thank you very very much for the inputs!
ok, yesterday i played again and had to deal with depth charges after attacking a convoy.
since the BDU sent me now in the vinicity of the rokall bank i had this time plenty of water under me and took the boat down to 160meters. after initial avoidment manouvers for some DC i slowed down to 50-60 rpm and this time i indeed was able to shake of the escort without using silent running. so it truly seems to depend on randomized knowledge of the crew, surely water depth , wethaer etc. (i,m using sh3 cmdr) since i like it hard core- realistic, auto reloads are off and i,m running without the simply "silent running button" . its a real pleasure of gaming when you carefully have to look at the weather, visibility, lightness at night and much more for a convoy attack, i need a whole evening to make one attack, but thats no problem!
i finally sent one cargo to the bottom and damaged two another. after avoiding the escorts i turned on the grammophone. later i realized that my window of the room was partly open and my neighbours listened a little sceptic since there came songs out of my speakers wchich are very delicate for the people where i live.... (i live in germany)
cheers!