In the 20th century, the shipbuilding industry moved from the Mystic River to the Thames River. Electric Boat is the town's largest employer. However, until 1931, submarines designed by Electric Boat were subcontracted to other shipyards, primarily Fore River Shipbuilding in Quincy, Massachusetts. Electric Boat commenced industrial operations in Groton with the establishment of the New London Ship and Engine Company (NELSECO) as a subsidiary in 1911. NELSECO was the primary engine manufacturer for Electric Boat-designed submarines 1911–1925. In 1931, was laid down as the first submarine built in Groton. During World War II, Electric Boat completed submarines every two weeks.