Every cargo operation is really four operations running at once: the strength and stability plan, the hazard screening on bulk cargoes, the atmosphere inside a tanker's tanks, and the securing and paperwork that follow the cargo ashore. This chapter works through the reasoning and the numbers behind each one.
A cargo plan is judged on its departure and arrival conditions, but ships are not damaged by their departure and arrival conditions — they are damaged by what happens in between. Every intermediate stage of loading, discharge or ballast shift has to satisfy the same constraints as the final one: deadweight not exceeded, GM adequate, trim within the range the port and pilots expect, hull girder shear force and bending moment within the class limits, and no tank top loaded beyond its local strength. A plan that only checks the two end conditions can walk straight through an unsafe intermediate stage without anyone noticing until the ship notices for them.
The sequence also has to respect discharge port rotation. Cargo for the last port on the itinerary cannot be stowed where it blocks access to cargo for an earlier port — the working principle is broadly first-port cargo loaded last and discharged first, so that no hold has to be part-restowed at an intermediate port to reach cargo underneath it. Getting the rotation wrong costs port time and, on a tight schedule, tempts a crew into an unplanned restow under time pressure, which is exactly when stability and strength checks get rushed or skipped.
A cargo plan is a sequence of conditions, not a single condition. Check deadweight, stability, trim, strength and tank top loading at every stage the loading or discharge sequence passes through, not only at the two ends of it.
The IMSBC Code sorts solid bulk cargoes into three groups by the hazard they carry. Group A cargoes can liquefy — fine, moist particulate materials such as some mineral concentrates and iron ore fines that behave as a solid when dry but can transition to a liquid-like state under the vibration and movement of a voyage once their moisture content is high enough. Group B cargoes carry a chemical hazard — combustion, toxicity, corrosivity — without a liquefaction risk. Group C cargoes have neither hazard. A cargo can appear in more than one group depending on its exact composition, which is why the shipper's declaration and certificate, not the cargo's name on the manifest, decide how it is handled.
For a Group A cargo the number that matters is the transportable moisture limit, and it is derived from a laboratory test, not a shipboard guess. The flow moisture point is the moisture content at which a sample of the cargo, under standardised vibration in the test apparatus, begins to develop a visible flow state. The TML is set at 90 % of that figure — a fixed margin below the point of actual failure, because the flow moisture point is itself a laboratory idealisation of what happens in a real cargo hold subjected to a real sea state.
The can test — striking a sample-filled cylindrical can against a hard surface and watching for free moisture or a change in consistency — is a legitimate shipboard check for a sudden change in condition, for example after rain during loading. It is not a substitute for the certified moisture content test, and a positive can test is a reason to stop loading and seek a proper reassessment, not a reason to argue with the certificate on file.
Non-cohesive, free-flowing bulk cargoes — grain-like materials with a low angle of repose — carry a different risk: they can shift bodily inside a part-filled hold as the ship rolls. Trimming a bulk cargo level, or as level as practicable, removes the void space a shift would otherwise use and is the standard mitigation whenever the cargo's own angle of repose is too low to hold a peaked surface stable.
Grain cargoes get their own regime because a part-filled hold of free-flowing grain can shift transversely as the ship rolls, and that shift produces a heeling moment that stability alone does not capture. The approved grain loading booklet, carried for the ship's actual hold geometry, gives two linked pieces of information for any loading condition: the maximum heeling moment permitted for the ship's actual GM, and — through its volumetric heeling moment tables — the heeling moment that a given stow in a given hold, at a given degree of fullness, will actually produce.
Both figures have to be checked, and they are not the same check. GM adequacy tells you the ship is stable enough in general. The heeling moment check tells you whether this particular stow, with its particular slack spaces, stays within what that stability can absorb if the grain shifts. Reducing a hold's free surface — by trimming, by strapping or bagging the surface, or simply by filling it fuller — reduces the heeling moment it contributes; increasing GM, typically with low ballast, raises the moment the booklet will permit. A stow that fails the check can be fixed by either lever, or both.
GM adequate and heeling moment within the booklet's permitted value are two separate pass conditions for a grain stow. A ship can pass one and fail the other.
A hydrocarbon vapour and air mixture is only dangerous within a band of concentrations — below the lower flammable limit there is not enough vapour to burn, above the upper flammable limit there is not enough oxygen relative to vapour to burn, and in between the mixture is flammable. The entire logic of inert gas operations is to avoid that band, not to cross it carefully.
Inert gas, generated on board or supplied ashore, is introduced to keep tank oxygen content below 8 % by volume and the tank atmosphere at a slight positive pressure, so that air cannot be drawn in through any leak path. At that oxygen level combustion cannot be sustained regardless of how much hydrocarbon vapour is present, which is why a tank can safely hold a cargo vapour space that would be lethal to ignite if air were present instead.
The purging-before-gas-freeing sequence exists because moving directly from an inert, hydrocarbon-rich atmosphere to a fresh-air atmosphere would take the tank straight through the flammable range on the way. Purging uses inert gas — not air — to dilute the hydrocarbon vapour down to a low concentration, conventionally referenced against a critical dilution point, while oxygen content stays low throughout because the diluting gas is inert. Only once hydrocarbon content is low enough that introducing air cannot reconstruct a flammable mixture does gas freeing with fresh air begin. Skip the purge and go straight to gas freeing, and the tank atmosphere is walked deliberately through the one concentration band the whole inerting regime exists to avoid.
Purge with inert gas until hydrocarbon content is low, then gas-free with air. Never introduce air into a tank that is still hydrocarbon-rich, however low its oxygen content — the flammable range depends on both figures at once.
The Cargo Securing Manual is ship-specific: it is drawn up for the actual lashing equipment, stack heights and hull structure of that ship, approved by the flag administration or a recognised organisation acting for it, and required to be carried and followed under SOLAS. The CSS Code sets the general framework the manual is built within, but it is the ship's own manual — not the Code in the abstract — that gives the numbers actually used: maximum stack weights for each bay and tier, permitted lashing patterns, and the maximum securing load each lashing component is rated to.
Those numbers come from the forces a rolling, pitching ship imposes on a container stack — principally the transverse acceleration from roll, which rises with roll amplitude and with distance from the ship's centre of rotation. A stiffer ship, with a larger GM, rolls faster and can impose higher accelerations on a high stack even though it is, by the ordinary stability measure, the more stable ship — stability and lashing load are not the same problem, and a plan that only checks the first can still overload the second.
None of the stack, lashing or corner-post calculations mean anything if the mass they are built on is wrong, which is exactly what a mis-declared container weight does. The SOLAS requirement for a verified gross mass before a packed container is loaded exists because the stow plan, the stack weight limit and the lashing calculation are all built on a mass figure that, without verification, is only the shipper's word.
Cargo documentation is not paperwork generated after the fact — much of it is a precondition for loading at all, and the rest is the record that follows the cargo through the voyage and into any dispute after discharge. Moisture certificates and flow moisture point data for Group A bulk cargoes, dangerous goods declarations and container packing certificates, verified gross mass figures, and the ship-specific grain loading booklet all have to be in hand before the relevant cargo is loaded, not chased afterwards.
The mate's receipt is the chief mate's own record of what was actually received, in what apparent condition, and it is the document a bill of lading is normally issued against. Clausing it — noting damage, shortage or any condition at odds with what is declared — is the point at which the ship's own assessment enters the paper trail, and it is far easier to defend a clause made at the rail than a claim raised weeks later at the discharge port with no contemporaneous record behind it. A letter of protest, issued when the ship's own observations are disputed by the shore side, serves the same purpose: it puts the ship's position on record at the time, not in hindsight.
Documentation is not filed after the cargo operation — the certificates that matter are conditions for loading, and the receipts and protests that matter are written at the time, not reconstructed from memory later.
The three examples below take the ideas above and put numbers through them the way an exam question — or an actual loading decision — would: a moisture check against a TML, a grain heeling moment check against a loading booklet, and a container stack weight recomputed once VGM disagrees with the declared figures.
A parcel of nickel ore concentrate (a Group A cargo under the IMSBC Code) is due to load. The shipper's certificate gives a flow moisture point (FMP) of 9.5 %. A shore laboratory oven-dries a representative sample: wet mass 500 g, dry mass 460 g. Decide whether the cargo may be loaded, and how much margin exists.
Flow moisture point (FMP), from certificate = 9.5 % Sample wet mass = 500 g Sample dry mass (after oven drying) = 460 g
First find the cargo's actual moisture content from the mass lost during drying.
This is what the cargo is actually carrying, not what the certificate assumes.
Then find the transportable moisture limit.
TML is always 90 % of the certified flow moisture point — a fixed safety margin below the point at which the cargo starts to behave like a liquid under vibration.
Compare the measured moisture content against the TML to see whether the cargo is transportable, and by how much.
AnswerMoisture content (8.0 %) is below the TML (8.55 %), so the cargo is transportable — but the margin is only 0.55 percentage points.
The trap: reading "below TML" as a simple pass and stopping there. A 0.55-point margin is almost nothing next to what a few hours of rain during loading can add — the certificate says the cargo may be loaded, not that it may be loaded uncovered.
A bulk carrier is finalising a grain stow across four holds. The ship's corrected departure GM is 0.30 m, at which the approved grain loading booklet permits a maximum heeling moment of 1400 t·m. The booklet's volumetric heeling moment tables give the following heeling moments for the proposed stow: Hold 1 — 420 t·m, Hold 2 — 510 t·m, Hold 3 — 350 t·m, Hold 4 — 180 t·m. Is the stow acceptable?
Corrected departure GM = 0.30 m Max permitted heeling moment at this GM (from booklet) = 1400 t·m Heeling moments for the proposed stow: Hold 1 = 420 t·m, Hold 2 = 510 t·m, Hold 3 = 350 t·m, Hold 4 = 180 t·m
The heeling moment from a grain shift is additive across holds.
So sum the four hold values to get the total assumed heeling moment for the whole stow.
Now check that total against the maximum the booklet permits for the ship's actual GM — the GM check and the heeling moment check are two separate tests, and both must pass.
Decide what to do about the exceedance.
Two independent levers exist: increase GM (raises the permitted moment) or reduce the heeling moment itself, typically by topping up the slackest hold so its free surface contributes less moment. Either way the stow as proposed cannot be approved unchanged.
AnswerThe stow exceeds the permitted heeling moment by 60 t·m at the planned GM — it must be revised (top up a slack hold or increase GM) before it is approved.
The trap: confirming GM meets the minimum required for grain and assuming that is the whole check. A ship can be perfectly stable and still fail the grain heeling moment check if the stow itself is wrong — the two figures are checked against each other, not in isolation.
A 20-container stack position on deck is approved in the ship's Cargo Securing Manual for a maximum stack weight of 148 t. Six containers are loaded into it. Their declared (shipper-provided) weights are 24.0, 22.5, 21.0, 25.5, 23.0 and 20.0 t. Before loading, the terminal's verified gross mass (VGM) check flags two of them as different from the declared figure: the 21.0 t container actually verifies at 24.8 t, and the 23.0 t container verifies at 26.1 t. Is the stack still within its limit?
Approved maximum stack weight = 148 t Declared weights (t) = 24.0, 22.5, 21.0, 25.5, 23.0, 20.0 VGM for container 3 = 24.8 t (declared as 21.0 t) VGM for container 5 = 26.1 t (declared as 23.0 t)
Start with the picture the declared weights give.
Since that is what a stow plan built on shipper declarations alone would show.
Two containers' verified gross mass differs from what was declared.
Replace those two figures with their VGM and recompute the stack total — this is the number that actually governs the lashing and corner-post loads.
Compare the verified total against the approved limit to see the true margin.
And judge whether it is still safe to load the stack on the strength of only two VGM corrections.
AnswerVerified stack weight is 142.9 t against the 148 t limit — 5.1 t of margin, not the 12.0 t the declared weights suggested; the stack is loadable once every container in it is confirmed by VGM, not only the two already flagged.
The trap: treating VGM as paperwork to be filed after the stow is planned. The stack weight limit is a physical lashing and corner-post limit — it has to be checked against verified mass before the stack is finalised, because one mis-declared box can erase the whole margin.
O₂ < 8 % by volumeInert gas atmosphere — prevents a flammable mixture formingTML = 90 % × FMPGroup A bulk cargo transportable moisture limitCan testShipboard indicator only — not a substitute for a certified moisture testGroup A / B / CA liquefies; B is a chemical hazard; C is neitherCSS Code + Cargo Securing ManualShip-specific, approved, mandatory lashing arrangementsVerified gross mass (VGM)SOLAS requirement before a packed container may be loadedAngle of reposeLow angle of repose means a bulk cargo needs trimmingGrain heeling moment ≤ booklet's permitted valueChecked separately from the GM requirement itselfUllage → trim correction → VCF → massTanker cargo quantity calculation chainPurge before gas-freeingDilute hydrocarbon with inert gas first; never introduce air into a hydrocarbon-rich tank