X - class submarine 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 . 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 , 500 miles ( 2 , 400 km ) distance after suitable training . Actual range of the X - Craft itself was surfaced and at submerged . Specification . The craft was about 51 feet ( 15 . 5 m ) long , 5 . 5 feet ( 1 . 68 m ) in maximum diameter and displaced 27 tons surfaced and 30 tons submerged . Propulsion was by a 4 - cylinder Gardner 42 hp diesel engine , converted from a type used in London bus es , and a 30 hp electric motor , giving a maximum surface speed of 6 . 5 knots ( 12 km / h ) , and a submerged speed of 5 . 5 knots ( 10 . 1 km / h ) . The crew initially numbered three — commander , pilot and ERA ( Engine Room Artificer , i . e . engineer ) but soon a specialist diver was added , for which an airlock , known as a " wet and dry " compartment , was provided . The ERA , usually a Navy Chief Petty Officer , operated most of , and maintained all of , 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 then escape . The charges were detonate d by a time fuse . The craft were fitted with electro - magnets to evade detection by anti - submarine detectors on the sea bed . Service . A number of development craft were built before it was felt that a realistic weapon had been produced . The first operational craft was HMS " X3 " ( or HM S / M X . 3 ) , launched on the night of March 15 , 1942 . Training with the craft began in September 1942 , with HMS " 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 . Their first deployment was Operation Source in September , 1943 , an attempt to neutralise the heavy German warships based in Northern Norway . Six X - Craft were used , but only 2 successfully laid charges ( under the German battleship " Tirpitz " ) ; the rest were lost , scuttled or returned to base . The " Tirpitz " was badly damaged and out of action until April 1944 . This was the only multiple X - craft attack . The lost craft were replaced early in 1944 with X20 to X25 and six training - only craft . On April 15 , 1944 HMS " X24 " attacked the Laksevåg floating dock at Bergen . X22 was intended for the mission , but had been accidentally rammed during training and sunk with all hands . The X24 made the approach and escaped successfully , but the charges were placed under the " Bärenfels " , a 7 , 500 ton merchant - vessel along the dock , which was sunk ; the dock suffered only minor damage . On September 11 , 1944 , the operation was repeated by X24 , with a new crew ; this time the dock was sunk . X - Craft were involved in the preparatory work for " Overlord " . Operation Postage Able was planned to take surveys of the landing beaches with HMS " X20 " spending four days off the French coast . Periscope reconnaissance of the shoreline and echo - soundings were performed during daytime . Each night , " X20 " would approach the beach and 2 divers would swim ashore . Soil samples were collected in condoms . The divers went ashore on two nights to survey the beaches at Vierville - sur - Mer , Moulins St Laurent and Colleville - sur - Mer in what became the American Omaha Beach . On the third night , they were due to go ashore off the Orne Estuary ( Sword Beach ) , but by this stage fatigue ( the crew and divers had been living on little more than benzedrine tablets ) and the worsening weather caused Hudspeth to shorten the operation , returning to " Dolphin " on 21 January 1944 . Hudspeth received a bar to his DSC . " X20 " and " X23 " acted as lightships to help the D - Day invasion fleet land on the correct beaches ( Operation Gambit ) , as part of the Combined Operations Pilotage Parties ( COPP ) . " X24 " is the only remaining intact example of an X - Craft . It can be found in the Royal Navy Submarine Museum . Operations continued in the Far East with the revised XE class submarine s . Builders . The numbering sequence of the X class began with " X3 " because the designations " X1 " and " X2 " had already been used previously - " X1 " had been a one - off submarine cruiser design from the 1920s while " X2 " had been assigned to a captured Italian submarine . In media . This type of midget submarine was portrayed in the 1955 war film , " Above Us the Waves " , featuring John Mills , which was based on both Operation Source , and the earlier " Chariot " attacks on the " Tirpitz " . This class of submarine was later featured in the 1968 movie " Submarine X - 1 " starring James Caan as a Canadian Royal Naval Volunteer Reserve officer who after losing his submarine and fifty crew members in a battle with a German ship during World War II , gets a second chance training crews to take part in a raid using midget subs .