Now let us look ahead some three decades to 1917, by which time we find a yard dramatically transformed. Eleven wharves described a great arc at the confluence of the Charles and Mystic Rivers. The familiar old shiphouses had been replaced by a large shipbuilding ways and steel plate storage yards. The timber basin that had long dominated the center of the yard was gone, replaced by a new dry dock twice as long as the first one. The other timber basin at the east end of the yard had been filled in and was now the site of gas and oil tanks, a locomotive shed, and a gas plant for acetylene torches.
It was a vital place, showing an intensity not seen since the Civil War. In fact it was again a wartime yard: after almost three years of neutrality the United States had entered the global conflict that was later called World War I. Some 4,500 workers worked two ten-hour shifts or around the clock in three eight-hour shifts, answering to a steam whistle instead of the bell that had summoned 19th-century yard workers. The wharves and docks were crowded with three- and four-stacker steel ships, some carrying the towering cage masts that were a short-lived experiment of the period. On the building ways, workers had laid the keel of the fuel ship _Brazos_.
Electric lights illuminated the thousands of men working on ships through the night. Vessels under repair were alive with the flare of welding torches and the tattoo of pneumatic rivet guns. Over them moved the arms of great cranes, including a 150-ton floating derrick and a colossus that traveled on tracks between dry docks. Materials and equipment were transported by yard locomotives that had replaced the oxen (although horses still did service). A mechanized coaling plant near the old dry dock helped ease the dirty and arduous task of fueling ships. But it was apparently undependable, and at times ships were coaled the old way.
Charlestown’s main responsibility was repairing the warships of a greatly enlarged fleet: steel destroyers, armored cruisers and battleships, submarines, and wooden sub chasers. The yard also outfitted and commissioned new vessels, converted civilian vessels to wartime use, armed merchantmen, and altered seized German passenger liners to transport U.S. troops to France.