Ship Construction and Stability: Basics for the Watchkeeper
Stability fundamentals
You must understand the relationship between centre of gravity, centre of buoyancy and the transverse metacentre. Know that initial stability is measured by GM, that GZ is the righting lever, and that the area under the GZ curve represents dynamical stability. Understand free surface effect, why it reduces effective GM, and how slack tanks worsen it. Know the meaning of list versus loll, and why a stiff ship rolls quickly while a tender ship rolls slowly and heavily. Be able to describe the purpose of watertight bulkheads, double bottoms and the collision bulkhead.
Traps
A frequent trap is confusing KG and GM: raising KG reduces GM and increases the period of roll. Another is assuming that adding weight on deck always increases stability; it raises G and can reduce GM. Watch for questions on angle of vanishing stability and the range of stability, and on why a ship with a large free surface can capsize in a beam sea. Also be careful with the difference between transverse and longitudinal stability, and why longitudinal GM is very large.
Worked rule of thumb
GM equals KM minus KG. If KM is 8.0 m and KG is 7.2 m, GM is 0.8 m, a comfortable positive value. If you add a heavy weight high up, recompute KG by taking moments: new KG equals total moments divided by total displacement. Remember that a small GM gives a long roll period and a sluggish, tender feel, while a large GM gives a short, snappy roll that can damage cargo. Always check that the GZ curve has adequate area and that the angle of vanishing stability exceeds expected heel.