A safe stow is never one calculation. It's cubic capacity, tank top strength and securing forces, each checked independently, because passing one of them proves nothing about the other two.
Stowage factor is simply the volume one tonne of a cargo takes up once it's stowed — cubic metres per tonne, quoted in the tariff or the cargo's specification sheet. It's not the same as the cargo's own solid volume: a tonne of loose grain packs tighter than a tonne of the same grain in sacks, because the sacks trap air and stack with gaps between them. Multiply the tonnage you're loading by the stowage factor and you get the bare volume the parcel needs if it packed perfectly — which it never does.
That's where broken stowage comes in: the space lost to the shape of the hold against the shape of the cargo, to dunnage, to gaps around drums and cases that won't nest, to access left for lashing and inspection. It's expressed as a percentage of the space, added on top of the bare figure, and it moves with how regular the cargo is — a few percent for palletised cartons, 10–15 % for bagged cargo, 25 % or more for anything irregular in shape such as machinery or odd-sized crates. Skip it, or underestimate it, and a stow that looked fine on a calculator won't fit on the ship.
Which of the two limits — weight or volume — runs out first decides how the ship is loaded, and it depends on how the cargo's own stowage factor compares with the ship's. A vessel has a stowage factor of its own: its total bale or grain cubic capacity divided by its deadweight. Cargo denser than that figure is deadweight cargo — the holds still have space left when the ship is down to her marks, and weight is what limits the load. Cargo lighter than that figure is measurement cargo — the ship runs out of cubic capacity long before she's anywhere near her marks, and freight is usually charged by volume rather than weight for exactly that reason.
Space required is never just weight × stowage factor — it's weight × stowage factor × (1 + broken stowage), and broken stowage is the number that catches people out, because it doesn't show up until the cargo is actually going in.
The deadweight/measurement split tells you whether weight or volume runs out first for the ship as a whole, but there's a second limit that has nothing to do with either: how much load the tank top itself, or a 'tween deck, can actually bear per square metre. It's a structural figure from the ship's loading manual or stability booklet, sometimes posted on a plate in the hold, and it has to be checked separately from both the total deadweight and the total cubic capacity — a parcel can be well within both of those and still overload the deck it's standing on.
The reason is that weight isn't spread evenly by nature; cargo is. Steel coils, machinery, stacked drums — anything dense stowed over a limited footprint — concentrate a lot of tonnes over a small area, and it's entirely possible for that footprint to exceed the permissible loading while the ship overall is nowhere near her deadweight and the hold overall is nowhere near full. The check is a straightforward load density calculation: total weight divided by the actual area it's standing over, compared against the permissible figure for that tank top or deck.
Get the area wrong and the whole check is meaningless — it has to be the actual footprint the cargo occupies, not the area of the hold, and dunnage that spreads a point load over a wider base genuinely changes the number. This is also why heavy-lift and unitised cargo plans specify dunnage and footprint as carefully as they specify the lashing: spreading the load is doing real structural work, not just tidying the stow.
Cubic capacity, deadweight and tank top strength are three separate limits. A stow can pass two of them and still fail the third, so all three get checked, every time, not just whichever one comes to mind first.
A lashing exists to resist a force, and that force comes from the ship's own motion. Rolling accelerates cargo transversely, pitching accelerates it fore-and-aft, heaving accelerates it vertically, and all three act together in a real seaway. The size of the transverse component depends on the ship's roll period and GM, and on where the cargo actually sits — the further a package is from the ship's centre of rotation, both across the beam and above the waterline, the larger the swing, and the larger the force it generates for a given roll angle. A container lashed on deck at the ship's side, high and wide of the centreline, sees a far bigger transverse force than the same container stowed low and near the centreline.
That force is what the lashing plan has to match, not guess at. Treat it as mass times acceleration, with the acceleration expressed as a fraction of g for the stowage position, and you get a restraining force in tonnes. But a lashing's rated safe working load is not automatically what it contributes to resisting that force — only the component of its tension that actually acts in the direction of the force counts, and that depends on the angle the lashing is rigged at. A lashing running nearly flat along the deck gives very little restraint against a steep transverse force no matter how strong its SWL; the geometry has to be resolved before the numbers mean anything.
None of this is left to be worked out fresh on deck for a routine stow. Every ship carries a Cargo Securing Manual, ship-specific and approved, setting out standard, pre-calculated lashing arrangements for the cargoes she normally carries — how many lashings, what pattern, what equipment. Departing from it, because the cargo doesn't match a standard entry or the intended stowage position is different, means the calculation has to be redone properly, and is a reportable departure, not a judgement call for whoever happens to be on deck at the time.
A lashing's contribution is its SWL resolved through the angle it's actually rigged at, not the SWL stamped on the fitting — and the Cargo Securing Manual exists precisely so this doesn't have to be recalculated from scratch for every routine stow.
Air holds moisture, and how much it can hold depends on its temperature — warmer air holds more. Cool that air down and, once it reaches its dew point, it can't hold what it's carrying any more, and the excess condenses out as liquid water on whatever surface is cold enough. That single mechanism produces two quite different problems in a cargo hold, depending on which side of the temperature difference the moisture starts on.
Ship's sweat is condensation on the ship's own steel — the deckhead, frames, shell plating — when warm, moist air inside the hold meets structure that's colder than its dew point. It typically shows up moving from a warm climate into a cooler one, air brought aboard warm and humid later finding cold steel. Cargo sweat is the reverse: the cargo itself is cold, and warm, moist air condenses directly onto its cold surfaces. That's the pattern moving from a cool load port toward the tropics, and it's cargo sweat, not ship's sweat, that ventilation itself can cause if it's done at the wrong time.
The rule that decides whether to ventilate is a comparison of dew points, not temperatures: ventilate when the outside air's dew point is lower than the dew point of the air already in the hold, because that means the outside air is genuinely drier and will reduce the moisture inside. If the outside dew point is higher, opening the ventilators brings in air that's wetter than what's already there and makes the problem worse, even if the outside air feels cooler to the hand. Dew point has to be read off proper tables from dry- and wet-bulb thermometer readings — it isn't something to be judged by comparing two temperatures directly, because relative humidity at each temperature is doing the real work in the comparison.
Two different sweat problems share one mechanism — warm moist air meeting a colder surface — and one decision rule fixes both: compare dew points, never temperatures, before deciding whether to ventilate.
Every dangerous goods shipment is fixed in place by four things that follow from each other: its class, its packing group, the segregation that class requires from other cargo, and the documentation and stowage position that follow from both. The IMDG Code sets all four out, class by class, and none of it is something to infer from the cargo's name or common sense — a flammable liquid and an oxidising substance can look equally unremarkable on a manifest line and be entirely incompatible in the same hold.
Packing group indicates the degree of danger within a class — broadly, how readily the hazard is realised — and it affects packaging standards and quantity limits more than it affects where the goods can be stowed. Segregation is the part that actually decides the stowage plan, and it works in graded steps rather than a single yes or no: requirements range from keeping cargo simply away from an incompatible class, up through separated from, segregated from, and separated by a complete compartment or hold from it, each step meaning a stricter physical separation than the last. Which step applies to a given pair of classes comes from the Code's segregation table, cross-referencing the classes involved — it is not something to estimate.
None of this is paperwork to be taken on trust. The officer planning the stow checks the declared class and segregation requirement against the actual stowage plan before the goods go anywhere near the hold or the deck — including proximity to accommodation, machinery spaces, fire-fighting access and escape routes — because a documentation error caught after loading is a much harder problem than one caught before.
A cargo passage leaves a paper trail that exists to prove what was agreed, what was loaded, and what condition it was in: the mate's receipt noting apparent order and condition at the point cargo is taken on board, the bill of lading built from it, the manifest listing everything carried, stowage and securing plans showing where each parcel sits and how it's restrained, and — for anything hazardous — the dangerous goods declaration and supporting certificates. Each of these documents is checked against the cargo actually presented, not simply accepted because it's been signed by someone ashore; a mate's receipt claiming good order for cargo that plainly isn't in good order is a false record, and it's the officer standing there who's asked to put a name to it.
Underneath all of that documentation sits one responsibility that never moves: the master's duty for a safe stow — safe for the ship's stability and structural strength, safe for the crew handling it, and safe for the cargo itself. Charter party terms that shift the cost or physical work of loading onto shippers or stevedores don't shift that duty; the master and the officers can, and are expected to, refuse a cargo, a quantity, or a stowage instruction that would endanger the ship, and to record that objection clearly if commercial pressure overrides it. A loading plan is signed off against stability, strength and securing together — not stability today and securing tomorrow — because a stow that passes each check on its own can still combine to leave the ship in a condition nobody actually intended.
Paperwork records what was agreed to be loaded; it does not replace checking what was actually loaded. The responsibility for a safe stow sits with the master and can be objected to, but not signed away.
The three worked examples below build on the checks above rather than repeating any one of them in isolation — each combines at least two of the section's ideas, the way a real cargo question does.
You are planning No. 2 tween deck. 800 t of bagged rice (SF 1.5 m³/t, 15 % broken stowage) is booked for the space, which has a bale capacity of 1500 m³. The agent then asks whether a parcel of drummed chemicals (SF 2.0 m³/t, 20 % broken stowage) can also go in alongside it. How many tonnes of the drums, if any, will fit?
Bagged rice: 800 t to load, stowage factor 1.5 m³/t, broken stowage allowance 15 % No. 2 tween deck: bale capacity 1500 m³, currently empty Drummed chemicals also nominated for the same space: stowage factor 2.0 m³/t, broken stowage allowance 20 %
How many tonnes of the drums, if any, will fit?
Find the space the rice needs.
Multiply the weight by the stowage factor to get the bare stowed volume, before any allowance for the gaps a bag stow always leaves.
Add the 15 % broken stowage allowance.
It is a percentage of the stowed space, not of the weight.
Compare against the tween deck's bale capacity to see what.
If anything, is left over.
Convert that spare volume into a tonnage of drums.
Build the drums' own effective stowage factor first, weight and broken stowage together, then divide the spare space by it.
Answer50 t of the drummed chemicals can be accepted alongside the rice in No.2 tween deck; the remainder of that parcel must be stowed elsewhere.
The trap: applying broken stowage to the weight instead of the volume, or checking the drums' space requirement without first adding their own broken stowage allowance, both make the spare capacity look bigger than it really is.
No.4 hold has a bale capacity of 1080 m³ and a tank top area of 150 m², with a permissible loading of 5 t/m². You are asked to load 600 t of bagged fertiliser (stowage factor 1.5 m³/t, broken stowage 20 %) into it as a single parcel. Does it fit, and is the tank top loading acceptable?
Bagged fertiliser: 600 t to load, stowage factor 1.5 m³/t, broken stowage allowance 20 % No. 4 hold: bale capacity 1080 m³, tank top area 150 m² Permissible tank top loading: 5 t/m²
Find the bare stowed volume of the fertiliser.
Add the broken stowage allowance and compare with the hold's bale capacity.
That zero margin is worth noting before checking the second limit.
A stow planned to the exact cubic leaves no allowance for bags that don't pack quite as neatly as the stowage factor assumes, or for a few damaged bags set aside on deck.
Check the tank top separately.
Cubic capacity and tank top strength are independent limits, and either one can bind first.
AnswerThe 600 t parcel exactly fills the hold's bale cubic, with no spare volume, but loads the tank top at only 4 t/m² against a 5 t/m² permissible — this hold is volume-limited, not strength-limited, for this cargo.
The trap: assuming that because the tank top loading has margin to spare, the hold itself must have spare space too — the two checks are independent and both have to be done.
A 20 t laden container is stowed on deck at the ship's side, where rolling gives a transverse acceleration of 0.5 g. The lashings available have an SWL of 5 t each, and at the angle they can practically be rigged, only 50 % of that SWL acts in the transverse direction. How many lashings does the stow need, and how many should actually be rigged?
Deck container, laden mass 20 t, stowed athwartships at the ship's side Transverse acceleration at this stowage position, from the vessel's roll: 0.5 g Lashing safe working load (SWL): 5 t each Effective transverse component of each lashing, at its rigged angle: 50 % of SWL
How many lashings does the stow need, and how many should actually be rigged?
Turn the ship's motion into a force.
Rolling doesn't lift the container clear of the deck, it pushes it sideways — the restraining force needed is the container's mass times the acceleration, expressed as a fraction of g.
Find what one lashing actually contributes in that direction.
Its rated SWL is not the same as its transverse holding power once it's rigged at an angle.
Divide the force to be restrained by what each lashing actually delivers.
Four is the bare calculated minimum, not the number to rig.
A static estimate like this doesn't capture the snatch loading of a real seaway, and lashings work best rigged as opposing pairs so each one can be set up taut without fighting the others. Add a working pair beyond the minimum.
AnswerCalculated minimum is 4 lashings; rig 6, arranged as three opposing pairs, to give a working margin over the static figure.
The trap: quoting a lashing's rated SWL as its restraining contribution without resolving it through the angle it's actually rigged at — a shallow lashing can look strong on paper and hold almost nothing transversely.
Space required = W × SF × (1 + bs%)bs% as a fraction of the stowed space, not of the weightShip's own SF = total bale/grain cubic ÷ deadweightcargo denser than this loads by weight; lighter loads by volumeTank top load = W ÷ areacheck the local t/m² against the loading booklet figure, not just total tonnesRestraining force ≈ mass × transverse accel (as a fraction of g)resolve each lashing's SWL through its actual rigged angle firstVentilate only when outside dew point < hold dew pointcompare dew points, never air temperatures aloneShip's sweat: warm moist hold air + cold steeltypically a warm-to-cool passageCargo sweat: cold cargo + warm moist airtypically a cool-to-tropical passage, or ventilating at the wrong timeIMDG segregation, increasing strictnessaway from → separated from → segregated from → separated by a complete compartment/hold from