1:300 "X" class prototype. The tube was an induction pipe for the diesel engine, but could not be used as a snorkel.© Andrew Arthur

1:300 "X" class. The frame amidships was for tying the officer onto the casing!© Andrew Arthur

1:300 "XT" class, the hull of the "X" class but with less extensive equipment. © Andrew Arthur

1:300 "XE" class. A major improvement was a full length casing, and they also had an air-conditioner.© Andrew Arthur

The famous "X" craft were designed to attack heavily protected enemy shipping in inshore anchorages or harbours, particualarly German capital shipping lurking in Norwegian Fjords and Italian shiping in mainland ports.
The 'midget' submarines were towed by an "S" or "T" class submarine to their target area, with a passage crew on board. They only surfaced at night for a short while to change their air supply. When in the taget area, the crew waschanged, and a diver added to it, and the submarine made it's own way to the target vessel. The diver's job was to cut any torpedo nets or booms, clear anything snared in the propellor or help place the charges - with a timed detonator.
They were divided into four compartments; a control room, the diver's air/water lock ( the "W&D" or wet & dry ), the battery compartment and the machinery compartment. Due to a freeboard when surface running of only 1 foot, a 6 foot indution pipe was fitted to the diesel's intake, and this was also used to tie any crew-members on the casing to. ( on trials, "X4"'s officer was washed overboard and the crew inside remained oblivious and the boat almost sank! ).
They had a single screw, rudder & hydroplane, but were none-the-less very agile.
The "X" class were the fighting ships, and the "XT" class the training vessels. The "XE" class were enlarged and air-conditioned units for Far-East service. They had a full-length casing to provide much needed storage space.
Other than attacking shipping, most famously crippling the Tirpitz, they carried recconaisance parties to survey the D-Day beaches and acted as beacons for the first flights of gliders. For the Far-East they performed the vital task of cutting telegraph cables between the multitude of islands.