Hi again
thanks to your good advices and links, i managed to successfuly sink 2 merchants on the same lane and successfuly escape the DDs. I even think i understand the maths behind this technique ^__^ So each time i meet a convoy, i sink 2 of their ships, which is fair enough.
Now, its another story to sink the whole convoy

I think i have to better plot my escape course so i end up somehow behind the fleeing boats. Since they are on a zig-zag pattern, they will present me their flank at a time or another. Working on this technique now.
But i have 2 other concerns :
1. Planes. Its a real pain. every 5 or 10 seconds real time, my radar detects a plane and i have to dive to 30m. Is there any tips about this ? like travel with a slow time compression ? underwater only on daytime, then surface and flank speed at night ? i read that TMO1.9 reduces the planes spawn rate ... i guess there is a problem there.
2. i had a stange encounter last night. I still have the savegame if you guys want. i was escaping from singapore and had a report of a ship 5000m away. I dive to periscope depth and plot interception course. Its a japanese tanker, nothing special here. Problem is this ship is really slow, les than 2Kn, for a lonely tanker on daytime with a calm sea. Well, i hit time compression and wait for the ship to cross my 340. But, it stops at my 320. it stops completly, 0Kn, and just do nothing.... Well, i set up my TDC and fire torpedoes at him. And the torpedoes just cross it without hitting it. wtf ? i reload, set the torps to minimum depth, contact, launch and ... nothing, they just run past it and dont explode.... i think its just a bug, the ship actually dont exist, so i surface the boat, but it fires real bullets at me and i take real damage ^^ so what's up with that ? anyone had the same problem ?