[Aktungbby]

https://www.subsim.com/radioroom/sho...d.php?t=168671 Too bad they didn't have superglue back then; the 4mm panels did not hold up to the rigours of the frigid Atlantic...but I always
liked the black coated appearance in SHV

Although the principle was a sound one, problems were encountered with the adhesive coat which was not strong enough to adhere the rubber sheet to the hull. This resulted in the sheets being partially washed off, which flapped in the wake of the ocean current, causing hydrodynamic resistance and noise. Further research into more reliable adhesives were conducted, but up to the end of the war, only a few U-boats had received this treatment. A further contribution was that treating the hull was a time consuming and laborious task.

The first U-boat to receive Alberich was U-11, a Type II coastal boat for trials on its sound absorbing properties. On April 1940, the first operational U-boat was treated, U-67 a Type IXC which was just being laid down. Thereafter, problems with the adhesive prevented further treatments until late 1944, when U-480, a Type VII was tested again using a new adhesive. The results were satisfactory enough and it was decided that all new Type XXIIIs and XXVIs would receive this coating, but ultimately only one Type XXIII, U-4709 had been completed with the coating.

In any case, U-480 the first vessel to be coated for combat, had some succes but it wasn't mine proof:

German submarine 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 skin of anechoic tiles (codenamed Alberich, after the German mythological character who had the ability to become invisible), that made it difficult to detect with Allies' ASDIC (sonar).
The U-boat was laid down in the Deutsche Werke in Kiel as yard number 311 on 8 December 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.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 50°22′4″N 1°44′10″W, 20 kilometres (12 mi) 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' sometime between 29 January and 20 February. A mine had damaged the stern of U-480, sending it to the bottom 55 metres (180 ft) down. The entire crew of 48 was lost.