X-class submarine

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 1,000 nmi (1,900 km), after suitable training. Actual range of the X-Craft itself was 600 nmi (1,100 km) surfaced and 80 nmi (150 km) at 2 knots (3.7 km/h) submerged.

The craft was about 51 ft (16 m) long, 5.5 ft (1.7 m) maximum diameter and displaced 27 long tons (27 t) surfaced and 30 long tons (30 t) 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 6.5 knots (12.0 km/h; 7.5 mph) and a submerged speed of about one third of that. The crew initially numbered three – commander, 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 electromagnetic field generators to mask their inherent magnetic field to avoid detection by anti-submarine detectors on the sea bed and also with sonar and a periscope.

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, identical externally but with a completely reworked interior.

These operations were part of a longer series of frogman operations; see human torpedo.

The operational base and training establishment was HMS Varbel at the former Kyles Hydro Hotel at Port Bannatyne on the Isle of Bute in the Firth of Clyde, Scotland.