Vessel Design, Construction, Stability and Buoyancy
Core concepts
Know hull forms (displacement, semi-displacement, planing), bow and stern shapes, deadrise, flare, sheer, camber and rocker. Understand principal dimensions: length overall, length waterline, beam, draught, freeboard, depth, block coefficient. Learn construction materials (GRP, steel, aluminium, timber, composites) and methods (moulding, welding, riveting, cold moulding), plus structural members: keel, frames, stringers, bulkheads, floors, deck beams.
Stability is central. Understand Archimedes' principle, centre of buoyancy (CB), centre of gravity (CG), metacentre (M) and metacentric height (GM). Know initial stability (GM positive = stable), righting moment = displacement × GZ, and the GZ curve including angle of vanishing stability. Distinguish transverse and longitudinal stability, free surface effect, freeboard and reserve buoyancy. Understand heeling versus listing, and the effect of moving weights (adding weight high reduces GM).
Traps in questions
- Confusing centre of buoyancy (moves with immersed volume) with centre of gravity (fixed by mass distribution).
- Assuming larger GM is always better; excessive GM causes a stiff, uncomfortable, hazardous roll.
- Forgetting free surface effect reduces GM regardless of weight position.
- Mixing list (asymmetric loading) with heel (external force such as wind).
- Using LOA instead of LWL in hydrostatic calculations.
Rule of thumb: righting moment
Righting moment = Δ × GZ, where GZ = GM × sin θ for small angles.
Worked example: Displacement 5 000 kN, GM 0.8 m, heel 10°. GZ ≈ 0.8 × sin 10° = 0.139 m. Righting moment ≈ 5 000 × 0.139 = 695 kN·m. State assumptions (small angle) explicitly.