Ship Construction and Damage Control
Ship Construction and Damage Control tests your grip on the structure that carries the loads and on what happens to strength and stability once water is inside. Expect hull girder bending, subdivision and damage stability in the same question.
The hull girder
The ship behaves as a box girder, with longitudinal material above and below the neutral axis carrying bending stress, σ = M/Z. In hogging, excess buoyancy amidships (a wave crest at midships) bends the hull with the ends down: deck in tension, bottom in compression. In sagging, a trough amidships leaves the ends more buoyant: deck in compression, bottom in tension. Still-water bending moment plus wave-induced bending must stay inside the class allowable limits, which is what the loading computer checks.
Watch concentrated loads - heavy blocks, single heavy lifts, ballast transferred in one pass - because shear force, not just bending moment, can be exceeded locally. Hatch corners, openings and bracket toes are stress raisers, and fatigue cracking starts there.
Watertight subdivision
Key concepts: collision bulkhead (not less than 0.05L and normally not more than 0.08L from the forward perpendicular), floodable length, permeability, margin line, and the probabilistic subdivision index R under SOLAS II-1 Part B-1. The damage control plan and booklet show compartment boundaries, closing appliances and cross-flooding arrangements. Progressive flooding through pipes, vents and open doors is usually what turns a survivable breach into a loss, and the downflooding angle is set by the lowest opening that cannot be closed.
Worked example
A box barge 60 m × 12 m × 6 m floats at 3.0 m draught with KG 3.0 m. KB = 1.5 m and BM = B²/12d = 144/36 = 4.0 m, so KM = 5.5 m and GM = 2.5 m. A 6 m full-breadth, full-depth amidships compartment floods (100 per cent permeability). By lost buoyancy, displacement is unchanged, so new draught = 2,160 / (54 × 12) = 3.33 m; KB = 1.67 m and BM = 7,776 / 2,160 = 3.60 m, giving KM 5.27 m and GM 2.27 m. Freeboard has also fallen by 0.33 m.
Traps
Reversing hogging and sagging; assuming 100 per cent permeability when typical cargo spaces are 60–95 per cent; forgetting the free surface added by a flooded space; and quoting subdivision rules without naming the downflooding opening that actually governs.