Ship Stability and Cargo Operations
Ship Stability and Cargo Operations covers the intact and damage stability calculations you sign off as chief mate, plus the cargo knowledge that keeps a laden ship inside those limits. Examiners test numbers, and they test whether you know which limit governs when two are close together.
Intact stability criteria
The 2008 Intact Stability Code sets minimum standards for most cargo ships of 24 m and over:
- Area under the GZ curve to 30°: at least 0.055 m·rad.
- Area to 40°, or to the angle of flooding if less: at least 0.09 m·rad.
- Area between 30° and 40°: at least 0.03 m·rad.
- Maximum GZ at least 0.20 m, occurring at 30° or more.
- Initial metacentric height GM at least 0.15 m. The severe wind and rolling (weather) criterion must also be satisfied. Grain ships under the International Grain Code need a heel of not more than 12° from an assumed grain shift.
Free surface
Free surface reduces effective GM: FSC = i/Δ, where i is the second moment of area of the tank's free surface about its centreline. For a rectangular tank, i = l·b³/12 - the athwartships breadth is cubed, not the length.
Cargo knowledge
Stowage factor drives deadfreight; heavy cargo in the wrong place produces unacceptable stress. IMSBC Group A cargoes may liquefy, so you need the transportable moisture limit and must reject cargo above it. Group B cargoes present a chemical hazard; Group C presents neither, if properly handled. Securing follows the CSS Code, packaged dangerous goods segregation follows the IMDG Code.
Worked example
A double-bottom tank 20 m long and 10 m wide is slack. i = 20 × 10³ / 12 = 1,667 m⁴. At 8,000 t displacement, FSC = 1,667 / 8,000 = 0.21 m. A solid GM of 0.75 m becomes an effective 0.54 m. That still clears the 0.15 m minimum, but the GZ curve must be re-checked, because losing 0.21 m of GM flattens the whole curve.
Traps
Cubing the length; forgetting that two slack tanks add their i values; confusing list with angle of loll, which is corrected by filling low spaces first and never by shifting weight high; and accepting a cargo declaration without checking moisture content against the TML.