Ship Construction and Stability
The paper tests transverse and longitudinal strength and stability: GM and free surface, the GZ curve against the IMO intact stability criteria, angle of loll versus list, damage stability, load line and shear force and bending moment.
The core quantities
- GM (fluid) = KM − KG − free surface correction, where FSC = free surface moment ÷ displacement.
- Transverse metacentre KM comes from the hydrostatics for the actual draft; KG comes from the loading condition.
- Angle of loll arises from negative GM: the ship flops to port or starboard and the GZ curve starts below zero. Correct it by lowering KG (filling low double bottoms) and removing free surface first. Do not shift weight across the deck.
IMO intact stability criteria (IS Code 2008)
- Area under the GZ curve to 30° at least 0.055 m·rad.
- Area to 40°, or the flooding angle, at least 0.09 m·rad.
- Area between 30° and 40° at least 0.03 m·rad.
- Maximum GZ at least 0.20 m at an angle of at least 30°.
- Initial GM at least 0.15 m.
Longitudinal strength and load line
Keep shear force and bending moment inside the loading manual limits. The load line marks (S, W, T, F, TF, plus timber and passenger marks) fix the minimum freeboard; the deck line and the assigned summer freeboard come from the International Convention on Load Lines 1966 as amended.
Worked example
KM 8.20 m, KG 7.35 m, free surface moment 1,440 t·m, displacement 9,600 t. Solid GM = 8.20 − 7.35 = 0.85 m. FSC = 1,440 ÷ 9,600 = 0.15 m. Fluid GM = 0.85 − 0.15 = 0.70 m, comfortably above the 0.15 m minimum, so this condition is acceptable.
Traps
- Forgetting the free surface correction and quoting solid GM.
- Reading KM at the wrong draft.
- Trying to correct loll by shifting weight across the deck, which may throw the ship to the other side.