The V-boats were a group of nine United States Navy submarines built between World War I and World War II from 1921 to 1934. These were not a ship class in the usual sense of a series of nearly identical ships built from the same design, but shared authorization under the "fleet boat" program. The term "V-boats" as used includes five separate classes of submarines. They broke down into three large, fast fleet submarines (V-1 through V-3), three large long-range submarines (V-4 through V-6), and three medium-sized submarines (V-7 through V-9). The successful fleet submarines of World War II ( Tambor class through Tench class) were descended from the last three, especially V-7, though somewhat larger with pure diesel-electric propulsion systems.