WARTIME EXIGENCIES PIONEERED MODERN SHIPBUILDING TECHNIQUES

BUILT for the war effort, the so-called Liberty Ships were prefabricated in parts and then welded together, enabling US yards to mass-produce 2 710 of these vessels within a few years, one vessel being assembled from prefabricated blocks in an astonishing 112 hours!

- BRIAN INGPEN

Most Liberties were 135m long, had an 8m draught and were about 10 800 dwt. The triple expansion steam engine with two oil-fired water-tube boilers and a single screw gave them a speed of 11 knots, and remarkably, they had a 15 000-nautical-mile range.With heavy shipping losses - especially from U-boats - a Liberty was built literally for one trans-Atlantic voyage. Additional crossings constituted a bonus, and their wartime survival was miraculous, given wild winter seas, their slow speed, ubiquitous submarine and aerial hazards, and inherent structural weaknesses in these all-welded ships.

Scientific investigations into the cause of structural fractures suffered by over 400 Liberties showed the cause lay in the grade of steel with which they were built. Cracks in the steel originated at stress points in heavy weather, such as hatch corners in the main deck, and at discontinuities in the sheer strake on the ships' sides. A fracture either could creep or spread rapidly with great force, sometimes causing the ship to sink.

Existing ships were strengthened by riveting crack-arresting steel “straps” about a half-metre wide and two-thirds of the ships’ length to the outboard edge of the main deck, and to the upper edge of the sheer strake, on both sides of the ships. On ships under construction, builders riveted a heavy right-angle girder to connect the deck to the sheer strake. Reinforcing the hatch corners on existing ships addressed the cracking there, and on new ships, a gentle radius to the corners was designed to spread the stress.