Liberty ships were a class of cargo ship built in the United States during World War II. Though British in concept,the design was adopted by the United States for its simple, low-cost construction. Mass-produced on an unprecedented scale, the Liberty ship came to symbolize U.S. wartime industrial output.
The class was developed to meet British orders for transports to replace ships that had been lost. Eighteen American shipyards built 2,710 Liberty ships between 1941 and 1945 (an average of three ships every two days), easily the largest number of ships ever produced to a single design.

Their production mirrored (albeit on a much larger scale) the manufacture of "Hog Islander" and similar standardized ship types during World War I. The immensity of the effort, the number of ships built, the role of female workers in their construction, and the survival of some far longer than their original five-year design life combine to make them the subject of much continued interest.

In 1940 the British government ordered 60 Ocean-class freighters from American yards to replace war losses and boost the merchant fleet. These were simple but fairly large (for the time) with a single 2,500 horsepower (1,900 kW) compound steam engine of obsolete but reliable design. Britain specified coal-fired plants, because it then had extensive coal mines and no significant domestic oil production

The predecessor designs, which included the "Northeast Coast, Open Shelter Deck Steamer", were based on a simple ship originally produced in J.L. Thompson & Sons based on a 1939 design for a simple tramp steamer, which was cheap to build and cheap to run. Examples include SS Dorington Court built in 1939. The order specified an 18-inch (0.46 m) increase in draft to boost displacement by 800 long tons (810 t) to 10,100 long tons (10,300 t). The accommodation, bridge, and main engine were located amidships, with a tunnel connecting the main engine shaft to the propeller via a long aft extension. The first Ocean-class ship, SS Ocean Vanguard, was launched on 16 August 1941.

The design was modified by the United States Maritime Commission, in part to increase conformity to American construction practices, but more importantly to make it even quicker and cheaper to build. The US version was designated 'EC2-S-C1': 'EC' for Emergency Cargo, '2' for a ship between 400 and 450 feet (120 and 140 m) long (Load Waterline Length), 'S' for steam engines, and 'C1' for design C1. The new design replaced much riveting, which accounted for one-third of the labor costs, with welding, and had oil-fired boilers. It was adopted as a Merchant Marine Act design, and production awarded to a conglomerate of West Coast engineering and construction companies headed by Henry J. Kaiser known as the Six Companies. Liberty ships were designed to carry 10,000 long tons (10,200 t) of cargo, usually one type per ship, but, during wartime, generally carried loads far exceeding this.

On 27 March 1941, the number of lend-lease ships was increased to 200 by the Defense Aid Supplemental Appropriations Act and increased again in April to 306, of which 117 would be Liberty ships.

The basic EC2-S-C1 cargo design was modified during construction into three major variants with the same basic dimensions and slight variance in tonnage. One variant, with basically the same features but different type numbers, had four rather than five holds served by large hatches and kingpost with large capacity booms. Those four hold ships were designated for transport of tanks and boxed aircraft.

In preparation for the Normandy landings and afterward to support the rapid expansion of logistical transport ashore a modification was made to make standard Liberty vessels more suitable for mass transport of vehicles and in records are seen as "MT" for Motor Transport vessels. In that case four holds were loaded with vehicles while the fifth was modified to house the drivers and assistants.Propulsion

By 1941, the steam turbine was the preferred marine steam engine because of its greater efficiency compared to earlier reciprocating compound steam engines. Steam turbine engines required very precise manufacturing techniques and balancing and a complicated reduction gear, however, and the companies capable of manufacturing them already were committed to the large construction program for warships. Therefore, a 140-ton vertical triple expansion steam engine of obsolete design was selected to power Liberty ships because it was cheaper and easier to build in the numbers required for the Liberty ship program and because more companies could manufacture it. Eighteen different companies eventually built the engine. It had the additional advantage of ruggedness and simplicity. Parts manufactured by one company were interchangeable with those made by another, and the openness of its design made most of its moving parts easy to see, access, and oil. The engine—21 feet (6.4 m) long and 19 feet (5.8 m) tall—was designed to operate at 76 rpm and propel a Liberty ship at about 11 knots (20 km/h; 13 mph). Construction The ships were constructed of sections that were welded together. This is similar to the technique used by Palmer's at Jarrow, northeast England, but substituted welding for riveting. Riveted ships took several months to construct. The work force was newly trained—no one had previously built welded ships. As America entered the war, the shipbuilding yards employed women, to replace men who were enlisting in the armed forces.