BML Revision: Ship Stability (Section 7, part 2)

Revision card 8 of 12 — Boatmaster's Licence, Tier 1 Level 1 Underpinning Knowledge, MSN 1853 Annex 9. Stability is one of the densest sections in this syllabus — worth its own card.

Basic Principles

• Principles of floatation
• Water and weather tightness, watertight integrity and reserve buoyancy — watertight doors, ports, windows, deadlights

Definitions You Must Know Cold

• Mass, volume, density, relative density
• Displacement, deadweight, buoyancy — and waterline length, breadth, draught, length overall, length between perpendiculars, freeboard
• Centre of Buoyancy (B), Centre of Gravity (G), free surface, transverse metacentre (M), righting lever, righting moment at a small angle of heel

Equilibrium & Loading

• Stable, neutral and unstable equilibrium; what makes a vessel 'stiff' vs. 'tender'
• How loading, discharging, moving weights, ballast or bunkers changes G during a voyage
• The danger of free surface effect at small angles of heel — this is the one examiners come back to

Quick Self-Check

• Define righting lever in your own words — no looking it up.
• Why is free surface effect more dangerous at small angles of heel than large ones?
• What's the practical difference between a 'stiff' and a 'tender' vessel, and which would you rather be on in a seaway?

Test yourself: Stability Knowledge Checks.

Related Reading

• Nancy Glen — a real stability-modification fatality, exactly the kind of scenario these definitions exist to prevent
• Bayesian — a stability failure at anchor, worth understanding even at Boatmaster scale