Stability and Trim
Stability and trim fundamentals
This domain tests intact stability: KG, KM, GM, GZ, free surface effect, list and trim. The exam expects numerical reasoning, not just definitions.
- GM = KM − KG; positive GM indicates initial stability, but excessive GM gives a stiff, uncomfortable roll.
- Free surface effect raises the effective KG; the free surface correction is FSM divided by displacement.
- List is a steady heel; it may be caused by off-centre weight, negative GM or asymmetric flooding. Loll is a condition of negative stability.
- Trim uses TPC and MCTC; always state whether trim is by the head or stern.
- IMO intact stability criteria include an area under the GZ curve and a minimum GM.
Worked example
A ship displaces 6000 t with GM 0.45 m. A 60 t weight is shifted 12 m to starboard. Transverse shift GG' = (60 × 12) / 6000 = 0.12 m. List angle: tan θ = GG' / GM = 0.12 / 0.45 = 0.2667, so θ ≈ 14.9°. If free surface reduces effective GM to 0.40 m, tan θ = 0.12 / 0.40 = 0.30, giving about 16.7°. The OOW should correct the list by transferring or ballasting weight, not by assuming the ship is unstable.
Traps
Using KG and VCG as different quantities when they are the same reference. Omitting free surface correction in a partly filled tank. Assuming all list means negative GM. Mixing TPC and MCTC units.