Ship Construction, Stability and Damage Control
Core concepts
Know hull structure: keel, frames, stringers, bulkheads, double bottom, and the purpose of collision bulkhead and watertight subdivisions. Understand intact and damage stability terms: centre of gravity, centre of buoyancy, metacentre, GM, free surface effect, and the meaning of KG and KM. Know flooding, list, trim, and the actions to take after damage.
Traps in questions
- Confusing GM with GZ; GM is a stability parameter, GZ is the righting lever.
- Forgetting that free surface effect reduces effective GM and can be dangerous with slack tanks.
- Assuming list always means negative GM; list can arise from off-centre weight.
- Mixing up trim and list: trim is fore-and-aft, list is athwartships.
Rule of thumb and worked example
GM = KM − KG. If KM is 8.5 m and KG is 7.9 m, GM = 0.6 m, a positive and acceptable value for many coasters. Free surface correction reduces GM: FSC = (free surface moment)/(displacement). If free surface moment is 1,200 t·m and displacement is 3,000 t, FSC = 0.4 m, so effective GM = 0.2 m, which is marginal and demands attention. This illustrates why partially filled tanks must be minimised.
Study focus
Revise watertight door operation, damage control drills, and the purpose of cross-flooding. Know the difference between permeability and block coefficient, and understand how flooding a compartment changes draft, trim and list. Learn the significance of the load line and reserve buoyancy.