Ship Stability and Ship Construction: Criteria, Free Surface and Strength
What this domain tests
Intact and damage stability to the 2008 Intact Stability Code and SOLAS II-1, list and loll calculations, free surface effect, and the longitudinal strength and structural arrangements that keep the hull together.
Core rules
- IS Code 2008 Part A: area under the GZ curve to 30° ≥ 0.055 m·rad; to 40° ≥ 0.090 m·rad; between 30° and 40° ≥ 0.030 m·rad; GZ ≥ 0.20 m at 30° or more; maximum GZ at 25° or more; initial GM ≥ 0.15 m.
- GZ = KN - KG sin θ. Free surface correction always reduces effective GM: FSC = i·ρ / Δ, where i = l·b³/12 for a rectangular tank.
- Angle of loll occurs with negative GM; the ship lies at tan θ = √(2|GM| / BM) for a wall-sided form until corrected by lowering G.
- Damage stability for post-2009 ships is probabilistic (SOLAS II-1 Part B-1); older ships use deterministic margin-line criteria.
- Strength: shear force and bending moment curves from loading software; hogging and sagging limits; torsion in container ships with open decks.
Worked example
A rectangular tank 10 m long, 8 m wide, half full of seawater. i = 10 × 8³ / 12 = 426.7 m⁴. Free surface moment = 426.7 × 1.025 = 437.3 t·m. With displacement 5,000 t, FSC = 437.3 / 5,000 = 0.087 m. A solid GM of 0.60 m therefore becomes an effective GM of 0.513 m, still above the 0.15 m minimum.
Traps
- Confusing GM with KM, or adding FSC to GM instead of subtracting it.
- Checking only one IS Code criterion.
- Using moulded breadth for a tank with sloping sides.
- Ignoring the strength check while chasing a stability solution by pressing up double-bottom tanks.