Stability, Subdivision and Load Lines: Criteria, Damage Cases and Freeboard Corrections
This domain tests the numerical side of safety: intact stability criteria, the logic of subdivision and damage stability, and the assignment of freeboard under the 1966 Load Line Convention. Expect a calculation with a correction factor and a question that turns on which criterion applies to which ship.
Intact stability
The 2008 Intact Stability Code Part A is mandatory through SOLAS II-1 Regulation 5-1. The general criteria for cargo ships include: area under the GZ curve to 30 degrees at least 0.055 m rad; to 40 degrees, or the angle of flooding if less, at least 0.09 m rad; between 30 and 40 degrees at least 0.03 m rad; GZ at 30 degrees at least 0.20 m; maximum GZ occurring at not less than 25 degrees; initial metacentric height at least 0.15 m. The severe wind and rolling criterion applies to ships of 24 m and above. Grain ships add the Grain Code: heeling angle not exceeding 12 degrees or the angle of deck-edge immersion, whichever is less, and residual area of at least 0.075 m rad.
Subdivision and damage stability
Passenger ships use probabilistic assessment against a required subdivision index R. Cargo ships of 80 m and above built on or after 1 January 2009 must also satisfy the probabilistic index A, not less than R. Older cargo ships are assessed deterministically. The point of the surveyor's knowledge is to recognise when a change, such as a new door, a pipe penetration or a ballast rearrangement, invalidates the approved damage case.
Load lines
Freeboard is read from the tables by ship type (A or B) and length, then corrected. The block coefficient correction applies above 0.68: factor = (Cb + 0.68) / 1.36. Depth, superstructure and sheer corrections follow, and minimum bow height must be checked separately. Seasonal zones include Tropical, Summer, Winter and Winter North Atlantic, the last applying to ships under 100 m in length. The deck line is 300 by 25 mm and the load line ring 300 mm outside diameter, 25 mm thick.
Worked example
A Type B ship of 140 m has a tabular freeboard of 2,150 mm and Cb 0.76. Correction factor = (0.76 + 0.68) / 1.36 = 1.0588, so the corrected freeboard is 2,150 x 1.0588 = 2,276 mm before depth, superstructure and sheer adjustments.
Traps
Treating KM as dependent on loading (it depends on hull form only); confusing list with angle of loll; applying the Winter North Atlantic line to ships over 100 m; forgetting the flooding angle when the 40-degree criterion is replaced.