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. 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 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. 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. Hydrophones were fitted, one on each side near the bows facing outwards and one on the conning tower facing aft. 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. 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. 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. The superstructure and conning tower were built with free-flooding holes and storage for cables, anchors and sundry items. 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 new boats were the smallest built since the First World War. According to the recommendation of the Hopwood Committee of 1926 the boats had names beginning with the same letter in the alphabet. 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. In March 1934 he approved a specification for a "Small, Simple, Submarine, for Anti-Submarine Training etc". The Rear-Admiral Submarines, Noel Laurence, wanted a class of small, inexpensive boats for training, armed with torpedoes for short-range patrols. 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 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. 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. A further development was the British V-class submarine of 1942. The class is sometimes known as the Undine class, after the first submarine built. The British U-class submarines (officially " War Emergency 19 programmes, short hull") were a class of 49 small submarines built just before and during the Second World War. 6 × 21 inch (533 mm) torpedo tubes - 4 bow internal, 2 bow external (first group only) 8–10 torpedoes.2 Paxman Ricardo diesel generators + electric motors.540 long tons (550 t) standard, 630 long tons (640 t) full load surfaced. U-class submarine HMS Ultimatum departing Holy Loch, August 1943
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