Stowage Factor (SF)

Stowage Factor (SF) : amount of space occupied by a given quantity of any dry commodity in whatever mode of transport (eg: grain in bags or in bulk) is said to be its stowage factor – best expressed for ease of remembering in terms of cubic feet per long ton or per metric tonne, but often described as cubic metres per tonne. The difference between long tons and metric tonnes is slight, only 1.6%, and for practical purposes can be disregarded. The trend is to quote tonnages in metric units and thus it will usually be found convenient to work in metric tonnes rather than in imperial units. There is however, considerable ‘market’ resistance to working in units other than cubic feet when describing stowage factors. Not only are stowage factors expressed in cubic feet easier for most people to remember – dispensing with the fractions required when utilizing cubic metres – they are the traditional method of describing cargo-space requirements and unlikely to be abandoned for a long time, if at all. Consequently, it behoves all involved in trading goods at sea, whether they prefer to work in cubic feet or in cubic metres, to bear in mind that 1 Cubic Metre equals 35.3148 Cubic Feet, for undoubtedly the need will arise to convert one to another. Whether in cubic feet or in cubic metres, the lighter a commodity, the more space it will occupy, and therefore, the higher its stowage factor. Thus corn stows around 50 cubic feet (1.42 cubic metres) per tonne in bulk condition, whilst much heavier iron ore occupies 15 (0.42) or less. As a result, a ship with a “poor cubic” – that is to say with a low ratio of cubic cargo space in relation to available deadweight, would have difficulty in loading a full deadweight cargo of corn, because of insufficient space in her holds. Furthermore, awkward or squared shapes cannot usually fit into every nook and cranny in a ship’s holds, thus losing valuable space through what is known as broken stowage (the term also used to describe spaces between packages, and indeed, any commodity remaining unfilled). Moreover, with many non-bulk commodities shipped, for example, in bags or in bales, the necessity of keeping them away from ship’s sides and other metal areas so as to avoid ‘sweating’ and associated damages, and to create ventilation channels, causes more loss of cubic capacity. Consequently, ships are measured for two sets of cubic capacity. Grain Capacity describing the total volumetric space available for all bulk cargoes (not just for grain); and Bale Capacity recording unobstructed space available for non-bulk cargoes. It. follows that a ship’s bale capacity should normally be smaller than its grain capacity, although with some modern “box-type” hold designs used principally in the short-seat trades, the grain and bale capacities are similar, the holds being clear, regular-shaped and unobstructed. Another method of assessing cargo space requirements is that termed Bulk Density, whereby a small amount of a bulk cargo is closely packed into a box and thereafter accorded a “stowage factor” expressed in terms of pounds (lbs) per cubic foot, or kilograms per cubic metre, depending on the box dimensions. This figure, however, must be treated with a degree of caution, as it fails to take into account the shape or characteristic of the carrying vessel. Furthermore, densely packed material does not necessarily recreate the lay of cargo in a ship’s hold. Stowage factors based on bulk density therefore, are liable to be lower than the actual stowage factor of the commodity in a ship’s cargo compartment. Carriers running a regular scheduled service between certain ports (a liner service) normally charge freight on the basis of USD per cargo ton. But where the goods involved are light and occupy a large amount of space, such a system would restrict carriers’ income. Accordingly, as a general rule, most operators levy carriage charges on a volume basis for articles stowing at more than 40 cubic feet or, if charging on a metric basis, more than one cubic metre. Thus has arisen the term Measurement Ton. whereby operators elect to levy freight on a tonnage basis if the goods involved stow at less than 40 cubic feet; or on a volume basis per each 40 cubic feet, for cargo stowing above that figure; or if they work in metric units, per metric tonne or per cubic metre.