 m³. For a total bending moment of 250 MN·m, σ = 250/5.0 = 50 MPa. Rule of thumb: in conventional ships the wave-induced bending moment is often of the same order as the still-water moment, so both must be included.