@Stiebler:
No, actually it was invented and on trials already in april 1940, according to this link anyway:
http://www.uboataces.com/sonar-coating.shtml , I remember having read, I think it was at uboat net, not sure, that is was available already in the early 40's. It was then forgotten about for some years due to bad adhesive capabilities, but then improved and came back again late 44/early 45, so I just thought that if it was available already in 1940-42 why the hell not on my boat then
