Where candidates fail
The most common weakness is treating stability as a loading calculation exercise rather than a command decision. Candidates can quote GM and GZ but cannot explain what they would do with the information, or how they would recognise a deteriorating stability situation at sea. Examiners also catch candidates who do not know where their stability information lives on board, or who conflate the Inclining Experiment, the Stability Booklet, and the Loading Manual as if they are the same document.
The approved stability information
Every vessel must carry approved stability information — for yachts operating under the Red Ensign Group Yacht Code (REG YC Part A, which superseded LY3) this is provided in the Stability Booklet (sometimes called the Trim and Stability Booklet). This is a flag-state approved document produced from the Inclining Experiment or, for series-built yachts, from a sister-ship experiment with a lightweight survey. It is not optional and must be available to the Master at all times. The Master is responsible for ensuring the vessel is operated within its approved limits.
Key contents to know:
- Lightship displacement and LCG/VCG from the inclining experiment
- Maximum permissible KG or minimum permissible GM curves against displacement
- Hydrostatic tables and cross-curves of stability (KN curves)
- Sample loading conditions showing compliant and, where relevant, limiting cases
- Free surface correction methodology
- Any vessel-specific operational limits (e.g. maximum angle of heel in a turn, stabiliser restrictions)
Working knowledge in practice
At command level the examiner expects you to demonstrate that you use this information, not merely possess it.
Before departure: Calculate or verify the departure and arrival condition. Check KG (or GM) against the approved curve for the actual displacement. Verify the vessel is in positive, adequate stability — not just technically compliant.
Free surface effect: Any slack tank degrades GM. Minimise the number of slack tanks simultaneously. Filling or emptying tanks to a known state is a stability management decision, not just a fuel management one.
Trim: A vessel may be stable but trimmed in a way that impairs steering, propeller emergence, or watertight integrity of openings. Trim affects the freeboard at the bow and stern — a command consideration, not merely a comfort issue.
At sea: Loading changes (stores consumed, fuel burned, water made/used, catch embarked, anchors deployed) alter displacement, KG and trim continuously. The Master must understand the direction of change and when a recalculation is warranted.
Intact vs. damage stability: The booklet addresses intact stability. The Master must know the vessel's subdivision characteristics and, where a damage stability booklet or SOLAS equivalence documentation exists, where it is and how to interpret it.
Recognising a problem: Sluggish return from a roll, a persistent list with no obvious cause, unusual sensitivity to beam seas — these are the practical cues that send a competent Master to the stability booklet, not away from it.