but modern lakers are self unloaders, allowing them to unload faster and in more ports. 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. 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. Size The largest vessels on the lakes are the 1000-footers (300 m). These vessels are between long, wide and of hull depth. They can carry as much as 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 between 1976 and 1981, and all remain in service today. The most powerful is , which carried two Enterprise DMRV-16-4 diesel engines driving twin propellers and was rated at , making her the most powerful lake boat on the Seaway. This allowed a top speed of . MV Edwin H. Gott was repowered in 2011 with two MaK/Caterpillar 8M43C engines, each rated at , and other laker freighters have been repowered as well. is the largest boat on the lakes, at and capable of loading 68,000 tons of bulk cargo. Stewart J. Cort was 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