When attention is directed at port times, however, immediate difficulties are encountered. Unlike tankers, dry-cargo voyages vary enormously in their port time content. The difficulty is that often one cannot calculate port time until cargo quantity is known, and cargo quantity cannot be calculated until an assessment is made of the bunker quantity remaining on board at strategic points in the proposed voyage, and this cannot be properly calculated until voyage duration is assessed. Fortunately, even when working all cargo gear, dry-cargo vessels normally consume very small quantities of bunkers in port and so port bunker consumption can largely be overlooked for the purposes of cargo quantity estimation. Nevertheless, it may sometimes be necessary temporarily to postpone completion of the itinerary section of an estimate until the cargo calculation in Stage 2 has been concluded. Be very careful also over the route selected. Sometimes there are alternatives and only a marginal difference will tilt the balance in favour of one route or another. Bad weather at certain times of the year; high canal tolls on one route; cheaper bunkers on another. All factors must be considered. As an example, consider for a moment the alternatives for an estimator of a vessel proceeding from the United States Gulf (say, from New Orleans) to Singapore. On the face of it, Alternative 1 seems the better selection. But this is to overlook the cost of canal tolls and allowances for canal transit delay in comparison with the longer but probably cheaper Cape of Good Hope route. But is time of the essence? Is it necessary to complete the voyage as quickly as possible? In which case the estimator may have little choice but to proceed by the shortest, more expensive route.