Cargo Calculations

For cargo calculations it is necessary to consult the load line zones map. The port of Newcastle is in a permanent summer zone and this zone stretches all the way to Southern Africa and up into the Atlantic ocean. There is a permanent tropical zone straddling the Equator as far northwards as the Canary Islands before a transitting vessel re-enters a permanent summer zone. So far there is nothing to prevent a vessel loading to her summer marks as an intervening tropical zone is no hindrance. Problems only set in with the vessel’s reaching the northwestern tip of the Iberian Peninsula, near Vigo, where the next zone may remain as a summer area for part of each year or be classified as a winter zone for the remainder of the year, the affected area reaching all the way to Rotterdam and beyond. Thus at certain times of the year a northbound vessel adjacent to Vigo will be crossing from a summer into a winter loadline zone and her master/operators will have to ensure that winter marks are not submerged. In our case we are told that the ‘CURLEW’ is loading at Newcastle during April. Consequently, by the time she reaches the area of Vigo in May/June, the zone will be summer, and the effect of a winter zone can thus be disregarded. At other times of the year it may be necessary for an estimator to calculate whether the winter zone will affect the cargo quantity that can be loaded at Newcastle or not. To do this one has to ascertain whether the ship will have burnt off enough bunkers to raise her draft to winter marks and the way to do this is to run figures backwards from Vigo by calculating the distance back to Newcastle or to the previous bunkering port, and the vessel’s anticipated bunker consumption for that part of her voyage. This consumption tonnage is then added to the vessel’s winter deadweight, and the result equals the quantity of cargo the vessel can load at Newcastle, always providing this does not exceed the vessel’s summer deadweight. In other words, if the estimated bunker consumption exceeds the difference between the winter and summer deadweight, the summer deadweight remains the restricting factor.