The next generation – U2502, a Type XXI with a smaller Type XXIII alongside. They would have been a formidable threat but unreliability delayed their entry into service.
German efforts to bring fast submarines into the Battle of the Atlantic. None of these newer boats was operational in the battle but it is worth considering what might have happened. The development is a fine example of the adage, ‘Requirements pull, technology pushes.’
The initial push came from Professor Hellmuth Walter, who, from about 1933, put forward a number of proposals for fast submarines using very concentrated hydrogen peroxide as the oxidant for the burning of fuel oil while submerged. This combination could be used either in a diesel engine, recycling the exhaust gases, or in a turbine. Walter wanted peroxide with a concentration of 80 per cent, when the strongest solution available was 45 per cent. This problem was overcome and in 1939–40 the experimental boat V80 was built. It had a submerged speed of twenty-eight knots on trials in 1940, demonstrating the potential of the scheme.
Four larger prototypes were built in 1942, two by Germania and two by Blohm & Voss. They were of 260–300 tons submerged and had most features of an operational submarine, achieving speeds of up to twenty-four knots. It had not been appreciated how much of the total drag of a submerged submarine was due to appendages such as open bridge, misaligned hydroplanes, even freeing ports. A small series of Type XVII boats was started, with a submerged speed of twenty-five knots, but none became operational. (U-1407 became HMS Meteorite after the war.)
There were many problems still to be overcome, not least the in-fighting within the bureaucracy of the Third Reich and the more rational arguments of those who feared the delay to the building of conventional U-boats. However, by late 1942 a design had been completed for an operational ‘Atlantic’ U-boat, the Type XVIII. It was of 1,652 tons, with a design speed of twenty-four knots submerged and an endurance of 200 miles at that speed. Two were ordered in January 1943 but they were stopped in March 1944. There were other designs to follow but little had been done when the war ended. High Test Peroxide (HTP) is a very nasty substance, causing fire or explosion when in contact with many apparently innocuous materials. RN experience after the war, first with the ex-German Meteorite and later with the two British-built Explorers, suggested that this was not the way to go.
Concerned with the delays in the Walter boats, the design office proposed to use the slippery shape of the Type XVIII for a fast battery boat, which was to become the Type XXI. There was to be a double pressure hull in a figure-of-eight configuration. The lower hull was to hold an enormous battery, three times the size of the older conventional U-boats. There were motors delivering 4,200hp to give a hoped-for seventeen knots. There were smaller motors for quiet running. Submerged displacement was 1,819 tons.
An elaborate production scheme was devised with nine hull sections built and fitted out in many factories and brought together for rapid assembly in a shipyard. Some thirty-two firms built structural sections; these were passed to one of sixteen specialist companies, which would install main and auxiliary machinery, piping, etc. In most cases, the sections were transported by canal. Finally, one of three building yards would put the sections together and carry out acceptance tests. Difficulties soon became apparent; labour of any sort was scarce and skilled labour very much so. In consequence, it was found that sections did not align correctly and installation was difficult, among many additional problems. After the war, a joint team from the rival British builders of the A class, Vickers and Chatham Dockyard, thought that both their prefabrication schemes were superior to that in Germany. The Type XXI was the first U-boat with a complete hydraulic main (telemotor system) but it was poorly designed and bad workmanship led to many leaks. Since the system was mainly outside the boat, seawater contamination was common. There was also the Type XXIII, a small, fast conventional U-boat, but it played no part in the battle.
The design of the Type XXI’s pressure hull was intended to provide a collapse depth of 330 metres, which allowed for an operational depth of 135 metres and a test depth of 200 metres. The design methods of the day could not really cope with the novel section shape and it was recognised that there were problems. Several attempts at a test dive had to be abandoned but on 8 May 1945, U-2529 finally reached 220 metres. A post-war British study suggested 500 feet as a usable depth for the Type XXI.
This pictorial illustrates the shape of the detection area for the 144 ASDIC, the ‘Q; attachment and the 147 Asdic. An advanced Type 147 ASDIC set was developed later in the war that tracked U-boats in three dimensions, giving readouts of bearing as well as range and depth.
In 1939, most destroyers had Type 128 asdic sets, sloops had Type 127 and trawlers Type 123, though there were probably a few older sets in use. Brief particulars of the more important sets follow. (For more detail see Hackmann.) Note that there were frequent updates to sets while in service; for example, the ultimate 128 was virtually identical to 144. These updates took place when the ship was in hand for other work. Information on ship-fitting is scarce and often unreliable. While this note was written with care, its complete accuracy cannot be guaranteed.
Type 121. Prototype tested in Woolston in 1931. Fitted in D, E, F and G class destroyers, some cruisers and the sloop Enchantress. First production retracting dome.
Type 123. Trawlers and other auxiliaries. Introduced 1934, replacing earlier Type 122. Detachable dome.
Type 124. Updated 121. In 1934–7 fitted to C, H, J, K and Tribal class destroyers, coastal sloops and a few older destroyers. First with standard range recorder.
Type 127. Designed for sloops but very widely fitted from 1937 in older destroyers, frigates and in allied ships. Dome as 122 and electronics as 123. Some had Q (qv).
Type 128. From 1937 in destroyers; prototype in Acheron, then A (retro-fit), L and Hunt classes. Dome and directing gear as 121, electronics as 127. There were at least nineteen wartime variants, with improved recorders, helmsman display, etc. Type 128 XE became 144. Some had Q attachment.
Type 141. US set QCJ/QCL found in forty-seven flush-deck destroyers, fitted with RN range and bearing recorder. It had no dome and a few were given British domes as Type 141A.
Type 144. Started May 1941. Introduced in 1942 into destroyers and major escorts after trials in Kingfisher. It was the first set specifically intended for ahead-throwing weapons such as Hedgehog. It was a complete redesign, although many of its features were worked into later updates of 127/128.
Type 145 was similar to 144 but had portable, rather than retractable, dome for slower escorts.
Type 147. Sea trials in Ambuscade in May 1943. Very much part of a weapon system – Squid. Depth measurement.
Q attachment. 1943. Wedge-shaped beam only 3º wide in horizontal plane. Could measure depths 300–700 feet. Fitted to Types 127 and 145 without the need for docking the ship; vessels with Types 128 and 144 needed docking. Fitting took two to six days. Production from April 1943. In an attack, the ship would switch to Q at about 1,500 yards.