The Liberty ships were cargo ships built in the Unite States during World War II. They were British in conception but adapted by the USA, cheap and quick to build, and came to symbolize U.S. wartime industrial output. Based on vessels ordered by Britain to replace ships torpedoed by German U-boats, they were purchased for the U.S. fleet and for lend-lease provision to Britain. Eighteen American shipyards built 2,751 Liberties between 1941 and 1945, easily the largest number of ships produced to a single design.