High Quality Content by WIKIPEDIA articles! The X class was a World War II midget submarine class built for the Royal Navy during 1943?44. 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, the passage crew returning with the dinghy to the towing submarine.