Vessel Construction, Stability, and Load Line Requirements
Domain 3 combines structural strength, intact and damage stability, and load line assignment - the calculations that decide whether a loaded ship may sail.
Strength and structure
Hogging puts the deck in tension, sagging puts the bottom in tension; both are checked through shear force and bending moment against section modulus. Framing is transverse or longitudinal. The collision bulkhead sits between 0.05L and 0.08L from the forward perpendicular (SOLAS II-1 Reg 12), with the smaller absolute limit applying forward.
Intact stability - 2008 IS Code Part A
- Area under the GZ curve to 30° ≥ 0.055 m·rad; to 40° or the flooding angle ≥ 0.09 m·rad.
- GZ ≥ 0.20 m at 30°, and maximum GZ at an angle of at least 30°.
- Initial GM ≥ 0.15 m, after free-surface correction.
Load lines
The 1966 Convention and its 1988 Protocol assign freeboard by zone (tropical, summer, winter, winter North Atlantic) and set minimum bow height. Fresh water allowance = displacement / (4 × TPC), in centimetres.
Worked example
KM = 7.35 m and KG = 6.95 m give a solid GM of 0.40 m. A free-surface moment of 1,440 t·m on 12,000 t displacement gives FSC = 0.12 m, so fluid GM = 0.28 m - above the 0.15 m floor. That alone does not clear the ship: the GZ curve areas and the 0.20 m ordinate at 30° must also be verified before the loading computer result is accepted.
Traps
- Reporting solid GM when the criterion is applied to fluid GM.
- Mixing hogging with sagging.
- Assuming the master chooses the load line mark; it is assigned by or on behalf of the flag.
- Using the summer freeboard in a tropical zone without applying the permitted allowance.