The Hudson River National Defense Fleet (aka the Mothball Fleet) was established by Congress in 1946 and lasted until 1971 when most of the remaining vessels were sold for scrap. At its peak the fleet had 189 vessels, mostly WWII Liberty Ships and it was one of eight such fleets created by Congress at the end of the war in order to maintain a ready serve of Merchant Marine ships capable of supporting our military forces if needed.
During the Korean War about 130 ships were put into service, 35 put to sea during the 1956-7 Suez Crisis and more than 40 were mobilized during the Vietnam War.
The Liberty Ships came out of the emergency need to supply Britain and to carry the enormous amount of material necessary to support American military operations and millions of troops in Europe and the Pacific. When the Lend-Lease program was put into place in early 1941 America lacked the Merchant Marine capabilities for both that program and what increasingly became clear would be American entry at some point into the ongoing World War.
To meet the anticipated demand required a new way of building ships and the techniques were pioneered at the Kaiser Shipyards in the Bay Area of California and then replicated in shipyards across the U.S. The first Liberty Ship (also referred to as "ugly ducklings") was the Patrick Henry, launched on September 27, 1941 with President Roosevelt attending; it took 244 days to build. By the end of the war it took an average of only 42 days to build a Liberty Ship and one was built in 5 days! 2,710 Liberty Ships were launched in less than five years, more than one a day.
How was it done? Standardized part and assembly processes on multiple identical shipways with detailed planning to ensure availability of parts. The steel sheeting was welded, not riveted; not considered as durable but the ships were considered expendable, though fewer than 200 were lost during the war.
To speed the construction there was minimal attention paid to crew comfort. As described by Arthur Herman in Freedom's Forge:
Day Two (from Wikipedia) Day 24 (from Wikipedia)"There was no electricity or running water for the crew; their rooms and bunks were smaller than standard size. There was cement, not tile, in the toilet spaces, and no mechanical ventilation for the engine and boiler rooms and crew's quarters. The galley was lit with oil lamps and there was no fire detection system."
The result was a 441 foot long ship that was a "seagoing boxcar" capable of hauling eight thousand tons of materials and made an enormous contribution towards the winning of the war. In addition to supporting U.S. military needs in both theaters of war, the Liberty Ships supplied the United Kingdom and also traversed the extremely vulnerable North Atlantic route to Murmansk, the Soviet Union's port on the Arctic Ocean. Those who manned the ships faced constant danger. Of approximately 243,000 American merchant mariners about 9,000 died, a higher combat fatality rate than in the U.S armed forces.
Two Liberty Ships that have been preserved; SS John W Brown in Baltimore and SS Jeremiah O'Brian in San Francisco.
For more information on the Liberty Ships see this paper by Bill Lee.
(The Mothball Fleet also contained some ocean-liners converted to troop transports; from naval marine archive.com)