Naval Architecture - Hydrostatics, Stability, Freeboard and Tonnage
Naval Architecture
The Naval Architecture domain tests hydrostatics, intact and damage stability, freeboard and tonnage - the calculations that decide whether a hull may legally float and load. In the PRC system the governing technical texts are the MSA's 国内航行海船法定检验技术规则 and 内河船舶法定检验技术规则 for domestic ships, and the IMO instruments (IS Code 2008, SOLAS II-1, ICLL 1966/88, Tonnage 1969) as applied through CCS rules for sea-going ships.
Core relationships
- Displacement Δ = L·B·T·C_B·ρ; use 1.025 t/m³ for seawater, 1.000 for fresh water.
- KM = KB + BM, BM = I/∇ (for a box, BM = B²/12T); GM = KM − KG, with KG corrected for free surface: KG_eff = KG + FSC, FSC = Σ i·ρ/Δ.
- IS Code 2008 intact criteria: GM₀ ≥ 0.15 m; GZ area to 30° ≥ 0.055 m·rad; to 40° ≥ 0.09 m·rad; GZ_max ≥ 0.20 m at ≥30°.
- Damage stability for cargo ships follows the probabilistic attained index A ≥ R (SOLAS II-1 Part B-1).
- Freeboard is assigned from the ICLL tables and corrected for block coefficient, depth and superstructure; tonnage under the 1969 Convention uses GT = K₁V.
Worked example
Box pontoon, L 60 m, B 12 m, T 3 m, KG 2.2 m, seawater. ∇ = 60×12×3 = 2160 m³, Δ = 2214 t. KB = 1.5 m. BM = 12²/(12×3) = 4.00 m. KM = 5.50 m. GM = 5.50 − 2.20 = 3.30 m, well above 0.15 m. If a slack tank contributes a free surface moment of 2160 t·m, FSC = 2160/2214 = 0.98 m, so effective GM = 2.32 m - still compliant, but note the correction is applied to KG, not to KM.
Traps
Mixing moulded and extreme breadth; forgetting free surface; judging large-angle stability from GM alone; applying the wrong density; assuming domestic-rule criteria are identical to IS Code.