NATARAJAContainers & reefer — the box trade's arithmetic
Learning outcomes & permitted supports
- Read and write the box's postcode: bay (odd 20', even 40'), row (outward from the centreline), tier (80s ride on deck)
- Find who carries whom in a stack — the bottom box's burden is what the stack-weight limit protects
- Estimate the sideways force a roll puts on the lashings: weight × transverse acceleration, and why deck stacks cap harder
- Say what VGM is FOR: every downstream safety number — burden, lashings, KG — inherits the declared mass
- Work the reefer's pull-down as the exponential it is: the gap halves every τ·ln 2, height-to-time both ways
Supports in this lab: Calculator. Lashing force is the stated teaching model (w × a) — the ship's Cargo Securing Manual carries the full accelerations; VGM tolerance is the common administration line (±5% or 1 t). Golden 961/961.
Glossary: bay-row-tier · VGM · the cold chain · SWL & proof load
The postcode of a box: BAY — ROW — TIER
Every container on Earth answers to six digits. Bay from forward — odd for 20-footers, even for the 40' slot spanning the odd bays either side. Row outward from the centreline — odd to starboard, even to port, 00 on the line. Tier upward — 02, 04, 06 in the hold, then a jump to 82, 84 on deck, so a single glance at the code says whether the box rides under the hatches or in the weather.
Slot 210284 — a 20' position, port side, on deck. The codec round-trips in the golden suite, both ways.
The stack — who carries whom
Six high, the pile is a little building, and the bottom box carries everyone above it — that burden, not the total, is what corner-post strength and the plan's stack-weight limit protect. Load heavy under light (the natural order) and the burden falls tier by tier; put a heavy box on top and the bottom pays for it twice — in its posts and in the ship's KG.
The lashing arrows on the bench hold the DECK stack: sideways force = weight × transverse acceleration — a heavy roll runs about half a g at the top tiers. That force is why the lashing plan caps deck stacks harder than hold stacks, and why VGM lies break lashings before they break paperwork.
VGM at the gate
Since SOLAS VI/2, no verified gross mass, no loading. The terminal wants the number because the PLAN believed the declaration: stack limits, lashing forces and the ship's KG were all computed from it. The tolerance here is the common administration line — ±5% or one tonne, whichever is greater; the terminal's own rule governs at the gate.
Difference +1.1 t against a tolerance of 1.2 t — WITHIN: she loads.
Reefer — the cold chain is an exponential
Plug in a warm box and the machinery drags it toward the set point along Newton's curve: T(t) = Tset + (T₀ − Tset)·e^(−t/τ). The gap HALVES every τ·ln 2 hours — which is why the first ten degrees come fast and the last one crawls, and why "it was still warm at loading" is an argument about logarithms.
After 12.0 h the pulp reads -8.0 °C on the way to -18.0 °C. It crosses −15 °C (the frozen-cargo handover line) at 22.9 h — the exact inverse, golden-checked.
| Stack total vs plan limit | 94.0 t of 100 t — OK |
|---|---|
| Bottom box's burden | 66.0 t standing on it |
| Transverse force at the roll | 12.0 t on the deck-stack base lashings |