The Forces Acting on Cargo in a Shipping Container
The CTU Code defines the dynamic forces that act on cargo during sea transport as a multiple of the cargo weight (expressed as g-forces). The standard forces for a standard general cargo vessel voyage are:
| Direction | Force (CTU Code — standard sea voyage) |
|---|
| Longitudinal (forward and aft) | 0.5g — half the cargo weight in the direction of travel |
| Transverse (side to side) | 0.5g — half the cargo weight across the vessel |
| Vertical (upward) | 0.3g — 30% of cargo weight acting upward |
| Combined vertical (downward) | 1.3g — the cargo weight plus 0.3g downward surge |
These values are for general cargo on a standard voyage. The CTU Code provides enhanced values for cargo on open decks, in road vehicles, or in high-sea-state shipping lanes. DunLash can advise on the appropriate force values for your specific cargo, vessel type, and route.
The Simplified CTU Code Calculation Method
The simplified method calculates the required total securing force (in daN) for the cargo weight and transport conditions, then divides that by the MSL (Maximum Securing Load) of the individual lashing product to determine the number of lashings required.
Step 1 — Determine the Cargo Mass
Establish the gross mass of the cargo to be secured in kilograms (kg). This is the total mass of the cargo unit — including the pallet, packaging, and any ancillary materials.
Step 2 — Convert to daN
The CTU Code works in daN (decanewtons). To convert cargo mass in kg to daN: multiply by 9.81, then divide by 10. For practical purposes, 100 kg = approximately 98.1 daN (commonly rounded to 98 daN or 1 daN per kg for rough estimates).
Step 3 — Calculate the Required Securing Force
For each direction of potential cargo movement, the required securing force = cargo mass (daN) × the applicable g-force factor.
| Direction | Calculation (5,000 kg example) |
|---|
| Longitudinal | 5,000 kg × 0.5g × 0.0981 |
| Transverse | 5,000 kg × 0.5g × 0.0981 |
| Vertical (upward) | 5,000 kg × 0.3g × 0.0981 |
Note: the friction between cargo and floor also contributes to longitudinal and transverse restraint, reducing the lashing force required. The CTU Code provides friction coefficients by floor and cargo surface type. For palletised cargo on a standard container floor, a friction coefficient of 0.3 is commonly applied, reducing the required lashing force by 30%. DunLash recommends using the full un-reduced force for conservative calculations unless friction is verified.
Step 4 — Determine the MSL of the Lashing Product
The Maximum Securing Load (MSL) of a lashing is 50% of its system breaking strength (the tested breaking strength of the lashing combined with the buckle). All DunLash lashing products are SGS certified — the MSL is derived from independently witnessed break tests.
| DunLash Lashing | System Breaking Strength |
|---|
| DunLash 105 (32mm) | Up to 2,850 daN |
| DunLash 200 (42mm) with Dynablock 12 | 9,580 daN |
| DunLash 200 (42mm) with 4040H buckle | 9,450 daN |
| DunLash 750 (50mm) with Dynablock 15 | 13,850 daN |
| DunLash 750 (50mm) with 5050H buckle | 13,710 daN |
Step 5 — Calculate the Number of Lashings Required
Number of lashings required = Required securing force (daN) ÷ MSL per lashing. Lashings are applied in pairs for most applications — one from each side — to address forces from both directions simultaneously.
Worked example — securing a 10-tonne (10,000 kg) steel coil in a container for sea transport:
- Cargo mass: 10,000 kg = 981 daN (using 0.0981 conversion)
- Required longitudinal force: 981 × 0.5 = 491 daN
- Required transverse force: 981 × 0.5 = 491 daN
- Using DunLash 750 (MSL = 6,925 daN): 491 ÷ 6,925 = 0.07 — one lashing per direction is mathematically sufficient for a 10-tonne coil
- In practice: a minimum of two lashings per direction are applied for redundancy, angle compensation, and to address the combined vector of longitudinal and transverse forces
- For the heaviest steel coils (20–30 tonnes), four to six lashings of DunLash 750 are typical depending on lashing angle and arrangement
The Effect of Lashing Angle on Securing Force
Lashings are rarely applied perfectly horizontally. The angle of the lashing relative to the horizontal plane affects how much of the lashing’s MSL contributes to horizontal restraint. A lashing at 30° from horizontal delivers cos(30°) = 86.6% of its MSL as horizontal restraint. At 60°, only cos(60°) = 50% applies as horizontal force.
The CTU Code recommends lashing angles between 20° and 65° from horizontal for standard cargo. Steeper angles are less efficient for horizontal restraint but may provide useful vertical downforce on the cargo. DunLash recommends keeping lashings as close to horizontal as practical, and multiplying the required number of lashings upward to compensate for steep angles.
Composite Strapping for Lower-Force Applications
For lighter cargo where the calculated securing force falls within the range of composite polyester strapping, DunLash composite strapping provides a cost-effective and efficient solution:
| DunLash Strapping | System Breaking Strength |
|---|
| 13mm composite | 529 daN |
| 19mm composite | Approx. 900 daN |
| 25mm composite | Approx. 1,800 daN |
| 32mm composite (DC 105) | 2,771 daN |
| 32mm AAR (DC 105 AAR) | 2,771 daN |
