Marine Engineering Systems and Ship Stability
This domain combines propulsion and auxiliary systems with intact stability. It tests practical understanding of how machinery failures affect control and how loading affects the GZ curve.
Stability criteria
Under the 2008 Intact Stability Code, initial GM must be at least 0.15 m; the area under the GZ curve to 30° at least 0.055 m·rad; to 40° at least 0.09 m·rad; between 30° and 40° at least 0.03 m·rad; maximum GZ at least 0.20 m at an angle not less than 30°, and preferably not less than 25°. Free surface reduces GM by FSC = iρ/Δ.
Worked example
A box barge is 100 m long, 20 m beam, 10 m draught, KG 6.0 m. KB = 5.0 m; BM = B²/12T = 400/120 = 3.33 m; KM = 8.33 m; GM = 2.33 m. A tank 20 m × 10 m × 2 m has free surface i = lb³/12 = 20 × 1000/12 = 1,667 m⁴. Displacement = 100 × 20 × 10 × 1.025 = 20,500 t. FSC = 1,667 × 1.025 / 20,500 = 0.083 m, so fluid GM is 2.25 m.
Traps
Using KG as GM; forgetting free surface; believing a very large GM is always good (it produces a stiff, uncomfortable ship); confusing list with trim; and ignoring steering-gear or emergency-generator limits in manoeuvring.