GATE NM: Ship Design and Construction

Ship Design and Construction covers the design spiral, principal dimensions, form coefficients, capacity, freeboard, tonnage and the construction sequence from block assembly to launch. GATE NM favours definitions and simple coefficient calculations rather than open-ended design.

Core rules

• The design spiral iterates mission requirements, dimensions, power, weight and cost until convergence.
• Block coefficient C_B = ∇/(LBT); prismatic coefficient C_P = ∇/(A_M L); midship coefficient C_M = A_M/(BT). For a box, all equal 1.
• Displacement Δ = ρ C_B LBT in consistent units.
• Freeboard is the distance from the summer load line to the freeboard deck; it is not the draft.
• Gross tonnage is a dimensionless measure of enclosed volume, not a weight; net tonnage relates to cargo capacity.
• Construction: planning, nesting and cutting, sub-assembly, block erection, welding sequence, testing and launch. Distortion control depends on balanced welding and fairing.

Worked example

A ship has L = 120 m, B = 18 m, T = 6 m, Δ = 9000 t in seawater of density 1.025 t/m³. ∇ = 9000/1.025 ≈ 8780 m³. LBT = 120 × 18 × 6 = 12,960 m³. C_B = 8780/12,960 ≈ 0.677. The check is that a fuller hull has C_B closer to 1; a slender fast vessel has a lower C_B. If the same ship were in freshwater, it would displace the same mass at a slightly larger volume and draft.

Traps

C_B is based on the displaced volume, not on the cargo volume. Gross tonnage is often mistaken for displacement. Freeboard and draft move in opposite directions when loading changes. Do not use LOA where LBP is required in coefficient formulae. In construction questions, a weld sequence that pulls freely causes angular distortion; balanced sequences and restraints control it.