U class submarine (1937)

The U-class submarines, also known as Undine-class, were a group of diesel-electric submarines built for the Royal Navy during the 1930s and saw extensive service throughout World War II. They were relatively small and designed for operations in the North Sea and Mediterranean, where their compact size allowed them to manoeuvre more easily in shallower waters. The U-class submarines were known for their reliability and versatility and were particularly effective in anti-submarine warfare and reconnaissance missions.

The U-class submarines were initially built as small, experimental submarines intended for training but proved to be quite effective operationally. This led to their eventual mass production and their use in active combat. The class went through several iterations, with each group improving upon the design of the previous one.
Specifications (varied slightly by group)
Displacement was Approximately 545 tons surfaced, 740 tons submerged, length About 58 meters (190 feet), powered by a Diesel-electric system, with two diesel engines driving electric motors for a to speed of 11.25 knots surfaced, 9 knots submerged and range of 4,500 nautical miles at 10 knots surfaced. Operational depth was around 60–90 meters, with some later boats capable of deeper dives
As far as armament went they had four 21-inch torpedo tubes in the bow, capable of carrying up to 8 torpedoes. Later versions were fitted with a 3-inch deck gun for additional surface attack capability and certain variants could also carry and deploy mines.

The U-class submarines saw extensive combat service during WWII, particularly in the Mediterranean and North Atlantic, where they conducted a variety of missions, including classic Anti-Submarine Warfare missions against enemy submarines, particularly in the North Sea. They were used for coastal Patrols, performing reconnaissance, patrolling close to enemy coastlines. They were used also for Convoy Protection by patrolling shipping lanes and were used for Special Operations, supporting covert missions and transporting Allied commandos behind enemy lines or VIPs in occupied territories.

HMS Ultimatum, of Group III, Batch 1940

These submarines were particularly effective in the Mediterranean, where their smaller size and shallow draft allowed them to operate close to enemy-held shores and in narrow straits, such as the Strait of Gibraltar. Their contributions were significant, as they helped contain the threat of Axis supply lines and contributed to the overall success of Allied naval operations.
A total of 49 U-class submarines were completed, with 20 being lost in action. Despite their limitations, U-class submarines were seen as a success due to their adaptability and combat effectiveness. Some submarines were transferred to other navies, including the Royal Netherlands Navy, Polish Navy, and Free French Naval Forces.

The U-class set the foundation for later British submarine designs and contributed valuable lessons in diesel-electric submarine warfare. They were succeeded by V-class, a final refinement of the U-class with 34 ordered, 21 built by Vickers under the 1941-42 War Emergency Programmes and the remainder cancelled. The hull was further lengthened to avoid vibrations and “singing propellers” issues whereas the bows were more streamlined. Hull Welding was introduced on thicker steel for the pressure hull, so they could dive down to 300 ft (91 m). None V-class were lost in action and they served for may years in the coldwar or were sold.

Development

The Royal Navy limited itself to 52,700 long tons (53,500 t) of submarines under the London Naval Treaty of 1930, a tonnage limit that favored smaller submarines. This was comounded by numerous reports of earlier large submarines trials showing they were slower to dive, handled poorly and were easy to see and hear. The First World War H-class submarines still around for training in anti-submarine warfare arrived at the end of their career, and this justified replacement in the eyes of the parliament. The Rear-Admiral Submarines, Noel Laurence wanted to simply replaced these with the same type of simple and inexpensive training boats but with enough torpedoes to found usefulness by making short-range patrols in wartime.

In March 1934 he approved the future U class specifications. He called for a “Small, Simple, Submarine, for Anti-Submarine Training (…).” The first three Unity-class boats were eventually ordered once finance was obtained, as HMS Undine, Unity and Ursula on 5 November 1936, from Vickers-Armstrongs, Barrow-in-Furness. The 1926 Hopwood Committee confirmed that names were preferrable to numbers due to crew morale and command and control, but that these names would begin with the same letter in the alphabet. As the plans and detailed blueprints were approved, the new U class would appear as the smallest built since the last war.

Predecessors: The H class

H4 at Brindisi in 1916
The British H-class submarines were Holland 602 types, constructed between 1915 and 1919 and becoming Britain’s largest class of submarine before WW2. They were designed and built in response to minelaying U-Boats and also to perform similar operations in German waters or to hunt German submarines in British waters at large.
They were considered as cramped and many lacked a deck gun but these remained popular amongst submariners. In addition many were sent to the Adriatic but most arrived late and saw little service and further batches saw no service at all, but on the interwar. They still sank U-51 and UB-52, loosing four of their own.
Post-war, many were retained for training purposes, four more lost in accidents in the 1920s. They were obsolete in 1939 and so they served in training and for coastal patrol, mitigating the heavy RN submarine losses in the early years. Two were sunk by the Germans. Canada also built some, which were equipped with Fessenden transducers.

The U class were completely different beasts from the S-class, desptite the fact the latter were also light submersibles. They found their origin in the Shark class, with more made in the 1941 programme, 1942 and 1943 programmes. They had a much longer range and much heavier tonnage of 810-990t for the latter series versus 500/700 tonnes for the U class.

Unity class boats

This class, laid down in 1937 and completed in 1938, were intended as replacements of the ‘H’ class for training and local patrols. They were distinguished by electric drive when surfaced, the main motors being driven from the dieselpowered generators, and were of single-hull type with an operational depth limit of 200ft. The external TT were in the bows and resulted in a bluff form which was not very satisfactory in a seaway.
The UNDINE class (also called “U” class prewar programme was designed at first for a 500t displacemnt, which looked unrealistic, and was later ported to 540t standard. She displaced 630t/730t on “normal” and fully loaded displacement for hull just 191 feet long and to be powered by Paxman-Ricardo diesels coiupled with Vickers electric motors. The former actually developed less than the latter and both surfaced and submerged speeds were about the same between 11-10kts. On trials Undine made 12.45kts surfaced and 8.58kts submerged.

Range was an issue as they only carried 38t of oil, in two 19 tonnes fuel tanks. The initial armament was to be only in the bow and initially of six tubes plus 10 torpedoes including those in the tubes already, leaving 4 spares. It was impossible to fit all six in the pressure hull so two remainder were placed in the outer hull and not reloadable. They were also given a 3 inches/20cwt deck gun from the start. Ursula had only 8 torpedoes to compensate for the weight of the 3in gun, which only she carried, and she could subsequently carry 6 M2 mines as an alternative. The crew amounted to just 27 compared to 48 for an S-class boat (almost double) and to 61 on a T-class (2.5 times larger).

This was due a clever way to manage ergonomics so that some crew members could take on several roles. Still, in their tiny pressure hull they felt extremely cramped. Yet, they appeared in general to be very reliable and agile despite lacking speed and range.
In all three boats were made, UNDINE (Vickers, Armstrong-Barrow, launched 5.10.1937), UNITY (16.2.1938) and URSULA (16.2.1938) the only one to survive the war, discarded in 1950. She was loaned to USSR as B4 from 1944 to 1949 but was allegedly handed over in such a poor state that she was no use to the RN and quickly decommissioned. See later for the historical records of all three.

Design of the class

Hull and general design

The U class were the smallest British submarines built since the D class back in 1908. They displaced 540 long tons (550 t) standard as well as 630 long tons (640 t) full load but surfaced and 730 tons submerged so a 100t buoyancy difference. The weight of a submarine under water needed to always equal its buoyancy, with extra weight compensated for by removing an equal amount of ballast near the keel and it was linked to the density of the water around, from fresh to highly saline.
They were also moderately compact at 191 ft (58 m) long overall for a beam of 16 ft 1 in (4.90 m) and a draught of 15 ft 2 in (4.62 m). To compare the 1908 D-class measured only 50.2 meters long but were beamier at 6.2m, with a 3.5 m draught. Theor conning tower was a simple bathtub style

The U-class boats had a hull of riveted steel, 0.5 inches (13 mm) thick to be able to dive down to 200 feet (61 m) while having the fuel tanks and ballast tanks inside rather than outside the outer hull. Still they were of a partial double hull type. The superstructure and conning tower were built with free-flooding holes as well as storage spaces for cables, anchors and sundry items which was found handy in service. No spare space was left inoccupied. The pressure hull was divided into five bulkheads. The main acess was from the conning tower with an hatch in the torpedo-stowage compartment. However the engine room had drop-down canvas trunks usabled in emergencies. Lessons of the losses of the H class were well retained.

After closing all water-tight doors, the trunks were lowered, connected to the deck. The hatch was opened slightly until air–water pressure was equal. The crew breathing trapped air until escaping. Each crewman had a Davis Submerged Escape Apparatus of nose-clip and mouthpiece connected to the short duration oxygen bag, only there for the pressure balance and rise up. The first man in trunking opened the hatch and left the boat, then the others in turn. Escape from the conning tower was saw two entering the tower and closing the lower hatch, then opening the upper one, flood this before leaving. So at least now there were ways to two escape.
Among interesting equipments were their 200 mm bifocal periscope fitted with low and high magnification and a second 6 inches (150 mm) low magnification periscope for attack only, more discreete. The periscopes could rise 12 ft (3.7 m) but they had a very shallow periscope depth and were easily spotted from the air in the Mediterranean.

Powerplant

These small boats were powered by two 400 hp (300 kW) Paxman diesel-electric engines. Each had an outout of 615 bhp (459 kW). Tis was completed by two Vickers electric motors rated for 825 shp (615 kW). The diesels totalled 1230 bhp, enough for a surface speed of 11.25 knots (20.84 km/h; 12.95 mph) and the two electric motors a submerged speed of 10 knots (19 km/h; 12 mph). The diesels drive directly the propellers via generators also charging the battery composed of 112 cells under the control room/crew accommodation. There was a small hatch for checking these. The propellers were the object of great care, made not to perform best on the surface but rather submerged in order to reduce propeller noise. However destpite of this, these “singing propellers” remained a problem for the class. They had a fuel capacity of 38 long tons, enough for a range of 3,800 nmi (7,000 km; 4,400 mi) at 10 knots (19 km/h; 12 mph) surfaced, 120 nmi (220 km; 140 mi) submerged at 2 knots (3.7 km/h; 2.3 mph). She needed to surface to recharge her battery. In 1944 however some of the wartime U-class tested dummy snorkels solely for ASW training. For quick diving the U class had six ordinary ballast tanks and a special quick-diving “Q tank”. These ballast tanks as well as the hydroplanes and rudder were all hydraulically operated. The forward hydroplanes were placed high on the bow, folding upwards for docking.

Armament

As seen above the small pressure hull was too constrained to fit more than four tubes. Still, specs called for six, so the two extra ones were relocated to the outer hull, and so considered “external”. This was done also for the T class. These four internal bow torpedo tubes could be reloaded underway. Each had a single spare torpedo, the other being already present in tube when loaded at port. The two external tubes were only reloadable in port. They were located in a specific bulged housing forward, creating also a “forecastle” of sorts. Between the internal and external tubes this amounted to ten. The prospect of having two stern tubes was dropped early on due to the lack of space and trimming complications.

Before the war, the first second group boats, HMS Unique, Upholder, Upright and Utmost only had eight torpedoes for weight reasons, and kept their external tubes, but later groups dispensed of these, as bulge generated a large bow wave. Dive stability at periscope depth was problematic and trimming even more sensible. These boats had a shallow 12 ft (3.7 m) persicopic depth, which conducted captains to also to discard the two external tubes. This loss was felt fully justified in operations. Plus when firing these two external tube torpedoes, sudden loss completely break stability and trim, making them to porpoise, break the surface under enemy guns. It was already feared in 1938 and so the Group two boats of the War Supplementary Emergency Programme had the bulg but not the external torpedo tubes already. Group III boats were the first with a straight bow.

21 inches Mark VIII**

The Mark VIII were the first burner-cycle torpedo in service and the most common became the VIII**, with 3,732 fired by September 1944, 56.4% total of those made. It was exported and maintained in service as late ass 1983. A truly fast and dependable model. When correctly used, itw as a sure kill in stark contrast to the US Mark 14. The model suggested below was standard when completed. They were designed betrween 1925 and 1927, and were mass produced for all British subs from the O class. They had a negative buoyancy of 804 lbs. (365 kg).

3 in (76 mm) QF 20 cwt

HMS Ursula and most of the U class built afterwards carried a 3 in (76 mm) Mark V deck gun. It was not protected by the conning tower and unshielded. Ther crew was completely exposed. To compensate for its weight the ten torpedoes carried on paper was reduced to eight. This deck gun was originally a dual purpose or AA model, but the deck gun mount had a more limited elevation than surface ship’s 90°. This gun was shared by the S-class, U-class and V-class submarines. Approximate Barrel Life was 1,250 rounds.

Sensors

The U class had Hydrophones fitted on each side near the bow and facing outwards. There was an extra one on the conning tower, facing aft to cover all angles. Next, an Asdic Type 129 was installed forward of the keel (from 1937). For communication the U class had two wireless aerials plus a jumping aerial on the conning tower usable to detect low frequency signals at periscope depth. The also had a WT mast raised above the water spread for conventional wireless signals.

Group I profile general appearance, HMS Unity.

Rendition of HMS Ursula, Group I

Group II (Umpire class)

(To come)

Group III (Ullswater class)

11 March 1940 batch

23 August 1940 batch

12 July 1941 batch

(More to come)

Succession: V class

(More to come)