German Type XXIII submarines were the first so-called elektroboats to become operational. They were small coastal submarines designed to operate in the shallow waters of the North Sea, Black Sea and Mediterranean Sea, where larger Type XXI Elektro boats were at risk in World War II. They were so small they could carry only two torpedoes, which had to be loaded externally. As with their far larger brothers — the Type XXI — they were able to remain submerged almost all of the time and were faster than all previous designs worldwide, due to the improved streamlining of their shape, batteries with larger capacity and the snorkel, which allowed the diesel engines to be used while submerged. The Type XXI and XXIII U-boats revolutionized post-war submarine design.
When development began on the Type XXI U-boat in late 1942, it was proposed to simultaneously develop a smaller version incorporating the same advanced technology to replace the Type II coastal submarine. Admiral Karl Dönitz added two requirements: as the boat would have to operate in the Mediterranean and the Black Sea, it had to be able to be transported by rail, and it had to use the standard 53.3-cm torpedo tubes.
The development of the Type XXIII was given a high priority, with an emphasis on using existing components as much as possible. To reduce development time, Hellmuth Walter designed the new submarine based on the previous Type XXII prototype. By 30 June 1943 the design was ready and construction began in parallel at several shipyards in Germany, France, Italy and German-occupied Russia. The lead contractor was Deutsche Werft in Hamburg.
As with the Type XXI, the Type XXIII was intended to be constructed in sections, various modules being produced by different subcontractors. Some were to be assembled at foreign yards, including U-2446 through U-2460 at the Deutsche Werft yard at Mykolaiv. These were reassigned to the Linzner yard on 1 May 1944 and subsequently cancelled. In the end, circumstances meant that construction was concentrated at Germaniawerft in Kiel and Deutsche Werft in Hamburg, Germaniawerft building 51 and Deutsche Werft 49. Of the 280 submarines ordered, only 61 entered service, and only 6 ever carried out a war patrol.
The Type XXIII had an all-welded single hull design, the first submarine to use a single hull. It had a fully streamlined outer casing and apart from the relatively small conning tower and a fairing which housed the Diesel exhaust silencer, it had an uncluttered upper deck. In line with Walter's design practice, there were no forward hydroplanes, although these were added later.
The submarine was propelled by a single three-bladed propeller and steered by a single rudder. As with the Type XXI, the lower section of the figure-of-eight hull was used to house a large 62-cell battery.
In order to allow the boat to be transported by rail, the hull sections had to be limited in size to fit the standard loading gauge. For transportation, the hull was broken into four sections and the bridge was removed. Due to the space restrictions, the forward bow section had to be made as short as possible, which meant that only two torpedo tubes could be fitted and no reload torpedoes could be carried. The torpedoes were loaded by ballasting the submarine down at the stern so that the bow lifted clear of the water and the torpedoes could be loaded directly into their tubes from a barge.
The Type XXIII proved to have excellent handling characteristics, and was highly maneuverable both on the surface and underwater. Its crash dive time was 9 seconds, and its maximum diving depth was 180 m (590 ft). Speed submerged was 12.5 kn (23.2 km/h; 14.4 mph), while surfaced speed was 9 kn (17 km/h; 10 mph). A submerged speed of 10.5 kn (19.4 km/h; 12.1 mph) could be attained while snorkeling.
The first Type XXIII, U-2321, was launched from Deutsche Werft in Hamburg on April 17, 1944. It was one of six XXIIIs that went on operational patrol around the British Isles in early 1945. Forty-eight others followed from Deutsche Werft and 13 from Germaniawerft of Kiel. U-4712 was the last one launched, on April 19, 1945.
None of the six operational Type XXIIIs — U-2321, U-2322, U-2324, U-2326, U-2329 and U-2336 — were sunk by the Allies ships but they sank or damaged five ships for a total of 14,601 tons.
The first war patrol of a Type XXIII began late in the war when U-2324 sailed from Kiel on 18 January 1945. Although she was to survive the war, she sank no enemy vessels. The first Type XXIII to achieve combat success was U-2322, commanded by Oberleutnant zur See Fridtjof Heckel. Sailing from a Norwegian base on 6 February 1945, she encountered a convoy off Berwick, Northumberland, and sank the coaster Egholm on 25 February. U-2321, operating from the same base, sank the coaster Gasray on 5 April 1945 off St Abbs Head. U-2336, under the command of Kapitänleutnant Emil Klusmeier, later sank the last Allied ships lost in the European war on 7 May 1945, when he torpedoed and sank the freighters Sneland and SS Avondale Park off the Isle of May inside the Firth of Forth.
The Sneland and the Avondale Park were sunk around 23:03, less than an hour before the official German surrender, and the Avondale Park was the last merchant ship to be sunk by a U-boat. At the time it was felt that Kapitänleutnant Klusmeier, who was on his first patrol, had deliberately ignored Donitz’s ceasefire order, however Klusmeier claimed that he had never received the order.