Stability and Shipboard Safety
What the exam tests
Stability and Shipboard Safety tests initial stability, the GZ curve, free surface effect, trim and list, damage stability, and practical safety controls such as drills, permits and enclosed-space entry.
Core rules
- KM = KB + BM, where BM = I/V for the waterplane moment of inertia I and displacement volume V. GM = KM − KG. Positive GM means initial stability; negative GM means an unstable ship that will loll.
- A free surface reduces effective GM by FSC = Σ(ρ_liquid i) / (ρ_ship V), where i is the free-surface moment of inertia of each tank surface. The correction is always subtractive.
- Large-angle stability is judged from the GZ curve: area under the curve, maximum GZ and range of positive stability matter, not GM alone.
- Trim is changed by moving weights; trimming moment = weight × longitudinal shift, and change of trim = moment / MCTC.
- Safety: SOLAS drills, muster lists, enclosed-space entry permits, atmosphere testing with oxygen about 20.9% in fresh air, rescue equipment and permit-to-work systems.
Worked example
A ship has KM = 8.00 m and KG = 7.20 m, so solid GM = 0.80 m. A slack tank produces a free-surface correction of 0.15 m. Effective GM = 0.80 − 0.15 = 0.65 m. The ship remains initially stable, but the reduced GM means a longer roll period and less resistance to small heeling moments. The GZ curve must still be checked for large angles.
Traps
Do not add FSC to GM. Do not use GM as the only stability criterion. Keep KG and KM in the same reference, normally the keel. In trim, check whether the weight shift is longitudinal or transverse; a transverse shift causes list, not trim.