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Container ships move standardised steel boxes (mostly 20 ft and 40 ft ISO units) through a cargo cycle that is planned ashore, executed by shore gantry cranes, and secured aboard with lashing gear rated against defined stack and lashing-force limits. Because the cargo is unitised and stacked in fixed cells, the entire operation depends on accurate positional data (bay/row/tier coordinates), accurate weight data (SOLAS VI Verified Gross Mass), and correct segregation of dangerous goods (IMDG Code) — errors in any of these three inputs can propagate into a structural or stability incident.
Every container slot aboard a cellular ship has a unique coordinate: bay (a cross-section from bow to stern, odd numbers on-deck/even in some numbering schemes, historically odd numbers for 20 ft bays and even for 40 ft), row (port-to-starboard position, numbered outward from the centreline), and tier (vertical position, below-deck tiers below 80 and on-deck tiers at 80 and above in common convention). The full set of occupied and empty slots for a voyage is the bay plan.
The bay plan is exchanged electronically between ship, terminal, and line planning systems using the BAPLIE EDIFACT message format (Bayplan/Stowage Plan Occupied and Empty Locations), which encodes container number, weight, size/type, port of loading/discharge, IMDG class where applicable, and reefer/temperature data for every slot. A COPRAR or MOVINS message set typically accompanies BAPLIE to instruct load/discharge sequencing at each port.
Ship-to-shore gantry cranes work a pre-planned sequence — discharge first, then load — bay by bay, to minimise crane travel and avoid re-handling. The terminal planning system executes the ship’s stowage plan but may adjust exact slot assignment within the constraints the ship’s chief officer or the line’s shore planner sets (weight limits, DG segregation, port rotation). Twin-lift and tandem-lift spreaders let modern cranes handle two 40 ft or four 20 ft boxes per cycle to increase productivity.
Containers are interlocked vertically with twistlocks (semi-automatic or manual cones fitted into the corner castings) and secured to the deck or to lashing bridges with lashing rods and turnbuckles. Lashing bridges — fixed steel structures between bays — allow lashing points higher up the stack, which lets vessels carry taller deck stacks within the same lashing-force envelope. Lashing patterns are calculated per bay, per voyage condition (GM, roll period, expected weather) against the limits in the vessel’s Cargo Securing Manual, which is itself built to the IMO CSS Code.
The CSS Code (Code of Safe Practice for Cargo Stowage and Securing) sets the framework for calculating the forces a stack and its lashings must withstand — racking, compression, and tension — under defined roll, pitch, and heave accelerations. A ship’s approved Cargo Securing Manual translates this into per-bay stack-weight and lashing-force tables that the planner and chief officer must not exceed. Exceeding stack-weight limits, especially on lighter boxes stowed under heavier ones, risks container collapse or lashing failure in heavy weather.
Refrigerated (reefer) containers require an onboard electrical supply and are stowed in dedicated reefer slots wired to the ship’s reefer plug points, with a monitoring system tracking set-point and alarm status. Reefer slot availability is a stowage constraint in its own right — reefer boxes cannot be stowed wherever space allows, only where power and monitoring exist — and is treated as a first-class planning input alongside weight and DG segregation.
Dangerous goods in containers are classified, packaged, and segregated under the IMDG Code (International Maritime Dangerous Goods Code), which sets segregation distances between incompatible classes (for example, oxidisers away from flammables), stowage category restrictions (on deck only, away from accommodation, away from machinery casings), and documentation requirements (dangerous goods manifest, container packing certificate). Planners must segregate correctly in three dimensions across the bay plan, not just within a single bay.
Historically, declared container weights were sometimes materially wrong, which fed inaccurate data into stability and stack-weight calculations and was cited as a contributing factor in a number of stack collapses and stability incidents industry-wide. In response, SOLAS Chapter VI, Regulation 2 made it mandatory, before a packed container may be loaded, for the shipper to provide a Verified Gross Mass (VGM) — obtained either by weighing the packed container (Method 1) or by weighing all contents and adding the tare weight (Method 2). A container without a VGM on file may not be loaded.
Large container ships are susceptible to parametric rolling — a resonance phenomenon where periodic changes in waterplane area as a vessel pitches in head or near-head seas cause roll amplitude to build rapidly, even in seas that do not look extreme. Combined with high, lightly-buffered deck stacks, parametric rolling and severe synchronous rolling have been implicated in container-loss-overboard events. Route and speed adjustment to avoid the resonant wave-encounter period, informed by weather routing, is the principal operational mitigation; see also vessel stability.
Modern container operations run on dedicated stowage-planning software that ingests BAPLIE data, checks stack-weight and lashing-force limits automatically, flags DG segregation conflicts, and calculates the resulting stability condition (GM, trim, stress) before a plan is approved. The chief officer aboard retains final sign-off responsibility even where the initial plan is generated by a shore-based planner or the line’s central planning team.
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