Container lashing on a ship is the system that holds container stacks against rolling, pitching, wind and green seas. This article covers the ship side: the equipment, stack weight limits, the forces at work and how planners check them. The general principles of cargo securing, including inside containers and on trucks, are in cargo lashing and securing.
Container lashing equipment
| Equipment | Function | Typical location |
|---|---|---|
| Twistlock (manual, semi-automatic, fully automatic) | Locks container corners, resists lifting and sliding | Between deck tiers, deck foundations |
| Midlock / stacking cone | Resists sliding between tiers | In holds with cell guides, 20ft positions |
| Lashing rod (long and short) | Resists racking and tipping | Deck stacks, from deck or lashing bridge |
| Turnbuckle | Tensions the lashing rod | Foot of the rod on deck or bridge |
| Bridge fitting | Links adjacent stacks at the top tier | Top tier of deck stacks (on some ships) |
| Lashing bridge | Platform so rods can reach higher tiers | Between bays on larger ships |
| Cell guide | Vertical rails that hold containers | Holds of cellular container ships |
In the holds of cellular ships, cell guides restrain the containers, so lashing rods are normally not used. On deck, twistlocks and rods work together. Lashing bridges let rods reach the bottom corners of the third or fourth tier, giving tall stacks more support.
Stack weight limits and container strength
Each stack position has a maximum total weight (stack weight) in the Cargo Securing Manual, for example for 20ft and 40ft stacks on deck and in the hold. The limit reflects the strength of the hatch covers or tank top, the strength of the container corner posts and the lashing pattern.
The container's own strength is shown on its CSC plate: the allowable stacking load and the racking test load. Containers built to the older ISO minimum values are designed for a superimposed stacking mass of 192,000 kg at 1.8 g and a racking test of 150 kN, but many newer boxes are rated higher.
Deck 40ft stack example (illustrative): the stack limit is 90 t. The first plan has five tiers, bottom to top, of 28 t, 26 t, 18 t, 12 t and 8 t, a total of 92 t, 2 t over the limit. The 8 t box on top is moved to another stack, bringing the total to 84 t. Beyond the total, lashing software often limits top-tier weights too, because heavy boxes high up increase forces on lashings and corner posts. The weights used must be the VGM; prepare the data with the VGM form.
Forces from roll and pitch
A rolling ship gives the stacks transverse accelerations. These grow with distance from the roll axis, so top tiers and outboard rows take the most load. Pitching adds vertical acceleration, largest in the foremost and aftermost bays. Wind on tall stacks and seas coming on deck add further load.
The roll period can be estimated with the approximation T ≈ 0.8 × B ÷ √GM (B is the ship's beam in metres). For an illustrative ship with B = 32.2 m:
- GM 1.5 m: T ≈ 25.8 ÷ 1.22 ≈ 21 seconds, a slow, easy roll.
- GM 4.0 m: T ≈ 25.8 ÷ 2.0 ≈ 13 seconds, a quick roll with larger accelerations.
A larger GM adds stability, but lashing forces rise with it. The trade-off is discussed in metacentric height (GM), and you can check the loaded GM with the basic GM calculator.
The Cargo Securing Manual and lashing software
SOLAS requires ships to carry a Cargo Securing Manual (CSM) approved by the flag state or class. On a container ship, the CSM gives the lashing patterns per bay, stack weight limits, the maximum securing load (MSL) of each item of equipment and how it is to be inspected and maintained. Its principles follow the CSS Code.
Modern container ships use lashing software as part of the loading computer. It calculates forces on lashings, twistlocks and corner posts for each stack from weights, heights, positions and GM. Class approves such programs against its rules and the relevant IACS requirements, including UR C6 for lashing software. Stacks over their limits are flagged, and the planner moves boxes, lightens the top tiers or changes the lashing pattern.
High cubes, 45ft units and mixed stacks
- High cube (9'6"). 2,896 mm tall, about 305 mm more than a standard box. The stack gets taller, its centre of gravity rises and the wind area grows. If a high cube sits in a lashed tier, rod length and angle change, so the matching rod must be used.
- 45ft. Usually stowed on deck on top of 40ft stacks, using extra corner fittings at the 40ft positions. The ends overhang the stack below, and the box will not fit a normal 40ft cell guide.
- Mixed 20ft and 40ft. A 40ft on two 20ft boxes is common where approved. A 20ft on top of a 40ft is generally not allowed because there is no support in the middle.
- Bridge visibility. Tall stacks forward of the bridge are limited by line-of-sight rules; see bridge visibility.
Parametric rolling and heavy weather
Parametric rolling occurs in head or following seas when the wave encounter period is about half the ship's natural roll period and the wavelength is close to the ship's length. As crests and troughs pass, the waterplane changes shape and stability varies in rhythm, so the roll can grow within a few cycles. Container ships and car carriers with pronounced bow flare and wide sterns are most at risk. IMO guidance MSC.1/Circ.1228 on avoiding dangerous situations in adverse weather helps masters assess course and speed. Even a compliant lashing pattern can be overwhelmed by rolling this extreme.
Common planning mistakes
- Putting heavy boxes in the top tier because they reached the terminal late, then finding the stack over its limit when the lashing software runs.
- Assuming every box is 8'6", when high cubes change stack height, rod length and wind load.
- Using booking weights instead of VGM, so the calculated forces are lower than the real ones.
- Overlooking the outboard rows in the forward bays, where accelerations and sea loads are highest.
- Checking lashing only for departure, although GM changes as bunkers and ballast are used on passage.
Checks before departure
On Indonesia's domestic container trades, feeder ships often call at many ports with short port stays, so lashings are released and refitted again and again. Practical checks:
- All twistlocks locked, including semi-automatic ones; damaged twistlocks replaced, not reused.
- Rods on the correct points for the pattern and the box heights.
- Turnbuckles tight but not over-tightened, with lock nuts set.
- Containers whose weight does not match the VGM reported to the planner.
Shore-side planners can open stowage data in the BAPLIE viewer, check stacks with the container stowage calculator and build bays using container ship bay planning. STOWAGE.ID Vessel Stowage helps prepare and check plans ashore as decision support; the ship's class-approved loading computer and lashing software, the CSM and the stability booklet remain the official reference.