Explain why a vessel's effective GM changes as it passes through waves in heavy weather, and what practical implications this has for the Chief Mate.

A. DYNAMIC GM VARIATION IN WAVES — CM EXPLANATION: In calm water, GM is a fixed value calculated from the vessel's mass distribution and geometry. IN WAVES: (1) WAVE CREST AMIDSHIPS: when a wave crest is at amidships, the waterplane area at the bow and stern is reduced (since the bow and stern are in the wave troughs). Waterplane area reduction → moment of inertia (I) of waterplane reduced → metacentric radius (BM = I/V) reduced → M moves down → GM decreases. At this moment the vessel is LESS stable than in calm water; (2) WAVE TROUGH AMIDSHIPS: when a wave trough is amidships (vessel at peak between two crests), the bow and stern waterplane areas are in the crests — waterplane area INCREASES → BM increases → GM increases. Vessel MORE stable than calm water; (3) PERIODICITY: this GM variation occurs at the wave encounter frequency. If the wave encounter frequency is approximately twice the vessel's natural roll frequency — PARAMETRIC ROLLING can develop; PRACTICAL IMPLICATIONS FOR CM: (a) The intact stability criteria are based on calm water GM — they do not account for this wave-induced GM variation; (b) A vessel that just meets the minimum GM requirement may be at risk in heavy weather; (c) Vessels with large variation in waterplane area (fine-bowed, wall-sided amidships) are most susceptible; (d) Container vessels with high GM are susceptible to parametric rolling because their GM amplification when a wave trough is amidships drives the parametric rolling phenomenon.

B. GM is a fixed property of a vessel's construction and does not change in waves. Only the vessel's angle of heel changes in waves, not the GM value.

C. In heavy weather, GM always increases because waves add buoyancy at the bow and stern. A vessel is therefore always more stable in rough seas than in calm water.

D. Wave-induced GM variation only affects vessels with a displacement over 20,000 tonnes. Smaller vessels have rigid waterplanes that do not change with wave patterns.

A. CM HEAVY WEATHER PREPARATION — PRE-DEPARTURE: (1) STABILITY CALCULATION: produce final departure stability calculation using the loading computer. Verify: GM exceeds loading manual minimum AND an appropriate margin above minimum for heavy weather (typical recommendation: GM at least 0.5-1.0m above minimum for Force 9+ North Atlantic); check GZ curve meets all intact stability criteria (2008 IS Code); verify trim is within acceptable limits; (2) FLOODING ANGLES: calculate the angle at which the vessel would be at risk of flooding — ventilators, air pipes, or other openings. Ensure this angle has adequate margin above the expected rolling in Force 9; (3) FREE SURFACES: minimise free surfaces in ballast tanks — either top up or press down. Partial tanks create large free surface corrections that reduce effective GM; (4) LASHINGS INSPECTION: physically check all cargo lashings on deck and in holds — direct the bosun and deck gang to retighten any slack lashings. Check hatch covers — all cleats tight, rubber seals intact, cross-joint seals good; (5) DECK FITTINGS: make rounds of the weather deck — secure all portable equipment; check anchor lashings; verify everything on deck is properly secured; (6) BILGES: sound all bilges and ballast tanks — confirm no unexpected water ingress; (7) CREW BRIEFING: brief the watch officers on what speed and heading changes to expect; advise the bosun to keep crew off deck in heavy weather; (8) PASSAGE PLAN REVIEW: review the passage plan with the master — identify alternatives if the weather is worse than forecast.

B. Stability in heavy weather is calculated automatically by the ECDIS loading computer. The Chief Mate only needs to check the final print-out, not independently verify calculations.

C. For a North Atlantic winter passage, the Chief Mate's preparation involves adding extra ballast to maximum capacity regardless of the effect on GM. Maximum ballast always equals safest vessel.

D. Heavy weather preparation is primarily the master's responsibility. The Chief Mate should focus on cargo documentation and hand-over notes rather than stability assessment.

A. CARGO SHIFT — CM INVESTIGATION AND MANAGEMENT: (1) INITIAL ASSESSMENT: 8-degree list unaccounted for by known weights — URGENT. Calculate: with current GM, what weight shift would produce 8 degrees? Formula: d = (W × GG1) / (Δ × GM). This quantifies the possible shift. Stability computer: input the assumed shift and see if it matches observed list; (2) INVESTIGATE CAUSES: (a) Reduce speed and alter course to a more comfortable heading — reduce rolling to minimise further shift; (b) If it is safe to do so, arrange for hold inspection to confirm or identify cargo shift. In Force 9, entry into holds may be impossible safely — wait for a reduction in conditions; (c) Check bilges in all holds — flooding can also cause a list; (d) Check ballast tanks — has a tank emptied or flooded unexpectedly?; (3) CORRECT THE LIST: options to reduce the list: (a) Ballast: pump ballast water to the high side to counteract the list. Use cross-filling of double bottom tanks; (b) Do NOT enter holds to reshift cargo in heavy weather — this is extremely dangerous; (4) STABILITY CALCULATION: recalculate stability with the assumed cargo shift. If GM appears to be reduced significantly — master to consider heading for nearest port of refuge; (5) NOTIFY MASTER: immediately — an 8-degree list is a serious stability situation; (6) MAYDAY READINESS: if list is increasing and stability is deteriorating, prepare for emergency.

B. An 8-degree list can be corrected by transferring fuel oil between port and starboard service tanks. No other investigation is necessary as a list of 8 degrees is within normal operating parameters.

C. Send the bosun's team into the cargo hold immediately to investigate and reshift cargo. Finding the shifted cargo source takes priority over safety considerations in Force 9.

D. Continue passage at full speed. An 8-degree list in heavy weather is caused by dynamic rolling only and will self-correct when the weather improves.