Naval Architecture, Ship Construction and Stability
Core concepts
Candidates must handle hydrostatics and ship geometry confidently. Know the definitions and relationships of displacement, deadweight, lightweight, block, midship, waterplane and prismatic coefficients. Stability content includes initial metacentric height, the metacentre, righting lever curve, angle of vanishing stability, free surface effect, and the effect of a suspended weight. Understand loading conditions, the International Grain Code basics, damage stability and the probabilistic subdivision concept, and the intact stability criteria of the IS Code. Construction content covers longitudinal and transverse framing, the role of the double bottom and duct keel, bulkhead types, watertight doors, hatch covers and their securing, and the purpose of the collision bulkhead. Hull stresses: hogging, sagging, racking, panting and slamming.
Traps
A classic error is assuming GM alone defines safety; a stiff ship with large GM can have a violent roll and high accelerations. Another is confusing the metacentre with the centre of buoyancy. Questions often test free surface effect: it reduces effective GM and is independent of the mass of liquid, depending on the free surface moment. Do not confuse hogging (excess buoyancy amidships) with sagging.
Worked rule of thumb
Free surface correction to GM equals the free surface moment divided by displacement. For a rectangular tank of breadth b and length l holding liquid of density rho, the free surface moment is rho times l times b cubed divided by twelve; halving breadth quarters the effect, which is why longitudinal subdivisions are so effective.