The Mark 37 Electric Torpedo was a high-performance, submarine-launched weapon considered the first successful torpedo developed for use against other submarines. It was designed to swim out of a 21-inch torpedo tube while remaining attached by a length of wire, allowing it to be steered to the target from inside the submarine. It was also equipped with a sophisticated homing system that used both active and passive sonar. Several versions served in the U.S. Navy from the late 1950's through the early 1980s before being replaced by the more advanced Mark 28. It weighed 1,700 pounds, measured 13.5 feet long, and had a diameter of 19 inches.