British X-craft were a critical player in operations leading up to D-Day. By , the Royal Navy had developed a foot "midget" submarine named the. X-Craft (Midget Submarine). Technical Data (X-5 to X series). Length, 15,76 meter (51,7 feet). Beam, 1,8 meter (5,9 feet). Draught, 1,62 meter (5,3 feet).Weight of side cargo: 4 tons.