Development By Nazi Germany
The technology of anechoic tiles was developed by the Kriegsmarine in the Second World War. Codenamed Alberich after the invisible dwarf from Germanic Mythology. The coating was made up of sheets approximately 1 m (3 ft 3 in) square and 4 mm (0.16 in) thick, with rows of holes in two sizes, 4 mm (0.16 in) and 2 mm (0.079 in) in diameter. It was manufactured by IG Farben from a material known as Oppanol. The material was not homogeneous but contained air cavities; it was these cavities that degraded the reflection of ASDIC. The coating worked in the 10 to 18 kHz range, reducing ASDIC return by about 15%. 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 coating had its first sea trials in 1940, on U-11, a Type IIB. U-67, a Type IX, was the first operational U-boat with this coating. After her first war patrol, she put in at Wilhelmshaven probably sometime in April 1941 where she was given the coating, it covered the conning tower and sides of the U-boat, but not to the deck. By 15 May 1941, U-67 was in Kiel performing tests in the Baltic Sea. During July, the coating was removed from all parts of the boat except the conning tower and bow. Further experiments and sound trials were made in the Little Belt but they presumably proved unsatisfactory, as all the coating was subsequently removed. Problems were encountered early-on, when the adhesive was found to be not strong enough to stick the synthetic rubber sheets to the pressure hull and casing of the U-boat. This resulted in the sheets loosening and creating turbulence in the water, making it easier for the submarine to be detected. Furthermore, the coating was found to have considerably decreased the speed of the boat.
It was not until late 1944 that the problems with the adhesive were mostly resolved. The coating required a special adhesive and careful application; it took several thousand hours of glueing and riveting on the U-boat. The first U-boat to test the new adhesive was U-480 a Type VIIC. With good results with the new adhesive, the Oberkommando der Marine intended that it would be widely used on the new Type XXI and Type XXII U-boats. However, the war ended before it could be put into large scale use. Ultimately only one operational Type XXIII, U-4709, was coated with the anechoic tiles. U-boats with the anechoic tiles coating include: U-11, U-480, U-485 U-486, U-1105, U-1106, U-1107, U-1304, U-1306, U-1308, U-4704, U-4708 and U-4709.
The Japanese I-400 super-submarine also used anechoic tile technology supplied by Germany.
Read more about this topic: Anechoic Tile
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