Page 147 - Ship Construction.DJ Eyres 6Ed

Ch14-H8070.fm Page 136 Wednesday, October 18, 2006 6:56 AM

14
Prefabrication

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During the Second World War a large number of merchant and war ships were
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required to be built in a short period of time. These requirements speeded
the adoption of welding in shipyards, and often led to the application of mass
production techniques in shipbuilding. Prefabrication of ship units, that is
the construction of individual sections of the ship’s structure prior to erection,
became a highly developed science. Often the units were manufactured at a
location remote from the shipyard, and erection in the shipyards was carried
out with schedules which still look very impressive today. Many of the more
spectacular achievements in this field were obtained in the USA where
much of the tonnage required during the war period was constructed using
these new advances. Unfortunately the results of this crash building pro-
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gramme were not always entirely satisfactory; the reputation of welded
structures for example suffered for quite a time as a result of wholesale
application without a background of experience. Since the war prefabrication
has gradually been applied to merchant ship construction as shipyards in this
country have been modified to employ this technique and have gained experi-
ence of unit fabrication. Today all vessels are to varying degrees prefabricated.
With riveted construction unit fabrication was rarely employed, the intro-
duction of welding lent itself more favourably to this form of construction.
With welding, simpler unit shapes (no staggered butts are required in the
side shell for example) and less critical tolerances can be applied. Units may
be constructed under cover, which is an attractive advantage in the British
climate, not only because of working conditions, but because of the better
welding conditions. It is possible to turn units over to allow downhand welding
which is easier to perform and likely to provide better results. There is great
advantage in keeping vertical and overhead welding to a minimum. Also
central services are more readily available at the shop, with gases for cutting,
air for chipping, and electric current for welding, being placed where needed.
Production planning techniques may be adopted with prefabrication sequ-
ences, the material and labour being planned in unit groups and the whole
shipbuilding sequence being controlled to fit the time allowed on the berth,
or in the building dock.
Until well into the 1970s prefabrication in British yards was primarily
concerned with the construction and erection of 2-dimensional and
3-dimensional steel units to form the ship structure into which the outfit