Liberty Ships

* Liberty Ships were a class of cargo ships built in the United States during World War II under the Emergency Shipbuilding Program. 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. The design was modified by the United States Maritime Commission, in part to increase conformity  to American construction practices, more importantly to make it even quicker and cheaper to build. The new design replaced much riveting, which accounted for one-third of the labor costs, with welding.

* 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. The first ships required about 230 days to build, but the median production time per ship dropped to 39 days by 1943. The ships were made assembly-line style, from prefabricated sections.

* In 1943 three Liberty Ships were completed daily.

Day 1…Slip preparation. Day 2…Laying of the keel plates. Day 6…Bulkheads and girders below the second deck are in place. Day 10…Lower deck being completed and the upper deck amidship erected. Day 14…Upper deck erected and mast houses and the after-deck house in place. Day 24…Ship ready for launching.

* Early Liberty Ships suffered hull and deck cracks, and a few were lost due to such structural defects. During World War II there were nearly 1,500 instances of significant brittle fractures. Twelve ships, including three of the 2,710 ships built broke in half without warning and sank to the bottom of the ocean. Suspicion fell on the shipyards which had often used inexperienced workers and new welding techniques to produce large numbers of ships in great haste.

* Fractures did not start in the welds, but were due to the embrittlement of the steel used; however, the same steel used in riveted construction did not have this problem. At a certain temperature, the steel the ships were made from changed from being ductile to brittle. This allowed cracks to form and propagate. That temperature is known as the critical ductile-brittle transition temperature. Ships in the North Atlantic were exposed to temperatures that could fall below this critical point. The predominantly welded hull construction, effectively a continuous sheet of steel, allowed small cracks to propagate unimpeded; unlike in a hull made of separate plates riveted together.

* One common type of crack nucleated at the square corner of a hatch which coincided with a welded seam, both the corner and the weld acting as stress concentrators. Furthermore, the ships were frequently grossly overloaded, increasing stress, and some of the problems occurred during or after severe storms that would further have increased stress.

* Minor revisions to the hatches and various reinforcement were applied to the Liberty Ships to arrest the cracking problem. A steel “band-Aid” was riveted to each side of the hull to solve the problem!



* Which supports and barriers were in play?

* What were the dynamics?

* Who, or What, won the Tug-of-war?

* Discuss the outcome with your friends and family!

* Use Post #4 as a reference for the dynamics, and the relationships, between supports and barriers.