Ship Construction, Stability and Damage Control for Engineers
What this domain tests
Engineers are examined on the structure they cut, weld and bolt to, and on the stability consequences of ballast, free surface and flooding: bulkheads, watertight boundaries, KG and GM, loll, and damage control.
Structure
- The double bottom, hopper and topside tanks, transverse frames and longitudinal girders carry the hull loads; a hot work repair needs the adjacent structure checked and the classification surveyor involved.
- Watertight doors, bulkhead penetrations and tank boundaries are part of subdivision; a new pipe run through a bulkhead needs the same watertight standard as the original.
- The collision bulkhead sits between 0.05L and 0.08L from the forward perpendicular and has no openings below the freeboard deck.
Stability basics
- GM = KM − KG. Positive GM rights the ship; too small a GM gives a tender ship with slow roll; too large a GM gives a stiff ship with violent roll and high accelerations.
- Free surface effect reduces effective GM by FSC = FSM ÷ displacement; it exists whenever a tank is slack and disappears when the tank is full or empty.
- List is an off-centre weight and is corrected by moving weight; loll is an unstable equilibrium, corrected by filling the low-side double bottom first and then the high side to lower G.
- Damage survival depends on floodable length, permeability and cross-flooding arrangements; the watertight subdivision index is the SOLAS measure of that protection.
Worked example
A rectangular tank 20 m long and 10 m wide holds liquid of density 1.0 t/m³. Free surface inertia = 20 × 10³ ÷ 12 = 1,667 m⁴, so the free surface moment is 1,667 t·m. With displacement 8,000 t, FSC = 1,667 ÷ 8,000 = 0.21 m. If the ship's GM was 1.10 m, the effective GM falls to 0.89 m. Reasoning: the moment depends on the free surface dimensions, not on how much liquid is in the tank, so slightly topping up the tank changes nothing until it is full.
Traps that cost marks
Treating loll as list and trying to correct it by moving weight across; adding weight high to cure a list; ignoring free surface effect after a partial transfer; leaving a watertight door open for access; cutting a beam or frame without a stress check and surveyor involvement.
Key points
- GM = KM − KG; positive GM rights the ship