metal ores—and most of this trade was coastal. However, on the Great Lakes, bulk carriers hauled vast amounts of iron ore from Minnesota and Michigan's northern mines to the steel mills. In 1929, 73 million tons of iron ore was transported on the Lakes, and an almost equal amount of coal, limestone, and other products were also moved. Two defining characteristics of bulk carriers were already emerging: the double bottom, which was adopted in 1890, and the triangular structure of the ballast tanks, which was introduced in 1905. After World War II, an international bulk trade began to develop among industrialized nations, particularly between the European countries, the United States and Japan. Due to the economics of this trade, ocean bulk carriers became larger and more specialized. In this period, Great Lakes freighters increased in size, to maximize economies of scale, and self-unloaders became more common to cut turnaround time. The thousand-footers of the Great Lakes fleets, built in the 1970s, were among the longest ships afloat, and, in 1979, a record 214 million tons of bulk cargo were moved on the Great Lakes. Categories Size categories Bulk carriers are segregated into six major size categories: small, handysize, handymax, panamax, capesize, and very large. Very large bulk and ore carriers fall into the capesize category but are often considered separately. Categories as per regions Categories occur in regional trade, such as Kamsarmax, Seawaymax, Setouchmax, Dunkirkmax, and Newcastlemax also appear in regional trade. "Kamsarmax" : Maximum length overall 229 meters refers to a new type of ships, larger than panamax, that are suitable for berthing at the Port of Kamsar (Republic of Guinea), where the major loading terminal of bauxite is restricted to vessels not more than 229 meters. "Newcastlemax" : Maximum beam 50 meters, and maximum length overall of 300 meters Refers to the largest vessel able to enter the port of Newcastle, Australia at about 185,000 DWT "Setouchmax" : About 203,000 DWT, being the largest vessels able to navigate the Setouchi Sea, Japan "Seawaymax" : LOA 226 m max / 7.92 m draft. Refers to the largest vessel that can pass through the canal locks of the St. Lawrence Seaway (Great Lakes, Canada) "Malaccamax" : LOA 330 m / 20 m draft / 300,000 DWT, Refers to the largest vessel that can pass through the Straits of Malacca. "Dunkirkmax" : Maximum allowable beam = 45 m / LOA 289