Nevertheless, Charlestown kept up its steady repair work, especially on destroyers, albeit at a more modest level and with a workforce reduced to below 3,000 by 1922. The addition of a marine railway in 1919 allowed the yard to more easily service smaller ships of up to 2,000 tons. By the end of the decade further developments seriously threatened the Charlestown yard. The London Naval Treaty of 1930 extended the moratorium on new capital ship construction for another six years. The treaty further required the U.S. to scrap three battleships and 94 destroyers—the latter a mainstay of Charlestown. The deepening Depression also hurt the yard, as the government’s austerity program in the early years of the crisis reduced work at naval shipyards. The Hoover administration threatened to close most federal yards, including Charlestown. Yet in the Depression itself we can trace the roots of the coming boom. Building a Steel Ship Beginning in the 1880s, steel rapidly supplanted wood as the primary material in U.S. naval vessels. Charlestown began building large steel vessels in 1915-20, the period depicted below. Stronger per pound than wood or iron, steel enabled naval architects to design bigger ships that could carry more armament. Steel was also better suited to bearing the massive weight of steam engines and boilers. The structural members of early steel vessels were riveted together, with limited gas welding in use by World War I. Shipyard artisans traded auger, saw, and mallet for pneumatic drill, gas cutting torch, and pneumatic rivet gun. Massive steam-powered cranes replaced the old hoisting shears. Yet, while a riveted steel ship demanded vastly more complicated plans and a higher level of coordination between shops, it was assembled in much the same way as a wooden vessel. From the keel rose the stem, sternpost, and frames. Transverse beams, longitudinals, vertical stanchions, watertight bulkheads, decking, and plating completed the hull, all held together by rivets. Electric welding (_below_), developed in the 1930s, allowed still lighter construction and the prefabrication of sections. Designers, however, still called for rivets for some parts of the hull throughout World War II. [Illustration: _When the Charlestown yard began constructing steel ships in 1915, a new building ways was erected on the site of the shiphouse in which the wooden screw frigate_ Merrimack _had been built 60 years earlier_ (see pages 32-33). _The yard built three 475-foot fuel ships (“oilers”) on this shipways between 1917 and 1921, reducing the time between keel laying and launch from two years for the first ship to less than a year for the last._] [Illustration: Electric welding] Chain for the Navy