British U-class submarine
The British U-class submarines (officially ""War Emergency 1940 and 1941 programmes, short hull"") were a class of 49 small submarines built just before and during the Second World War. The class is sometimes known as the "Undine" class, after the first submarine built. A further development was the British V-class submarine of 1942. Design and development. These small submarines, of around 630 tons, were originally intended as unarmed training vessels to replace the ageing H class, to be used as practice targets in anti-submarine training exercises. The first three boats, , and were ordered  in 1936, and during construction were modified to accommodate four internal and two external bow torpedo tubes. Apart from "Undine" and "Unity", all boats were also equipped with a three-inch (76 mm) gun, although they lacked a hatch for the gun crew, who had instead to use the main conning tower hatch. As war loomed, twelve more vessels were ordered, although only four had the external tubes (HMS "Unique", HMS "Upholder", HMS "Upright" and HMS "Utmost"). The external tubes were dropped from later vessels because they generated a large bow wave and made depth keeping more difficult at periscope  depth. They proved to be useful warships in the confined waters of the North Sea and particularly in the Mediterranean. A further 34 vessels, forming the third group, were ordered in 1940 and 1941. They were similar to the second group, but were lengthened by to provide a more streamlined shape. All but two of the 49 boats built were constructed by Vickers-Armstrong; the exceptions were HMS "Umpire" and HMS "Una", both built at Chatham Dockyard. The submarines were powered by Paxman diesels generating and electric motors that could put out 825 shp (615 kW) giving a surface speed  of and a submerged speed of . Service history. Most of the boats built served with the 10th Submarine Flotilla based at Malta. 19 were lost during the war; 13 in the Mediterranean and the remainder in the North Sea and Atlantic Ocean. In addition, one vessel, HMS "Untamed" sank in May 1943 but was salvaged and reconstructed to be recommissioned as HMS "Vitality". From 1941, some of the boats were transferred to Soviet, Free French and other allied navies. The design was developed into the welded V class later in the war. Group one boats. The initial three U-class  boats all entered service in the latter half of 1938. Initially designed as training vessels, they were effective enough to persuade the Admiralty to expand the class, and also to improve their offensive capabilities to pose a more effective threat to enemy shipping. Of these initial three, only "Ursula" would survive the war. In addition, "Ursula" spent some time in service with an allied navy, in this case, the Soviet Navy, a practice that would be continued with other ships of the class. "Unity" was the first British submarine to have a propeller designed for best submerged performance rather than  surfaced performance. It was thought this might reduce propeller noise. Group two boats. The second group of the U class consisted of twelve submarines, of a similar design to the original three, but most were modified to improve depth keeping. This group included a number of submarines that would go on to become particularly famous. "Urchin" was transferred to the Polish Navy, where, as the ORP "Sokół", she sank 55,000 tons of enemy shipping. One of the most famous U-class submarines was , commanded for its entire career by Lieutenant-Commander Malcolm Wanklyn. Wanklyn received a Victoria Cross for attacking  a well-defended convoy on 25 May 1941 and sinking the Italian liner . In her 16-month operational career in the Mediterranean, before she was sunk in April 1942, "Upholder" carried out 24 patrols and sank around 119,000 tons of Axis ships – 3 U-boats, a destroyer, 15 transport ships with possibly a cruiser and another destroyer also sunk. Losses in this group were high, however. Only three out of the twelve survived the war. Group three boats. These formed the largest group of U-class submarines, comprising 34 vessels ordered in three batches. Losses continued to be high. In  June 1940 the decision was taken, in view of the anticipated high number of submarines to be ordered, to drop the practice of naming submarines, so these vessels were initially labelled "P31" to "P39", "P41" to "P49", etc., instead of receiving names. At the end of 1942 Churchill personally ordered that all submarines were to receive names, but eight ships were lost whilst in service with the Royal Navy, before they could have their names officially assigned (six according to uboat.net). In addition, a number were assigned to Allied navies, both before and after the war. One assigned to the  Royal Norwegian Navy, HNoMS "Uredd", was also lost after hitting a mine. The last surviving U-class boats were scrapped in the 1950s. Ten submarines were ordered on 11 March 1940 under the 1940 Programme. Twelve submarines were ordered on 23 August 1940, still under the 1940 Programme. A final twelve boats were ordered on 12 July 1941, under the 1941 Programme. Note that four of the last batch received 'V' names rather than 'U' names; however, the last two were renamed upon transfer to the Free Norwegian and Free French Navies respectively, and the original names were reassigned to  new 'V'-class submarines. Other navies. As well as serving with the Royal Navy, a number of U-class boats were transferred to operate with allied forces. The Red Fleet of the Soviet Union received three boats: The Royal Norwegian Navy two: The Polish Navy two: The Free French Navy one: The Royal Netherlands Navy one: In addition, both and were loaned to the Hellenic Navy after the end of the war, becoming "Amphitriti" and "Xifias" respectively. The following boats were transferred to Royal Danish Navy after the war: