The X class was a World War II midget submarine class built for the Royal Navy during 1943–44. It was substantially larger than the original Chariot manned torpedo. Known individually as X-Craft, the vessels were designed to be towed to their intended area of operations by a full-size "mother" submarine – usually one of the T class or S class – with a passage crew on board, the operational crew being transferred from the towing submarine to the X-Craft by dinghy when the operational area was reached, and the passage crew returning with the dinghy to the towing submarine. Once the attack was over, the X-Craft would rendezvous with the towing submarine and then be towed home. Range was limited primarily by the endurance and determination of their crews, but was thought to be up to 14 days in the craft or 1,200 miles (1931 km) distance after suitable training. Actual range of the X-Craft itself was surfaced and at submerged. Specification The craft was about long, maximum diameter and displaced surfaced and submerged. Propulsion was by a 4-cylinder Gardner 4LK 42 hp diesel engine, converted from a type used in London buses and a 30 hp electric motor, giving a maximum surface speed of and a submerged speed of about one third of that. The crew initially numbered threecommander, pilot and ERA (Engine Room Artificer, i.e. engineer), but soon a specialist diver was added, for whom an airlock, known as a "wet and dry" compartment, was provided. The ERA, usually a Navy Chief Petty Officer, operated and maintained the machinery in the vessel. The weapons on the "X-Craft" were two side-cargoes – explosive charges held on opposite sides of the hull with two tons of amatol in each. The intention was to drop these on the sea bed underneath the target and then escape. The charges were detonated by a time fuse. The craft were fitted with electromagnets to evade detection by anti-submarine detectors on the sea bed and also with sonar and a periscope. Service A number of development craft were built before it was felt that a feasible weapon had been produced. The first operational craft was X3 (or HM S/M X.3), launched on the night of 15 March 1942. Training with the craft began in September 1942, with X4 arriving in October. In December 1942 and January 1943, six of the "5-10" class began to arrive,