Ship Construction and Damage Control
The examiner expects structural vocabulary tied to real consequences: how hogging and sagging load the hull, why the collision bulkhead position is fixed by regulation, and how a flooded compartment changes draft, trim and stability. Damage control questions are usually scenario-based and reward a systematic response.
Longitudinal strength
Hogging (excess buoyancy amidships) puts the deck in tension and the keel in compression; sagging reverses this. Loading instruments give the still-water shear force and bending moment, and the loading manual states the permissible figures for harbour and sea conditions. Exceeding the still-water bending moment, even within the load line marks, can crack the hull.
Structural elements to name
Collision bulkhead located between 0.05L and 0.08L from the forward perpendicular (SOLAS II-1); double bottom; transverse bulkheads forming watertight subdivisions; cofferdams between tanks of different contents; watertight doors with their closing arrangements; reserve buoyancy above the waterline.
Damage control sequence
Sound all tanks and bilges; identify the flooded compartment and its boundaries; close watertight doors and ventilators; assess permeability and free surface; consult the damage stability booklet; shore or patch where possible; dewater under control; monitor list, trim and progressive flooding; report under the shipboard emergency plan and SOPEP if pollution is possible.
Worked example - sinkage from damage
The loss-of-buoyancy method: a compartment 12 m long x 8 m wide x 5 m deep is bilged at the waterline, with permeability 60%. Lost buoyancy volume = 12 x 8 x 5 x 0.60 = 288 m³. Intact waterplane area after damage = 1,200 m². Parallel sinkage = 288 / 1,200 = 0.24 m. If the compartment is not amidships, the same lost buoyancy also produces a trimming moment about the LCF, so a trim change must be calculated as well.
Traps that cost marks
- Using 100% permeability for a machinery space; typical assumed values are lower.
- Calculating sinkage and stopping there, ignoring trim and the free-surface effect of the flooded space.
- Quoting the allowable bending moment without checking shear force.
- Recommending dewatering before checking whether the bulkheads can take the resulting head of water.