Ship Stability, Ship Construction and Vessel Manoeuvring and Handling
Domain 4 tests whether you can judge stability from hydrostatic data and understand how hull form and machinery affect handling.
Stability basics
Displacement is the weight of water displaced. KG is the height of the centre of gravity above the keel; KM is the height of the metacentre above the keel; GM = KM − KG. Positive GM means stable equilibrium; negative GM means unstable and dangerous. GZ is the righting lever, and the area under the GZ curve indicates dynamical stability. Free surface effect raises the effective KG and reduces GM. List is a permanent transverse inclination; heel is temporary.
Construction and watertight integrity
A collision bulkhead is fitted forward; transverse watertight bulkheads subdivide the hull; the double bottom protects against grounding. Watertight doors, hatches and vents must be closed as required by the stability booklet. Know load lines, drafts and deadweight.
Manoeuvring
In a turning circle, advance is the distance along the original course to a 90° change; transfer is sideways movement; tactical diameter is the diameter for about a 180° change. Squat increases in shallow water and at higher speed, reducing under-keel clearance and increasing drag. Bank effect and ship-to-ship interaction can shear a vessel off course.
Worked example
KM 8.0 m and KG 7.4 m give GM = 0.6 m. If free surface in a partly filled tank produces a free surface correction of 0.25 m, the effective GM falls to about 0.35 m, making the vessel noticeably more tender.
Traps
Using GM alone without the GZ curve; confusing list and heel; forgetting free surface correction; assuming more ballast always improves stability; neglecting squat and interaction in shallow or confined water.