Ship Construction, Stability and Damage Control for Engineers: Structure, Free Surface and Flooding
This domain tests the engineer's part in hull integrity and survivability: structural arrangement, watertight subdivision, intact and damage stability, free surface effect, and the actions that limit progressive flooding.
Core rules
- Longitudinal members carry primary bending; transverse frames resist local pressure. Deep tanks and collision bulkhead position follow subdivision rules, with the collision bulkhead between 0.05L and 0.08L from the forward perpendicular.
- GM = KM - KG. Free surface effect reduces effective GM by FSC = i x density / displacement, where i is the second moment of the free surface area about its centreline.
- GZ curves show righting lever against heel; angle of vanishing stability and areas under the curve define survivability criteria.
- Damage control relies on watertight doors and hatches closed, cross-flooding arrangements used as designed, and prompt reporting of any breach.
- Loading instrument checks shear force and bending moment, not only draft and trim.
Worked example
A rectangular tank 20 m long and 10 m wide is slack, with contents of density 1.025 t/m3, on a ship displacing 8000 t. Second moment i = l x b cubed / 12 = 20 x 1000 / 12 = 1666.7 m4. FSC = 1666.7 x 1.025 / 8000 = 0.214 m. A reported GM of 0.30 m therefore becomes an effective 0.086 m, which explains a slow, tender roll.
Traps
Using breadth rather than breadth cubed; ignoring free surface when checking GM; and pressing up slack tanks during a damage emergency without checking the stress condition.