Known locally as BIW or The Yard, Bath Iron Works has built ships on the same site since 1890, producing more than 420 vessels including cargo vessels, fishing vessels, yachts, tugs, barges, and naval vessels. Between 1984 and 2009, it built naval ships exclusively and became one of Maine's largest private employers. As of May 2009, it was building Arleigh Burke-class AEGIS guided missile destroyers, each 509'6" long, with 9,200 tons full load displacement and a crew of 380; it had built the lead ship of the class and 29 follow ships, with four more scheduled for delivery by 2011. Each destroyer takes more than 3 years to build from keel laying to Navy delivery, and leaves fully functional, including armament. In 2009, the yard also began building the lead ship of the Zumwalt destroyer class (DDG-1000), and approximately 5,600 men and women worked there. For more than 400 years, ships have been assembled on this river, and over the past 60 years Bath Iron Works increased efficiency by building larger sections of ships inside buildings. The Ultra Hall can assemble complete sections from the keel to the main deck level; an Arleigh Burke-class ship is made of more than 20 such units, and future units were expected to be even larger, with some weighing more than two World War II-era destroyers. These units are often built upside-down for comfort and safety, then turned right-side up by cranes, and can be pre-outfitted with piping, electrical cables, lighting, ventilation, doors, paneling, engines, generators, machinery, electronics, and paint before being moved on self-propelled mobile transporters to the building slip. Bath Iron Works also adopted a floating dry dock in 2001, replacing the older slanted launching method in line with a worldwide trend. The dry dock is 750 feet long, 144 feet wide between the walls, 52 feet high, and can lift 28,000 long tons; it moves side to side along chains anchored in the river, aligns with three shipways, and can be pulled into the river, submerged, and used to float ships off after completed hulls are moved into it by hydraulic trolley and rest on temporary cradles and blocking. When not in use, it sits on a landing grid with ballast tanks open so the tide does not affect it. The shipyard's cranes include a white-and-orange-striped crane 400 feet tall with a 220-ton lift capacity, installed in 1973 and then the largest crane in the western hemisphere, along with three large blue cranes, two able to lift 300 tons and one able to lift 100 tons.