Why try to open the screen ? If you position yourself in front of the convoy you can easily let it pass you by and you can attack anything from the middle of the convoy then.
To be sure that the front escort(s) don't detect you, maybe you should dive deep and get up when surrounded by the convoy ships.
If you attack from the middle you can also create many confusion and get a better chance to sneak away. Especially in this early stage of the war.
Even in the beginning I had no problems evading the 4 escorts of a convoy hunting me so they are perhaps just not very experienced in using there asdic against you so you can avoid being detected by going deep@1kn.
After my first run, when reloading torpedoes I would definitely take care that after that tubes 1&3 and tubes 2&4 carry the same type of torpedo so that can shoot 2-torp-salvos without mixing type I and II's.
Also I would make a note to take care of an according loadout when I'm in home port next time
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