Vessel Speed

Speed is another important factor.  In some cases it might be better to proceed more slowly and economise on bunkers.  Particularly this is the case in coastal estimating, where voyages are dependent on tides.  There may be little point in steaming full speed only to await a suitable tide for some hours following arrival off a port. Initially one needs to know the vessel’s total deadweight, this on the assumption that there are no draft limitations anywhere on the voyage.  From this figure one must first deduct the constants, which consist of stores, water, lubricants, spares and even the weight of the crew.  This tonnage is rarely critical, and is in the region of 400 to 500 tons for most deep sea vessels of 30,000 tonnes dwt and above.  The other deductible, before arriving at the true cargo capacity, is the amount of bunkers on board.  This can be a complex calculation and is best explained by examples which are given later in this lesson.  For the meantime, assuming a suitable figure for deadweight cargo capacity is reached this is not the end of the problem.  The vessel may be able to lift a particular tonnage of cargo but has it the space to carry it?  This is where the importance of the stowage factor (SF) enters into our consideration and, although there are fewer very light cargoes around than there used to be, the SF is still a relevant factor.  In theory, by dividing the grain or bale cubic of the vessel by the SF we reach her volume capacity but, if there are several types of grades of cargo to be separated, it may be impossible even to fill the vessel, whilst trim and stability must also be watched. Knowledge of loadline zones is essential. All zones transited between loading and discharging ports must be considered; as a vessel cannot enter a winter zone, for example, when loaded to summer marks.  The restricting load line must be considered and the cargo calculated accordingly.