Lashing forces

Annex 13 of the IMO Code of Safe Practice for Cargo Stowage and Securing (CSS Code, Resolution A.714(17) as amended in 2011) provides a standardised method for calculating the forces on deck cargo and verifying that the securing arrangement is adequate. It defines external accelerations, equipment strength limits (MSL), and two calculation methods — balance-of-forces (basic) and the advanced method.

External forces (Fx, Fy, Fz)

• · Transverse (Fy): function of sea area (North Atlantic, North Pacific, Indian Ocean, or sheltered) and ship length (LOA). North Atlantic, LOA < 100 m: Fy = 0.8g; LOA 300 m: Fy = 0.5g. Values taken from Annex 13 Table 2.
• · Longitudinal (Fx): typically 0.3g forward, 0.5g aft for standard sea areas; Table 2 also provides these values.
• · Vertical (Fz): upward, reducing the cargo's effective weight. Typically 0.5g vertically in North Atlantic conditions. Combined with the cargo mass this gives the effective compressive force on friction surfaces.
• · Annex 13 Table 1 classifies sea areas (A = North Atlantic/North Pacific, B = other open seas, C = sheltered); the sea area used must match the intended voyage — not the most favourable.

Maximum securing load (MSL) by equipment type

• · Chain lashings: MSL = 0.50 × BL. BL from IACS/Class certificate or stamped on the chain.
• · Wire rope lashings: MSL = 0.70 × BL (higher because wire is more elastic and shares load better).
• · Fibre rope / synthetic strops: MSL = 0.50 × BL; must have documented BL; untested rope not accepted.
• · Webbing straps: MSL = 0.70 × BL when tested to EN 12195-2; check label on strap or certificate.
• · Shackles, turnbuckles, D-rings: MSL stamped or from SWL certificate; SWL as marked may equal MSL if so stated.

Balance-of-forces method

• · For transverse balance: ΣCS(transverse) + μ × (m × g − Fz × m) ≥ Fy × m, where m is the cargo mass, μ the friction coefficient.
• · CS contribution of each lashing = MSL × cos α × cos β (α = horizontal deviation from transverse axis, β = vertical angle from horizontal).
• · Friction coefficients: 0.3 (steel on steel, wet), 0.3 (timber on steel), 0.4 (rubber on steel), 0.5 (timber on timber).
• · Tipping check: lashings must also prevent tipping about the cargo's lower outer edge; this is a moment calculation about that tipping axis.

Advanced calculation method

• · Takes into account the elasticity of lashing equipment (stiffer materials carry more load), pre-tension, and the simultaneous action of transverse and longitudinal forces.
• · Produces a Calculated Securing Number (CS number) for the arrangement; CS ≥ 1.5 × external force requirement.
• · Requires software or skilled calculation; used for project cargo, heavy lifts, and non-standard configurations.
• · P&I clubs and hull underwriters may require an advanced-method calculation or a marine surveyor's endorsement for cargo above 100 t per unit.

Cargo securing manual requirements

• · Every cargo ship must have an approved Cargo Securing Manual (CSM) — IMO MSC/Circ.745 and SOLAS VI/5.6 requirement.
• · The CSM lists lashing points, MSL ratings, and standard securing patterns approved by the flag State administration.
• · Chief Officer must verify that actual lashing equipment matches the CSM specifications; substitute equipment must be rated equivalent or higher in MSL.
• · PSC inspectors check that the securing arrangement matches the CSM and that lashing equipment is certified and in good condition.

Sources: IMO CSS Code (Resolution A.714(17)) + 2011 amendments incorporating Annex 13; IMO MSC/Circ.745 — Guidelines for the preparation of the Cargo Securing Manual; SOLAS Chapter VI Reg. 5 — Stowage and securing. See also breaking load, cargo securing manual, hatch cover testing.