This book is a history of escort vessels, not of U-boats, but some mention must be made of the 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.1 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.2 A post-war British study suggested 500 feet as a usable depth for the Type XXI.3
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.
For all its faults, the production system delivered 120 boats in two years but, mainly because of teething troubles, only one set out on an operational cruise, aborted when the war ended. Surface speed was about fifteen knots but, in other than a flat calm, the bridge flooded at speed. They were designed for rapid diving of twenty seconds and to achieve this a very large number of freeing ports were fitted. It was found that, submerged, these added 28 per cent to the drag of the boat. A practical submerged speed for an hour was sixteen and a half knots. Using the schnorkel, ten knots was obtained on trial but eddy shedding from the mast and periscope caused such severe vibration that the usable speed was about six knots. The quiet speed on auxiliary drive was also six knots.
They would have been formidable opponents but would not have had it all their own way. The essence of the convoy system was that it forced the attacker to come to the convoy, where the escort force was concentrated. Once the submarine was contacted, the Squid and its asdic would still be lethal, and the more capable Limbo anti-submarine mortar was only just round the corner. The Lochs would have retained some capability, as would the Captains if rearmed with Squid.
On the air side, early sonobuoys and MAD (magnetic-anomaly detection) were already in use before the war’s end. Together with the Mk 24 ‘mine’ they would have been a formidable counter to the Type XXI. Perhaps even more portentously for the future of anti-submarine warfare, helicopters were already flying. The development of all these technologies would have been rapid.