The British naval fleet had to keep their battleships and carriers near or in home waters to protect the vital shipping lanes in the North Atlantic from the powerful German battleships stationed in Norway. The previous missions by the British to destroy the German capital ships in the North Atlantic had failed so a new weapon was needed. The "X class" craft were specifically designed and built to attack the key Nazi battleships KMS Tirpitz and KMS Scharnhorst as well as the pocket battleship KMS Lutzow.
In 1942, an ultra-secret submarine training base was established at Lock Erisort on the Isle of Lewis in Scotland. By 1943, the Royal Navy had developed a 52-foot "midget" submarine named the "X-craft". The X-craft could remain at sea for days while carrying its own supplies for the required 4-man crew and could reach distances of 1,200 miles at underwater speeds of 6 knots. Dive capabilities were 300 feet depths (91.5 meters). The X craft had only one access hatch which would eventually prove a relatively lethal design feature in the event of an emergency. Her periscope was small, restricted by her overall design, and ultimately proved unreliable. Navigation was through a Browns A Gyro Compass and Auto Helmsman. The direction indicator utilized an AFV 6A/602 and shipbuilder Vickers Barrow UK was contracted to construct the boat. As midget submarines went, she was rather roomy though still required the use of short-statured crew members.
The desired method of operation concerning the X-craft was to position the submarine as close to the target using one of two delivery systems developed: the boat could be either towed by a conventional submarine to be suspended underneath the target enemy ship or launched from the deck of a submarine to make its way to the target ship under its own power. The Admiralty chose two 3,570 lb conjoined mines of a high explosive nature to be attached to the sides of the boat via a bolt. These mines were then released by way of a hand crank when the vessel was positioned below the hull of an enemy ship. The explosive was equipped with a delayed fuse timer that allowed the boat to retire out of the blast area within time. The explosive used was Amatex, a standard and stable military explosive comprised of 51% ammonium nitrate, 40% TNT, and 9% RDX.
Depending on the mission, a three- or four-man crew was selected consisting of one commanding officer, a first lieutenant, and engineer and a diver. The diver was in some ways a novelty position and useful on in the case that the craft ran afoul in anti-torpedo "netting" used to protect harbors. If called upon, the diver utilized an attached airlock to exit the vessel, letting sea water in to pressurize the compartment. Upon return to the craft, air pressure was used to force the water out of the small area, allowing the diver to reenter the submarine as needed.
On September 11th, 1943 "Operation Source" began when six X-craft were towed by conventional Royal Navy submarines from Scotland into the North Sea, this voyage taking ten days. The submarines were manned by crossing crews during the trip while the attack crews stayed on the towing submarine until mission time. The KMS Lutzow was to be attacked by X-8, she being towed by HMS Sea Nymph. However, the side-connected cargo mines began to take on water and were forced to be jettisoned due to the extra weight. During the jettison the side cargo mines prematurely exploded, causing enough damage to the X-8 that her crossing crew was removed and the craft scuttled. The KMS Lutzow was free to fight another day.
X-9 and X-10's mission was to attack the battleship KMS Scharnhorst at her mooring. During the crossing, the X-9 tragically sank with the crossing hands crew aboard when her towing cable snapped. X-10 continued on the attack plan alone. Her tow submarine, HMS Sceptre, released X-10 close to the target area. However, the crew found that the KMS Scharnhorst had moved into the North Atlantic. The X-10 rejoined HMS Scepter and was towed back to the Scotland base without ever having attacked the Scharnhorst.
X-5, X-6 and X-7's target became the 58,000-ton battleship KMS Tirpitz moored in Kafjord. All three X-craft were released close to the target on September 20th, 1943 and began the attack independently. X-7 surfaced 4 miles from Tirpitz to charge batteries and open the hatch for fresh air before they attacked. In the meantime, X-6 was released by her tow, HMS Truculent, retreating the passage crew. At 2 am, X-6 came upon a torpedo net and a small German ship passing through. The X-6 commander decided to follow the ship through the open net and was successful. However, while on the other side of the net while still submerged, she hit a rock close to where the Tirpitz berthed. Not being able to move forward or reverse, the X-6 had to surface close to Tirpitz. She was ultimately detected by German look outs and was fired upon by small arms. The decision was made to set the timed fused before abandoning the craft and surrendering to the Germans.
Unknown to the crew of the X-7 was that the crew of X-6 was on Tirpitz so the attack mission proceeded as planned. As the craft descended under the Tirpitz, she also came into contact with the hull which alerting the German crew. Now under the Tirpitz, one charge was released under the hull around the area of A and B turrets. The X-7 then moved 300 feet further astern and released the second mine under the aft turrets, these set with a 1-hour delay. The X-7 turned back towards the open sea and, at 8:12am, the charges exploded and lifted the Tirpitz eight feet out of the water. During the explosion and escape, X-7 was forced to surface after becoming entangled in the torpedo netting. The craft was half submerged and taking on water when the decision was made to abandon her through the single escape hatch. Lt. Place and crewman Aitken got out while crewmates Whittam and Whitley sadly went down with the sinking X-7. X-5 was never able to attack the Tirpitz as far as Admiralty records indicate. She never returned from her sortie and was presumed lost with all hands onboard. It is speculated that X-5 was sunk by Tirpitz as she approached while on the surface.
Despite the relatively successful attacks, the KMS Tirpitz was not permanently knocked out of commission. She did take on an estimated 14,000 tons of sea water which, in turn, affected her electrical machinery and key systems. This kept her out of the war for six critical months while making the North Sea shipping lanes much safer during the time. Tirpitz was unable to leave Kafjord until April of 1944 and she was finally destroyed by "Tallboy" bombs dropped by British Lancaster heavy bombers on November 12th, 1944, bringing an end to her sea-going legacy.
Additional X-craft were built due to the losses during Operation Source - six training craft (Vickers XT-1 to Vickers XT-6) and two operational craft (Broadbent X-20 and Marshall X-25) in 1944. The X-craft continued to serve the British Royal Navy during the war in the North Sea and the Far East. The X-22 was in training for attacking the large Laksevag floating dock in Bergan, Norway when she was rammed by the submarine HMS Syrtis during a training exercise and sank with all hands onboard. The X-24 was then chosen to train for the Bergan mission and was towed to Norway on April 15th, 1944. She attacked as planned but miscalculated the approach and set her charges under a 7,500 ton freighter, the Barenfels, which sank as a result of the explosion. The targeted dock, however, was not destroyed. The X-24 returned to the tug submarine and was towed back to Scotland. The dock became a high priority and X-24 was reassigned to destroy it once more. A new crew was trained and, on September 11th, 1944, the X-24 was again towed to Norway and officially and successfully sank the dock.
The planners of "Operation Overlord" - the ultimate invasion of the European mainland - needed as much information on the French coast and its many beach approaches as could be found. Pictures were taken and authorities even asked for such post cards of the French coast and beaches to be donated. "Operation Postage Able" was conceived to survey the different beaches the Allies could land troops on and, as such, X-20 was chosen to do the job. Though designed with a crew of four in mind, an additional fifth crewmember would be added for this hazardous select mission. The mission was to lay off the French beaches for four days observing the shoreline during the day through the periscope while making echo sounding measurements.
At night, two divers would swim to the shore making the sounding measurements and recorded them on an underwater pad and pencil. Once ashore they would use a compass with a beach gradient reel and stake to measure distances. The divers carried a trowel and auger for soil and sand samples to be kept in condoms. For self-defense, each carried a 0.45 caliber pistol and an ammo bandolier. To combat the cold waters of the North Sea, they carried a flask of brandy. The divers swam ashore on two nights, surveying the beaches at Vierville, Moulins, St Laurent and Colleville - the beaches that would make up "Omaha" beach. On the third night, the plan was to go ashore on the Orne Estuary but fatigue, a lack of food and foul weather forced the acting commander to cancel the operation. X-20 returned to HMS Dolphin on January 21st, 1944 and was towed to Scotland.
For D-Day June 6th, 1944, the X-craft were needed again under "Operation Gambit". On June 4th, X-20 was joined by X-23 stationed off the British and Canadian Sword and Juno beaches. The X craft surfaced and put up an 18 foot high telescopic navigation mast with lights shining only seaward. Additionally a radio beacon and echo sounder were onboard and could send a message to Allied Minesweepers and troop and capital ships approaching "Sword" and "Juno" beaches. The Americans declined X-craft assistance and decided to rely on their navigation instruments. However, on D-Day, the American ships and landing craft headed for "Utah" beach were sent westward by the current and its assault troops waded ashore at the wrong location. Perhaps help from the X-craft midget submarine fleet should have been accepted after all.
By early 1945, the original X-craft were all either lost in combat or to training accidents. Only five X-20 series boats ultimately remained before falling to the scrapman's torch. Twenty-three men died serving with X Craft and fifteen of those were officers. All six trainers built by Vickers were scrapped in 1945. Other manufacturers of X-craft included Varley Marine, Portsmouth Dockyard, Broadbent, Markham and Marshall.