U-class submarine HMS Ultimatum departing Holy Loch, August 1943
Class overview
Name	U class
Operators	Royal Navy  Polish Navy  Free French Naval Forces  Soviet Navy  Royal Netherlands Navy  Royal Danish Navy  Hellenic Navy  Royal Norwegian Navy
Preceded by	T class
Succeeded by	V class
Completed	49
General characteristics
Type	Submarine
Displacement	540 long tons (550 t) standard, 630 long tons (640 t) full load surfaced 730 tons submerged
Length	191 ft (58 m)
Beam	16 ft 1 in (4.90 m)
Draught	15 ft 2 in (4.62 m)
Propulsion	2 shaft diesel-electric 2 Paxman Ricardo diesel generators + electric motors 615 hp (459 kW), 825 hp (615 kW)
Speed	11.25 kn (20.84 km/h; 12.95 mph) surfaced 10 kn (12 mph; 19 km/h) submerged
Range	4,500 nmi (8,300 km; 5,200 mi) at 11 kn (20 km/h; 13 mph) surfaced
Complement	27 to 31
Armament	6 × 21-inch (533 mm) torpedo tubes - 4 bow internal, 2 bow external (first group only) 8–10 torpedoes 1 × 3 in (76 mm) gun

The British U-class submarines (officially "War Emergency 1940 and 1941 programmes, short hull")^[1] 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.

At the start of the Second World War the U class was, with the British S and T-class submarines, the Dutch O 21 class and German Type VII one of the most advanced submarine classes in service.^[2]

The Royal Navy was limited to no more than 52,700 long tons (53,500 t) of submarines by the London Naval Treaty of 1930. The tonnage limit led to proposals for smaller submarines which was also prompted by trials with larger submarines demonstrating that they were easier to find and lacked manoeuvrability. By coincidence the First World War-vintage H-class submarines used for training in anti-submarine warfare were reaching the end of their useful service. The Rear-Admiral Submarines, Noel Laurence, wanted a class of small, inexpensive boats for training, armed with torpedoes for short-range patrols.^[3]

In March 1934 he approved a specification for a "Small, Simple, Submarine, for Anti-Submarine Training etc".^[3] The three Unity-class boats, HMS Undine, Unity and Ursula were ordered on 5 November 1936 from Vickers-Armstrongs, to be built at their Naval Construction Yard in Barrow-in-Furness. According to the recommendation of the Hopwood Committee of 1926 the boats had names beginning with the same letter in the alphabet. ^[4] The new boats were the smallest built since the First World War.^[5][a]

The U-class boats had a hull of riveted steel, half-an-inch thick for dives to 200 ft (61 m), with the fuel tanks and ballast tanks on the inside. The superstructure and conning tower were built with free-flooding holes and storage for cables, anchors and sundry items. The hull was divided by five bulkheads with access from the conning tower; hatches in the torpedo-stowage compartment and in the engine room had drop-down canvas trunks for emergencies.^[7][b] The boats had an 8 in (200 mm) bifocal periscope with high/low magnification for searching and a 6 in (150 mm) low magnification periscope for attack.^[7]

The periscopes could rise 12 ft (3.7 m) but such a shallow periscope depth could allow the boat to be seen from the air. Hydrophones were fitted, one on each side near the bows facing outwards and one on the conning tower facing aft. Asdic Type 129 was installed forward of the keel from 1937 and two wireless aerials were carried, a jumping aerial on the conning tower for very low frequency signals at periscope depth and a WT mast which could be raised above the water spread the second aerial for conventional wireless signalling.^[8]

The boats had six ordinary ballast tanks and a quick-diving, "Q tank", the ballast tanks, hydroplanes and the rudder being hydraulically operated; the forward hydroplanes were mounted high on the hull and folded upwards for docking. The submarines had two 400 hp (300 kW) Paxman diesel-electric engines generating 615 bhp (459 kW) and electric motors of 825 shp (615 kW) giving a surface speed of 11.25 kn (20.84 km/h; 12.95 mph) and a submerged speed of 10 kn (19 km/h; 12 mph). The diesels were linked to the propellers by two generators which kept charged the battery of 112 cells under the control room and crew accommodation.^[9] Submarine propellers had been designed to perform best on the surface until the Unity class which was the first submarine design with propellers giving their best performance submerged to reduce propeller noise but "singing propellers" were a constant problem for the class.^[10]

The boats had a fuel capacity of 38 long tons (39 t) giving a range of 3,800 nmi (7,000 km; 4,400 mi) at 10 kn (19 km/h; 12 mph) on the surface and 120 nmi (220 km; 140 mi) at 2 kn (3.7 km/h; 2.3 mph) submerged; battery recharging required the submarine to surface; in 1944 dummy snorkels were fitted to some boats for anti-submarine warfare training^[11] During construction the four internal bow torpedo tubes were supplemented by two external tubes in a bulged housing, four reloads being carried for the internal tubes. Ursula carried a 3 in (76 mm) gun but had no hatch for the gun crew, who had to use the conning tower; to compensate for the weight of the gun only eight torpedoes were carried. Just before the war, a second group of twelve vessels were ordered, HMS Unique, HMS Upholder, HMS Upright and HMS Utmost with the external tubes, the others without, because the bulge at the bow generated a large bow wave. Depth keeping was more difficult at periscope depth, a rather shallow 12 ft (3.7 m) which was more of a disadvantage than the six-torpedo salvo justified. The sudden loss of weight in the bows when the torpedoes were loosed in salvo made the boat porpoise and break the surface.^[12]

The three Unity-class boats entered service in the latter half of 1938. Designed as training vessels, they were effective enough to persuade the Admiralty to build more and to improve their offensive capacity. Ursula was launched on 16 February 1938, was loaned to the Soviet Navy from 1944 to 1949 as V 4 and sold in May 1950 and broken up. Unity was launched on 16 February 1938 and sunk on 29 April 1940 in a collision with SS Atle Jarl off the Tyne. Undine was launched on 5 October 1937 and sunk by German minesweepers on 7 January 1940 off Heligoland.^[13]

Group Two boats
Class overview
Name	Group two
Completed	12
Lost	9
General characteristics
Type	Submarine
Displacement	630 tons surfaced, 732 tons submerged 1940–1941 boats 658 tons surfaced, 740 tons submerged
Length	191 ft (58 m)[c]
Beam	16 ft (4.9 m)
Draught	10 ft (3.0 m)
Installed power	615 bhp (459 kW)
Speed	11.25 kn (20.84 km/h; 12.95 mph) surfaced 9 kn (17 km/h; 10 mph) submerged
Range	3,800 mi (6,100 km) at 10 kn (19 km/h; 12 mph), 1940 and 1941 boats 5,000 mi (8,000 km) at 10 kn (19 km/h; 12 mph)
Endurance	120 mi (190 km) submerged
Test depth	200 ft (61 m)
Complement	36 officers and enlisted
Armament	bow 4 × 21-inch (533 mm) torpedo tubes (8 × torpedoes) (later, 6 mines) 1 × QF 12-pounder[d] 2, later 3 × .303-calibre machine-guns
Notes	maximum complement 33[e]

The experience gained with the U-class boats was incorporated into the Group two boats of the War Supplementary Emergency Programme, consisting of twelve submarines, of a similar design to the original three. The external torpedo tubes were omitted and the boats had a redesigned stern to reduce cavitation and on some of the boats a new bow shape was introduced to reduce the bow wave; the hydroplanes were enlarged for better submerged handling. The First World War-vintage 12-pounder was retained but replaced on Unbeaten and Unique by a 3-inch gun. The boats ordered in 1940 and 1941 carried the 3-inch gun and more fuel. Most of the boats were built by Vickers at Barrow-in-Furness. MI5 investigated the loss of Vandal and Untamed during training operations but the report was kept confidential.^[17]

In June 1940 the Admiralty had stopped naming submarines and known them by their pennant numbers but on 4 November 1942 the Prime Minister, Winston Churchill, questioned the policy. The First Lord of the Admiralty replied that naming had been dropped to avoid confusion with the big increase in the number of destroyers, which usually had names with the same initial letter. Numbering submarines had been the practise in the First World War but that because of Churchill's views, the Admiralty had decided that it was better to be right than consistent and that naming was to be resumed. After a delay Churchill was told that it was difficult to find sufficient names beginning with U and that the surplus were being named with words beginning with "V" and a list was sent to Churchill on 27 December 1942. Submarines lost before they could receive names kept their pennant number.^[18]

The group included submarines that became well-known; Urchin was transferred to the Polish Navy as ORP Sokół and sank 55,000 long tons (56,000 t) of Axis shipping. In the 16-month operational career of Upholder (Lieutenant-Commander Malcolm Wanklyn) in the Mediterranean, Upholder carried out 24 patrols and sank around 119,000 long tons (121,000 t) of Axis ships, consisting of three U-boats, a destroyer, 15 merchant ships with possibly a cruiser and another destroyer also sunk before being lost in April 1942. Wanklyn was awarded the Victoria Cross for attacking a well-defended convoy and sinking the Italian liner Conte Rosso on 25 May 1941. Losses in this group were high, only three out of the twelve survived the war.^[19]

Group three boats
Class overview
Name	Group three
Completed	34
General characteristics
Type	Submarine
Displacement	662 long tons (673 t) surfaced, 740 long tons (750 t)
Length	205 ft (62 m)
Beam	16 ft (4.9 m)
Draught	10 ft (3.0 m)
Installed power	800 bhp (600 kW) engines, 760 bhp (570 kW) motors
Speed	12.75 kn (23.61 km/h; 14.67 mph) surfaced 9 kn (17 km/h; 10 mph) submerged
Range	5,000 mi (8,000 km) at 10 kn (19 km/h; 12 mph)
Endurance	120 mi (190 km) submerged
Test depth	300 ft (91 m)
Complement	37 (maximum)
Armament	bow 4 × 21-inch (533 mm) torpedo tubes (8 × torpedoes) (later, 6 mines) 1 × QF 12-pounder 3 × machine-guns)
Notes	Data from McCartney, p. 17, British Submarines 1939–45, 2006, Osprey Publishing, Oxford, ISBN 978-1-84603-007-9

The third group 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 and the vessels were called P31 to P39, P41 to P49. At the end of 1942 Winston Churchill ordered that all submarines were to receive names but eight of the U-class boats were lost before they could receive them, whilst on operations with the Royal Navy.^[25]

The V-class boats were the final refinement of the U-class submarines, 34 were ordered and 21 were built by Vickers for the War Emergency Programmes of 1941 and 1942, the rest being cancelled. The hull was further lengthened to try to eliminate the singing propellers and the bows were more streamlined. Welding of the hull frames was introduced to use thicker steel for the pressure hull, giving a diving depth of 300 ft (91 m). None of the V-class boats were lost and some did not see service. The boats were named HMS Upshot, HMS Urtica, Vagabond, Variance, HMS Venturer, HMS Vigorous, Viking, HMS Vampire, Varne II, Veldt, HMS Vineyard, Virtue, Visigoth, HMS Vivid, Voracious, Votary, Vox II, HMS Virulent, Volatile, Vortex and Vulpine.^[32]

• British V-class submarine
• List of submarines of France

• Brown, D. K. (2012) [2000]. Nelson to Vanguard (hbk. repr. Seaforth Publishing (Pen & Sword) ed.). Barnsley: Chatham Publishing. ISBN 978-1-84832-149-6.
• Chesneau, Roger (1992) [1980]. Gardiner, Robert (ed.). Conway's All the World's Fighting Ships 1922–46 (3rd ed.). London: Conway Maritime Press. ISBN 978-0-85177-146-5 – via Archive Foundation.
• Colledge, J. J.; Wardlow, B. (2006) [1969]. Ships of the Royal Navy: The Complete Record of all Fighting Ships of the Royal Navy (2nd rev. ed.). London: Chatham. ISBN 978-1-86176-281-8.
• McCartney, I. (2006). British Submarines 1939–45. New Vanguard (No. 159). Oxford: Osprey. ISBN 978-1-84603-007-9.
• Pol, E. van den (1989). "Aspects of Submarines - Part I: Some Notes on Development" [Ship and Yard]. Schip en Werf (in Dutch). Vol. 56, no. 10. Rotterdam: Wyt & Zonen. pp. 352–358. ISSN 0036-6099.
• "U class". uboat.net.
• "Untiring to Urge". British submarines of World War II. Archived from the original on 11 July 2007.
• Walters, Derek (2004). The History of the British 'U' Class Submarine. Barnsley: Pen & Sword. ISBN 978-1-84415-131-8.

• Friedman, N. (2019). British Submarines in two World Wars (e-Pub ed.). Barnsley: Seaforth (Pen & Sword). ISBN 978-15267-3817-2.
• Hutchinson, Robert (2001). Submarines, War Beneath The Waves, From 1776 To The Present Day. London: HarperCollins. ISBN 978-0-00-710558-8.

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• RN Submarines 1936–1958: U Class