Ship Construction and Stability
Ship Construction and Stability
The exam tests the structural members that keep the hull together and the stability numbers that decide whether a load or a weight shift is safe.
Structure
- Keel, floors, girders, longitudinals, frames, transverse and longitudinal bulkheads, double bottom, deep tanks, hatch coamings, watertight doors.
- Collision bulkhead between 5 and 8 per cent of L from the forward perpendicular, with the smaller-ship limits applied.
- Stresses: hogging, sagging, shear force, torsion in container ships, panting and pounding forward. Keep ballast transfer rates and loading sequences inside the loading manual.
Statics
- KM = KB + BM, with BM = I/V. GM = KM minus KG. Free surface correction FSC = i/displacement, and it reduces effective GM: GM fluid = GM solid minus total FSC.
- List from a transverse shift: tan theta = (w x d) / (displacement x GM).
- Large GM gives a stiff, fast-rolling ship with high accelerations; small GM gives a tender ship; negative GM produces an angle of loll, which must not be corrected by shifting weight to the low side.
Worked example
Displacement 8 000 t, solid GM 0.60 m, one rectangular tank 12 m x 8 m part filled, and 40 t shifted 12 m across the deck.
- Free surface moment i = l x b cubed / 12 = 12 x 512 / 12 = 512 m4.
- FSC = 512 / 8 000 = 0.064 m; GM fluid = 0.60 - 0.064 = 0.536 m.
- tan theta = (40 x 12) / (8 000 x 0.536) = 480 / 4 288 = 0.112, so theta is about 6.4 degrees.
Traps
Ignoring free surface; mixing units of moment; confusing list with loll; loading to the marks without applying the fresh water allowance or the seasonal zone limits.