Ship Stability and Ship Construction: Intact and Damage Criteria
Stability fundamentals
The candidate must handle initial stability (GM), righting lever curves, and the relationship between KG, KM and GM. Understand free surface effect, the correction to KG for slack tanks, and why a small free surface can be critical in a ship already near her minimum GM.
Statutory criteria
Know the shape of the intact stability criteria: area under the curve to 30 degrees, to 40 degrees, and to the angle of maximum GZ; the minimum GZ and its angle; and the initial GM requirement. Understand how these are checked against the worst loading condition, not the departure condition.
Damage stability and subdivision
Understand permeability, the floodable length curve, and the difference between deterministic and probabilistic damage stability. Know why the margin line and bulkhead deck matter, and how cross-flooding arrangements affect the final angle of heel.
Construction and stress
Be able to describe longitudinal strength and the still-water and wave-induced bending moments, the purpose of shear strakes, bilge keels and transverse frames, and the reason for cofferdams around tanks. Understand corrosion margins and the survey regimes that preserve them.
Traps
- Using GM alone as evidence of adequate stability when the GZ curve is deficient.
- Ignoring the free surface correction in a partially filled double bottom.
- Confusing the angle of vanishing stability with the angle of maximum GZ.
- Treating a damage stability result as static when progressive flooding is possible.
Rule of thumb
For a first estimate, a shift of weight w over distance d changes KG by w times d divided by displacement. A 100 t shift 6 m upward on a 10,000 t displacement raises KG by 0.06 m and reduces GM by the same amount.