Ship Stability and Ship Construction
What the exam tests
Ship Stability and Ship Construction tests intact and damage stability, load lines, hull stresses, watertight subdivision and structural failure. Calculations must use correct units and the fluid GM.
Core rules and concepts
- GM = KM − KG. Positive GM gives initial stability; the GZ curve gives the righting lever over a range of heel.
- Free surface effect reduces GM: FSC = iρ/Δ, where i is the free-surface second moment; in mass units use i/Δ.
- Shifting a weight w over distance d moves G by GG′ = wd/Δ.
- Load line marks include S, W, WNA, T and F; seasonal zones and dates decide which mark may be submerged.
- Hull stresses include hogging, sagging, racking and panting; keep tank and cargo distribution within limits.
- Damage stability relies on watertight subdivision, cross-flooding arrangements and IS Code criteria.
Worked example
Displacement 5,000 t, KM 8.20 m, KG 7.40 m. GM = 0.80 m. A 100 t weight is shifted 6 m transversely: GG′ = 100 × 6 / 5,000 = 0.12 m. A transverse shift causes list but does not change KG. If the same weight is raised 3 m, GG′ = 100 × 3 / 5,000 = 0.06 m; new GM = 0.80 − 0.06 = 0.74 m.
Traps
Using KG instead of fluid KG; forgetting free surface in a slack tank; mixing tonnes and cubic metres; placing a weight high and assuming GM is unchanged; and ignoring load line zone dates.
Key points: GM = KM − KG; free surface correction; weight shift formula; load line zones; hull stresses; damage subdivision.