Ship Stability and Construction
Ship Stability and Construction tests hydrostatics, intact and damage stability, trim, free surface, load line and structural knowledge. The master must be able to read stability data, calculate corrections and recognise when a loading condition is unacceptable.
Core rules
- GM is the metacentric height; positive GM gives initial stability. GZ curve area and range indicate reserve stability.
- Free surface reduces effective GM by FSC = free surface moment / displacement.
- Trim is changed by moving weights; list is athwartships imbalance. Angle of loll is unstable zero-GM equilibrium, not list.
- Construction: collision bulkhead, double bottom, watertight subdivision, longitudinal strength and load line zones.
Worked example
A rectangular tank 12 m long and 8 m wide is partly filled with seawater (ρ = 1.025 t/m³). Free surface moment = (l b³/12)ρ = (12 × 8³/12) × 1.025 = 512 × 1.025 = 524.8 t·m. For displacement 6,000 t, FSC = 524.8/6,000 = 0.0875 m. If initial GM is 0.60 m, corrected GM is 0.5125 m. The reasoning: the free surface moment depends on tank dimensions and density, and is divided by the ship's displacement.
Traps
Adding FSC to GM instead of subtracting it; using cargo density instead of water density; confusing list with loll; and ignoring trim when calculating UKC at the bow or stern. Load line zone and subdivision errors also cost marks.