Container with a doubtful verified gross mass
MV Nordkap, a 4,250 TEU container ship, is loading at a transhipment terminal in Southeast Asia. The planner has prepared bay 14, a deck stow at the forward end of the ship, as six tiers of 40-ft boxes. The bottom two tiers are 24.0 t each and the next three are 12.0 t each. The ship's approved lashing plan limits any 40-ft stack in this bay to 95 t total and 15 t for the topmost tier, because of the forward position and the high windage. A container presented for the sixth tier carries a verified gross mass document showing 18.0 t. The terminal's crane load cell shows 26.5 t, and the tally clerk reports that the box was re-stuffed yesterday, after the VGM was issued.
1. What does the discrepancy require of the master under the VGM regime, and can this container be loaded as planned?
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Under SOLAS Chapter VI, a packed container may be loaded only if a verified gross mass has been provided by the shipper, obtained either by weighing the packed container or by a documented calculation method approved by the competent authority. The document on this box is 18.0 t; a calibrated crane load cell says 26.5 t, a difference of 8.5 t, or 47%. The tally clerk's report explains it: the box was re-stuffed after the VGM was issued.
An inaccurate VGM is worse than no VGM, because the whole stow, lashing and stability calculation has been built on it.
- The figure is no longer a verified gross mass. The master should treat the container as having no valid VGM and refuse to load it until the shipper produces a new verified figure.
- The 26.5 t figure from the terminal's load cell is itself only an indication until confirmed, but it is credible evidence and should be recorded.
- Independently of the VGM, the stow fails the lashing plan's own limit (see below), so the container cannot go in this stack in any case.
- The master should report the matter to the terminal and the company, and ensure the discrepancy and the refusal are logged. If the terminal loads it anyway, a letter of protest is required before the ship sails.
So the answer is no, twice over: on documentation grounds and on stowage grounds.
2. Calculate the stack weight and set out a compliant alternative stow with figures.
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Current five tiers: 24.0 + 24.0 + 12.0 + 12.0 + 12.0 = 84.0 t against a 95 t limit, leaving 11 t of capacity.
Sixth tier options:
- Declared VGM 18.0 t → stack 102.0 t (7.0 t over the limit, and 18.0 t exceeds the 15 t top-tier limit).
- Load cell 26.5 t → stack 110.5 t (15.5 t over the limit, and 11.5 t over the top-tier limit).
Even at the declared weight the stack is illegal, which is the point the planner has missed: the sixth tier was never available for this box.
Compliant alternatives, in order of preference:
- Stow it low in a different deck stack. A bottom tier of a lighter stack, where the lashing plan's tier limit is higher and the box's weight is carried near the deck, is the least disruptive answer.
- Re-shuffle bay 14. Move two of the 12.0 t boxes from the fifth tier to another bay and put the heavy box in the fourth tier, with 12 t boxes above it. Check that the four-tier stack (24 + 24 + 12 + 26.5 + 12) is within limits before moving anything.
- Stow below deck in a hold where the stack design limits and tank-top or deck strength permit the point load, accepting that a Class 5.1 or similar segregation requirement may rule this out.
- Reject the box and ask for it to be re-stuffed or re-issued with a valid VGM and a lighter stow plan.
Whatever is chosen, the deck load per unit area and the lashing plan must be re-checked, and the revised plan approved before the crane lifts.
3. Why does the position of a heavy box in the stack matter so much more than its weight alone?
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A deck stack is a lever. The horizontal force on each lashing and on the stack's bottom corners is the mass times the acceleration, times the height of the centre of gravity above the deck. Raising a heavy box from tier two to tier six roughly triples that height, so the overturning moment on the stack rises with the same factor, while the lashing capacity does not change.
The accelerations themselves are not constant:
- Pitching produces the largest vertical and longitudinal accelerations, and the CSS Code allowances rise steeply toward the ends of the ship. Bay 14 is right forward, which is why the lashing plan is so restrictive here.
- Rolling gives transverse acceleration that increases with the ship's GM and with height above the waterline. A ship stiffened by loading heavy boxes low will roll quickly and throw the top tier sideways.
- Windage: a box sitting proud of the stack at six high adds area and a wind lever of its own, and the forward bay is the most exposed.
Practical consequences to check:
- Lashings, twistlocks and stackers are rated for a design load; overloading a stack does not show until a lashing parts in a seaway, and then the failure is progressive and dangerous to the crew.
- An over-weight box also affects the ship's stability and trim; 8.5 t placed 18 m above the keel is a real KG penalty on a small ship.
- The master should record the discrepancy and note the terminal's load cell reading, so that any subsequent damage cannot be attributed to a stow the ship did not plan.