Thursday, June 8, 2017

WWII Liberty Ships

John Abbott posted
US could not keep Britain supplied at the beginning of WWII because German's U-boats were sinking our ships three times faster than we could build them [kiel-kap_3]. So revolutions were needed in ship building.

One revolution was to build a lot of shipyards. America had eighteen shipyards building Liberty Ships. Many of those shipyards included a steel plant so that the supply chain was short. Another aspect of the revolution is that they were all using a standard design that was a modification of a British design. No time was wasted by a design phase. And a standardized design allowed the efficiency of interchangeable parts no matter which shipyard built the ship. For example, a part developed by a subcontractor could be used by all of the shipyards during construction, and then used later, if needed, for repairs. That design also used an obsolete coal-fired steam engine instead of a steam turbine. One reason is that Britain had plenty of coal mines, but very few oil wells. Another reason is that America had very little industrial capacity to build the gear reduction trains needed by steam turbines. Furthermore, ship crews had decades of experience maintaining steam engines and the open design also made maintenance easier. [Wikipedia-Liberty]

By United States Maritime Commission photo - Scanned from Live (program published by Project Liberty Ship for cruises of the Liberty ship SS John W. Brown), 2013 edition, page 11. Additional descriptive information from p. 10 of the same publication., Public Domain, Link
I learned from the TV show Impossible Engineering that someone invented the modular or prefabrication technique of constructing ships to make speed up production. The problem with learning something from a TV show is that I cannot go back and look up the name of the ship builder who pioneered that technique. Instead of tying up a dry dock with piece-by-piece construction of the hull, sections of the hull were built on land and then moved, lifted and joined to the ship in the dry dock. Of course this required the development of cranes that could handle many tons of steel rather than just a few tons of steel.

One of the modifications the Americans made to the British design was to use welding instead of rivets. This is the change credited by [kiel-kap_3] for the improved output. Using rivets did not avoid having to train the workers how to weld because they were recently hired women who needed to be trained anyhow. So they might as well be taught the new technique instead of the obsolete technique. Another modification was that American ships switched from coal to oil boilers.

As the at the top shows, an alternative to prefabrication construction techniques was to build a shipyard with lots of dry docks.

One issue with using a standardized design is that, if there is a design flaw, then you have built a lot of problems. In this case, the ships cracked. At first the welds were blamed, especially since they were made with an inexperienced work force. "The cause of the failures was discovered by Constance Tipper, an engineering professor at Cambridge. She found that the grade of steel used to make Liberty ships suffered from embrittlement." [BrightHubEngineering] Cold temperatures change the mechanical properties of steel from ductile to brittle. And these ships operated in the North Atlantic, which is cold.

The cracks where not at the welds as was first assumed, but the welded joints did contribute to the problem of broken ships because a weld propagates a crack. In riveted ships the crack stopped at the end of a plate so it did not grow to a catastrophic length. The other issue was that some of the steel plants built for the Liberty Ship program produced steel that had a high ductile-to-brittle transformation temperature. (Actually, it wasn't the plants themselves, it was their source of raw materials. The steel had too much phosphorus. What caused embrittlement below a threshold temperature was not determined until about 1950. [kiel-kap_9] Modern steels have a threshold below freezing, which is good enough for ships. But now I understand why they talk about steel getting cold in northern Alaska as such a big deal.)

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