The craft used against the Tirpitz were developed from a prototype, known as X 3, built by Commander Cromwell Varley, a former member of the Royal Navy who had set up a marine engineering business near Southampton. After her launch in 1942, the X 3 underwent a series of successful trials, and Vickers Armstrong Ltd was contracted to supply six production models by early 1943.

In many respects the X craft (shown above) were scaled-down versions of conventional submarines, but with two important differences. First, they were constructed with a watertight space, the ‘wet and dry’. Situated between the forward compartment and the control room, the wet and dry allowed each crew’s diver to make underwater sorties against obstacles. The second difference lay in armament.

Unlike larger submarines the X craft did not carry torpedoes, but were fitted with two detachable charges on either side of the hull. Each charge was filled with two tons of high explosive and could be detached from inside the craft. A timing device allowed the crew up to 36 hours leeway before detonation.

The X-craft were not designed for comfort or speed. With a length of 51ft, they were extremely cramped and the crews had to work in a pressure hull that was a mere five and a half feet high. The midget submarines displaced 27tons on the surface and had a diving depth of 300ft. Speed on the surface was some six knots; underwater, this was reduced to two knots. Propulsion was provided by a Gardner diesel engine. With full fuel tanks and charged batteries, the operational range was some 1500 miles, but on board conditions meant that no crew could work efficiently for any great length of time.