The Barbel class of submarines, the last diesel-electric propelled attack submarines built by the United States Navy, incorporated numerous, radical engineering improvements over previous classes. They were the first production warships built with the teardrop-shape hull first tested on Albacore (SS-569), and the first to use an "attack center" within the hull rather than a conning tower in the sail. This class of submarine became part of the United States Navy's fleet in 1958 and was taken out of service between 1988 and 1990, leaving the Navy with an entirely nuclear-powered submarine fleet.