Before World War II, Mare Island had been in a continual state of upbuilding. By 1941, new projects included improvements to the central power plant, a new pattern storage building, a large foundry, machine shop, magazine building, paint shop, new administration building, and a huge storehouse. The yard was expected to be able to repair and paint six to eight large naval vessels at a time. Several finger piers had recently been built, as well as a new shipbuilding wharf, adding one and one berth. It employed 5,593 workers at the beginning of 1939, and rapidly increased to 18,500 busily engaged by May 1941, with a monthly payroll of $3,500,000 (1941). Then came the attack on Pearl Harbor. In 1941, the drafting department had expanded to three buildings accommodating over 400 naval architects, engineers and draftsmen. The hospital carried 584 bed patients. Mare Island became one of the Navy's shipbuilding sites in World War II specializing in building diesel engine-powered submarines; they eventually built 32 of them. After the war was over, Mare Island became a premier site for building nuclear-powered submarines, building 27 of them.