Stowage Factor: How Weight and Space Efficiency Impact Cargo Transport

Stowage Factor: Any cargo vessel’s freight valuation is closely correlated with its cargo capacity. Whether they operate a tanker, bulker, or containership, the goal of all ship owners and operators is to maximize the amount of cargo that can be accommodated in the ship holds while optimizing other aspects like flotation, stability, design strength, and service parameters. The more cargo slots a ship fills, the more money it makes travelling between two ports of call in succession.

Nevertheless, it is quite rare for a space intended for cargo to be completely occupied. Put another way, a cargo tank on a ship that is exactly cuboidal, for example, cannot be loaded to capacity. Why?

Two criteria determine this, the most significant being the first.

Types of cargo

Types of cargo
(Credit: ResearchGate)
  • The layout and configuration of the cargo area

Taking care of the second factor first, cargo space organization, is important. For instance, a tank with a hexagon or diamond form works well for one kind of cargo but can hold less of another sort of cargo than a tank with a perfect cuboidal shape.

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In addition, other elements that affect the amount of space available for cargo include the positioning of stiffening members, plumbing, electrical connections, equipment, ducting, ventilation, etc.

Additionally, even though the cargo capacity of a given vessel designed to carry a particular kind of cargo is maximized during loading, it is not filled for several reasons, such as reducing the risk of flammability and overflow for liquid cargo carriers like tankers, maintaining some clearance at the top to allow for ventilation and prevent the build-up of high air pressure in fully confined volumes, and other operational considerations.

However, the type of cargo itself is the most significant component that determines how it is arranged in a space or hold. The stowage factor, a crucial phrase in the marine industry, is used to measure this.

Stowage Factor

Stowage Factor 
(Credit: Marine Insight)

A cargo’s stowage factor is the weight to the amount of space needed for stowage under typical circumstances. It shows how many cubic meters, accounting for inevitable stowage losses in the transport vehicle or the CTU (Cargo Transport Unit), one metric ton of a specific kind of cargo takes up in a hold.

For instance, the stowage factor for iron pyrites is 0.53 m³/t [1], and the stowage factor for coffee in 60 kilogram bags is 1.9 m³/t [1]. These figures include unavoidable stowage loss, or the space needed for particular stowage techniques and stowing, as well as the inevitable gaps between cargo components.

A cargo’s stowage factor might change depending on the type of cargo and how it is packaged. The stowage factor for pelletized wheat bran is 1.45 to 1.90 m³/t, as stated in [1]. Grain size, water and oil contents, and bag filling levels are the key determining factors for this number.

Whether or whether bale items are transported compressed or uncompressed is a crucial factor to consider, for example:

hemp in bales, uncompressed: 7.65 m³/t [1]
hemp in bales, compressed: 2.55 – 3.40 m³/t [1]

Stowage factors for other items change based on how much processing they’ve undergone, such as:dry, loose hides:

hides, dried, loose: 5.10 m³/t [1]
hides, wet-salted, loose: 1.27 m³/t [1]

Two main criteria determine the stowage factor

  • Density
  • Characteristics of the substance

As far as we can tell, density has a direct bearing on specific gravity and, thus, on how a given material settles in space. Higher density, heavier materials settle more readily and have a lower average center of gravity than lighter materials when piled high, as in cargo tanks or ship holds.

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For instance, iron ore might occupy less volume for the same weight when heaped in the same place since it has a considerably higher density than coal or sugar and a lower stowage factor. Similarly, they can be packed in larger quantities in the hold or compartment than sugar or coal, which occupy the same amount of space but weigh significantly less.

Given that the vessel’s construction limitations allow it, the same vessel can therefore transport more iron ore than coal or sugar. The type of material content is also a significant factor.

Granular or finer materials, as one might expect, typically occupy less room and have a lower stowage factor. On the other hand, coarser materials have a higher percentage of occupied volumes that are made up of gaps and free space. Consider the straightforward example of putting sand and stone chips in the same jar.

Even though the stone has a larger unit density, you will find that the later weighs more than the former when you fill the jar and weigh each one separately. This is also the idea behind the stowage factor. For example, if you fill a vessel with ten tonnes of iron ore and ten tonnes of huge stone chips, you will find that, even if their densities are not very different, there is still a significant amount of room left over once the contents of the former are emptied.

Thus, the interaction of these two elements determines the stowage factor. Petroleum products and other liquid cargo have significantly lower stowage factors than bulk cargo due to their unique characteristics.

Stowage factor values between 0.4 and 0.5 cu are typical. For iron ore, meters per tonne, 1.2–1.4 cu. Coal in meters, 0.3–0.6 cu. 2.5 cu. mètres of rolled steel. For wood, mètres, etc.

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