Hi all not familiar with this method of covering a Uboat's hull with a rubber material but do know that the German army used a substance known as zimmeritt which was applied to the hulls of tanks and other A.F.V.s by hand. it was a sort of rubbery ceramic paste with a trowled on appearance,the idea being a thick layer of this would prevent magnetic anti tank charges from sticking to the steel hull also deflecting small arms munitions. It is Quite possible that another benefit of this coating would have perhaps made the vehicle harder to detect by early airborne radar systems or mobile ground radar systems, Perhaps this may shed some light on the origins of covering a subs hull with such a substance. Please note that I am not a scientist and may be incorrect , but looking at Modern stealth technologies on aircraft and nuclear subs this must have originated from previous theories and naval experiments of the post war years perhaps gained from captured enemy data. Seems possible . regards to all.