GM, GZ, KG, KM, free-surface, angle of loll — the stability numbers that decide whether your vessel floats right-side up.

Ship stability is the single topic most likely to fail an otherwise-strong deck-officer candidate. The math is unforgiving, the terminology is Victorian, and the consequences of getting it wrong in real life are measured in capsize events rather than demerit points. Examiners know this, which is why stability problems show up on every management-level written paper and every Master's oral across every major flag state. MMCE.app's stability module is built to drill the calculations until they become reflexive, and to teach the underlying physics well enough that you can answer scenario questions even when the examiner twists the setup.

What the exam actually tests

Stability exams test three things in roughly equal measure. First, the vocabulary — can you distinguish list from heel, loll from list, permeability from buoyancy? Second, the calculations — given a loading condition, can you compute GM, KG, free-surface correction, and evaluate the result against IMO Intact Stability Code criteria? Third, the narrative judgement — given a damaged-stability scenario or an angle-of-loll situation, can you describe the correct corrective action and explain why the wrong action makes things worse?

Which credentials test this

• USCG — Master 500-1600 GRT, Master Unlimited, Chief Mate (Stability and Trim module)
• MCA — Chief Mate (Stability, Construction and Operations written and orals), Master Unlimited
• Transport Canada (TC) — Master Near Coastal, Master Unlimited, Ship Construction and Stability examination
• DG-IN (India) — Master FG Function 3 (Controlling the Operation of the Ship) written and orals
• AMSA (Australia) — Master <3000 GT and Master Unlimited stability papers
• Maritime NZ (MNZ) — Master Foreign-Going
• MARINA (Philippines) — Management-level stability assessment
• RMI — Flag endorsements building on any of the above

Core subject-matter breakdown

• Metacentric height (GM): relationship between GM, KM, and KG; positive vs zero vs negative GM; typical operational ranges
• List vs heel vs loll: list from unbalanced weight, heel from external force (wind, turning), loll from negative initial GM with positive form stability further out
• Free-surface effect (FSE): loss of GM from slack tanks, FSE correction = i·ρ/W, transverse subdivision to reduce
• Permeability: fraction of flooded space available to water, cargo vs engine room vs accommodation values
• Righting arm (GZ) curve: initial slope = GM, maximum GZ and angle of max, range of positive stability, area under curve to 30° / 40° / downflooding angle
• Angle of loll: recognition (vessel lolls either side equally on small disturbance), correction (fill low tanks on the low side first, never the high side first)
• KG, KM, BM, BG: centre-of-gravity and metacentre relationships, effect of loading and ballasting
• Damage stability: deterministic (SOLAS factor-of-subdivision) and probabilistic (SOLAS 2009, Attained and Required Subdivision Index A and R) methods
• Watertight subdivision: margin line, bulkhead deck, floodable length, permeability of compartments
• Loadline zones and conditions: summer, winter, tropical, freshwater allowance

Common pitfalls & traps

The classic trap is correcting an angle of loll by ballasting the high-side tank first — this momentarily reduces GM further and can capsize the vessel through the lolled side. Always fill low-side low tanks first. Another pitfall is forgetting the free-surface correction when computing effective GM from a loading condition; examiners deliberately give you tank sounding tables to see if you apply i·ρ/W. A third is confusing GZ maximum with angle of vanishing stability (they are different points on the curve). Candidates also routinely botch the direction of KG change when loading high weight versus low weight.

How MMCE.app prepares you

MMCE.app's stability module includes worked numerical problems, hydrostatic curve interpretation drills, and scenario-based damage stability questions. Each wrong answer triggers an AI-tutor walkthrough that shows the error in your calculation chain — critical for problems where a sign error in line three becomes a capsized vessel in line nine. The adaptive engine tracks your mastery on intact versus damage separately, so if your GZ curves are strong but your probabilistic damage stability is weak, you'll see more of the latter. Pass threshold is 70% for stability; every module except Rules of the Road uses that benchmark.

Related credentials on MMCE.app

Stability knowledge underpins the Chief Mate cargo operations exam, the Tankerman-PIC cargo-transfer planning questions, and the IMO Model Course 7.03/7.01 written assessments. If you are working toward Master Unlimited, MMCE.app's stability bank pairs naturally with the Ship Construction, Cargo Operations, and Bridge Resource Management modules.