<P> Lakers are generally bulk carriers; that is, they carry cargoes of rock, ore, salt or grain in large contiguous holds, not packed in containers . The earlier ships required rail cars unloading on ore docks and unloading machinery at the receiving docks, but modern lakers are self unloaders, allowing them to unload faster and in more ports . </P> <P> The most common cargoes on the Great Lakes are taconite, limestone, grain, salt, coal, cement, gypsum, sand, slag, and potash . Much of the cargo supplies the steel mills of the auto industry, centered around the Great Lakes because of the ease of transport . Other destinations include coal - fired power plants, highway department salt domes and stone docks, where limestone is unloaded for the construction industry . U.S. - flagged freighters carried the largest portion of the trade, accounting for two - thirds of all cargo by weight . U.S. hulls carried most of the iron, limestone and cement while Canadian boats carried most of the potash and almost all of the salt and grain moved on the lakes . </P> <P> Destination harbors, ship sizes and legal restrictions greatly affect the pattern of haulage . Large U.S. ships hauled most of the iron ore on the lakes (79%) from U.S. mines to U.S. mills . This reflects the requirement of the Jones Act, as well as the industry using large volumes of material while being concentrated in a few large harbor locations . Salt and Canadian grain can be hauled to numerous smaller ports of either country on smaller, mostly Canadian, ships which can also enter the St. Lawrence Seaway with the Canadian ports of Montreal and Quebec City . </P> <P> The largest vessels on the lakes are the 1000 - footers (300 m). These vessels are between 1,000 and 1,013.5 feet (304.8 and 308.9 m) long, 105 feet (32 m) wide and of 56 ft (17 m) hull depth . They can carry as much as 78,850 long tons (80,120 t) of bulk cargo although their loading is dependent on lake water levels especially in the channels and ports . A dozen of these ships were built, all constructed between 1976 and 1981, and all remain in service today . The most powerful of these is MV Edwin H. Gott, which carried two Enterprise DMRV - 16 - 4 diesel engines driving twin propellers and was rated at 19,500 brake horsepower (14,500 kW), making it the most powerful lake boat on the seaway . This allowed a top speed of 16.7 miles per hour (14.5 kn; 26.9 km / h). MV Edwin H. Gott was repowered in 2011 with two MaK / Caterpillar 8M43C engines, each rated at 9,650 brake horsepower (7,200 kW), and other laker freighters have been repowered as well . MV Paul R. Tregurtha is the largest boat on the lakes, at 1,013 feet 6 inches (308.91 m) and 68,000 gross ton capacity . Stewart J. Cort, is both the first 1000 - footer to be put into service on the Lakes, and also the only one built in the traditional wheelhouse - forward Great Lakes style (although all accommodations are forward, and the stern deckhouse is occupied by self unloading equipment and the engines). Stewart J. Cort started life in Mississippi, and was sailed as a much smaller vessel consisting of only the bow and stern sections (named just as Hull 1173 at the time and appropriately nicknamed "Stubby"), to Erie, Pennsylvania, where she was cut in half and an additional 800 + feet of hull were added . Another interesting 1000 - footer is Presque Isle, an integrated tug and barge combination . Presque Isle is the largest tug / barge composite in the world . </P>

Are there oil tankers on the great lakes