Stowage factors of coal vary depending on; a) the size of coal; b) whether it is wet or dry c) the type of vessel; d) the trimming process; and e) its origin.
a) The size of coal: Coal is sometimes graded by size (screened), the smaller the coal the less stowage space required by one tonne. Care must be taken to avoid undue breakage of pieces of the larger coal sizes, which lessens its value.
b) Whether coal is wet or dry: Not only will this affect the stowage factor, wet coal will dry-off whilst on passage and is therefore likely to substantially reduce the tonnage of cargo discharged – perhaps by as much as 4%.
c) Type of vessel: As with many bulk cargoes, those vessels lacking specially constructed holds complete with self-trimming facilities, will lift less coal for a given cubic capacity. Most coal today, however, is transported in bulkcarriers eminently suitable for the carriage of this commodity. Very little is now carried in vessels such as tweendeckers.
d) Trimming: Because the angle of repose of coal exceeds 35°,trimming of coal is essential. Most bulkcarriers have self-trimming facilities built in to their construction, but certain vessels, particularly small, general-purpose ships operating in coastal waters, are unlikely to be equipped with such facilities and will need careful trimming to secure the cargo. Much coal business is fixed on the basis that the cargo be loaded, trimmed as necessary and discharged free of expense to shipowners, but the AMWELSH Charterparty states (line 20) that loading, dumping and trimming costs are to be for shipowners to bear. Loading and trimming costs are self-explanatory. Dumping costs apply usually to certain loading ports of the United States, and are charges to cover the expenses of unloading railway wagons that fetch the coal from mines to the dockside.
e) Origins: The stowage factors of coal loaded in suitable bulk carriers from certain areas:-
- Australia Coal 38/44
- Australia Coal Briquettes 54
- Continent Coal Briquettes 54/55
- Poland Coal 42/45
- Poland Coal Briquettes 55/60
- South Africa Coal 40/44
- United States Coal 41/48
- United States Coal Low Ash 55
- West Africa Coal 50/56