What is the CSS Code and how does Annex 13 apply?

The Code of Safe Practice for Cargo Stowage and Securing (CSS Code, IMO A.714(17)) provides guidance for securing cargo on deck and in holds. Annex 13 provides a simplified method to calculate the external forces on cargo units using acceleration coefficients derived from the vessel's service route. The balance-of-forces method then checks that the total securing force from lashings and friction exceeds the computed external force.

What is the Maximum Securing Load (MSL)?

MSL is the maximum force that a securing device may be subjected to. It is determined by the manufacturer and marked on the equipment. For wire rope, the MSL is typically 50% of the breaking strength. For lashing chains, MSL is often 50–75% of breaking strength. The CSS Code Annex 13 method requires that the sum of all lashing MSLs, plus friction, equals or exceeds the external forces on the cargo. Never confuse MSL with Breaking Strength (BS).

How does sea area affect the acceleration coefficients?

CSS Code Annex 13 Table 2 lists acceleration factors (a_x, a_y, a_z) as fractions of g for different sea areas and ship lengths. Short voyages in sheltered waters have lower coefficients (a_y ≈ 0.5g) while exposed North Atlantic winter passages can reach a_y ≈ 0.9g. Longer ships (L > 200 m) generally experience lower motion accelerations than short ships for the same sea area.

What is the balance-of-forces method?

In the balance-of-forces method, all lashing forces are resolved into their vertical, transverse, and longitudinal components. The sum of upward lashing components plus the cargo weight must exceed the vertical external force; the sum of transverse lashing components plus friction must exceed the transverse external force; and so on. The CSS Code Annex 13 simplified method pre-computes the required securing force (CS) so the planner only needs to verify that total MSL × 1.5 ≥ CS.

CSS Code lashing force calculator

Estimates the external forces (Fx, Fy, Fz) on a cargo unit using the simplified method in CSS Code Annex 13 (IMO A.714(17)). Formula: F = m × a, where m is in tonnes and a is in m/s², giving F in kN. Use these forces to verify that the total MSL of your lashings and friction exceeds each force component.

Cargo mass m (tonnes)

Sea area / voyage type

Ship length L (m)

Longer ships have lower motion accelerations (length factor applied).

Result

Sea area: North Atlantic winter / exposed ocean. Length factor: 0.97. Effective a_y = 0.776g, a_x = 0.339g, a_z(dyn) = 0.436g.

Fy — transverse force: 152.3 kN
Fx — longitudinal force: 66.6 kN
Fz — vertical downward (max): 276.0 kN
Fz — vertical upward (tipping): 104.7 kN

Securing check: Total MSL of all lashings (resolved into transverse component) + friction force must equal or exceed Fy. Apply the same check for Fx and Fz. Per CSS Code, use a friction coefficient of 0.3 for smooth steel on steel; 0.4–0.5 for timber dunnage.

Notes & caveats

· This calculator uses a simplified linear interpolation for the length correction factor. CSS Code Annex 13 Table 2 provides tabulated values for L = 50, 100, 150, 200, 300 m — use those for formal securing plans submitted to class or flag state.
· The acceleration coefficients here are representative values. For the authoritative figures, refer to CSS Code Annex 13 Table 2 directly. Some flag states (e.g., Germany, Japan) publish supplementary guidance with additional sea-area classifications.
· Securing plans for project cargo, heavy lifts, or vessels trading under a Cargo Securing Manual (CSM) approved by the flag administration must follow the CSM rather than Annex 13 alone.
· MSL must never be confused with Safe Working Load (SWL). MSL is the lashing-specific term under the CSS Code; SWL is used for lifting equipment. They are not interchangeable.

Sources: CSS Code Annex 13 (IMO A.714(17)); SOLAS VI/5–6; see also stowage factor calculator, passage time & ETA, maximum securing load.

CSS Code lashing force calculator

Cargo mass m (tonnes)

Sea area / voyage type

Ship length L (m)

Longer ships have lower motion accelerations (length factor applied).

Result

Sea area: North Atlantic winter / exposed ocean. Length factor: 0.97. Effective a_y = 0.776g, a_x = 0.339g, a_z(dyn) = 0.436g.

Fy — transverse force: 152.3 kN
Fx — longitudinal force: 66.6 kN
Fz — vertical downward (max): 276.0 kN
Fz — vertical upward (tipping): 104.7 kN

Notes & caveats

· This calculator uses a simplified linear interpolation for the length correction factor. CSS Code Annex 13 Table 2 provides tabulated values for L = 50, 100, 150, 200, 300 m — use those for formal securing plans submitted to class or flag state.

· The acceleration coefficients here are representative values. For the authoritative figures, refer to CSS Code Annex 13 Table 2 directly. Some flag states (e.g., Germany, Japan) publish supplementary guidance with additional sea-area classifications.

· Securing plans for project cargo, heavy lifts, or vessels trading under a Cargo Securing Manual (CSM) approved by the flag administration must follow the CSM rather than Annex 13 alone.

· MSL must never be confused with Safe Working Load (SWL). MSL is the lashing-specific term under the CSS Code; SWL is used for lifting equipment. They are not interchangeable.