Sorry, i stand corrected. this was my source (i guess i shouldnt beleive everything i read!):
"Greatly interested for reduction of the sonar echo of their U-boats, the Germans experimented with sound absorbing synthetic rubber-like laminated coating of about 4mm thickness, intended for efficiency reduction of Allied active sonar devices, which also possessed significant sound absorbing properties.
The material was called Oppanol, an transparent, versatile polymer (in essence form of polyisobutene, even today highly renowned chemical with applications assortment varying from chewing gum, adhesive plasters and double-glazing sealants through to cable insulation, roofing sealants and protective coatings applied to pipelines), impervious to water and gases, resistant to chemicals, physiologically harmless and with elevated adhesive properties. Renowned German company BASF patented a synthetization process for polyisobutene in 1931, and the product was later called Oppanol after the Ludwigshafen suburb of Oppau, where it was developed and produced. However, another seven years of research and development work were needed before it could be produced on an industrial scale. The first Oppanol plant went into operation in 1938, but in those times its military potentials still were completely overlooked.
This laminated material was secured to the outer hull with different adhesives. Although no conclusive tests were performed, Germans claimed that the sonar echo reflection of a U-boat equipped with Alberich coating was reduced by some 15 percent, although absorption varied with depth, temperatue and salinity. In addition, this maetrial also acted as a sound dampener, containing the U-boat’s own engine noise. Basically, the thin two-ply synthetic coating slightly oscillated while the submarine moved through the water, thus providing partial absorption and phase-shifting of reflected sonar impulses.
Although the principle was a completely scientifically sound one, numerous problems were encountered with the adhesion capacity of the special glue (ethylene-vinyl), which was not strong enough to keep the rubber panels in place. This resulted in the sheets being partially washed off, or flapped in the wake of the ocean current, causing hydrodynamic resistance and additional noise. Over time wave action and salt made the rubber panels come lose and actually created more noise than a boat that did not have the coating. It was also discovered that coating is decreasing the speed of the vessel by 1.5 knots. A further deficiency was the fact that adequate treating of the submarine hull was a highly time consuming and very laborious task.
Further research into more reliable adhesives was conducted, but up to the end of the war only a few U-boats had received this treatment. These coatings were intensively investigated in the US after the WW2, but they were not employed due to constantly present adhesion complexity.
Nevertheless, it is highly intriguing that in the early fifties the Soviet Union began providing its own Malyutka-class (Малютка = little one) of coastal submarines with these anti-sonar and anti-echoic coatings. They successfully evolved into real multi-purpose coatings, capable not only to reduce echo-footprint of the submarine, but also to absorb noise of the internal machinery. Important distinction, however, is that these solutions were feasible in application on double-hall submarines only, where coatings could be placed upon multiple sufaces."
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