German submarine U-480

U-480 was an experimental "Kriegsmarine" Type VIIC U-boat of World War II. Considered by many to be the first stealth submarine, it was equipped with a special rubber coating (codenamed "Alberich", after the German mythological character who had the ability to become invisible), that made it difficult to detect with British ASDIC (sonar).
The U-boat was laid down in the Deutsche Werke in Kiel as yard number 311 on 8 August 1942, launched on 14 August 1943 and commissioned on 6 October 1943 under "Oberleutnant zur See" Hans-Joachim Förster. "U-480" carried out three war patrols, all under Förster's command. Because of its coating, the boat was sent to the heavily defended English Channel. The "Alberich" worked; "U-480" was never detected by sonar.
Design.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. "U-480" had a displacement of when at the surface and while submerged. She had a total length of , a pressure hull length of , a beam of , a height of , and a draught of . The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of for use while surfaced, two Siemens-Schuckert GU 343/38–8 double-acting electric motors producing a total of for use while submerged. She had two shafts and two propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . "U-480" was fitted with five torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one SK C/35 naval gun, (220 rounds), one Flak M42 and two twin C/30 anti-aircraft guns. The boat had a complement of between forty-four and sixty.
Service history.
On its first patrol, the boat was attacked by a Canadian PBY Catalina flying boat of 162 Squadron RCAF, piloted by Laurance Sherman. The aircraft was shot down.
On the second patrol, Förster departed from Brest in occupied France on 3 August 1944, and sank two warships and two merchantmen:
For his success, Förster was awarded the Knight's Cross on 18 October 1944.
Fate.
"U-480" left Trondheim, Norway, on 6 January 1945 for its third and last patrol. It did not return. In 1997, the wreck of a Type VIIC U-boat was discovered by accident by divers at , southwest of the Isle of Wight. The following year, it was correctly identified as the Alberich-coated "U-480" by nautical archaeologist Innes McCartney. Subsequent research by the Naval Historical Branch established that it had fallen victim to the secret minefield 'Brazier D2' some time between 29 January and 20 February. A mine had damaged the stern of "U-480", sending it to the bottom down. The entire crew of 48 was lost. Helmsman Horst Rösner only survived because he had been left behind in Norway for training.
The coating.
The Germans developed a 4-millimetre (0.16 in) thick sheet of synthetic rubber anechoic tile. The coating reduced echoes by 15% in the 10 to 18 kHz range. This frequency range matched the operating range of the early ASDIC active sonar used by the Allies. The ASDIC types 123, 123A, 144 and 145 all operated in the 14 to 22 kHz range. However, this degradation in echo reflection was not uniform at all diving depths due to the voids being compressed by the water pressure. An additional benefit of the coating was it acted as a sound dampener, containing the U-boat’s own engine noises.
The rubber contained a series of holes, which helped break up sound waves. There were problems with this technology: the material performed differently at different depths, due to the holes being compressed by water pressure, and securing the tiles to the submarine's hull required a special adhesive and careful application. The first tests were conducted in 1940, but it was not used operationally until 1944, with "U-480". According to the "Naked Science" television episode "Stealth Submarine", "U-480" had a perforated inner rubber layer covered by a smooth outer one. This formed air pockets with the right separation and size to muffle sonar waves.