I made two more attacks on a different convoy, this time in BE 36. Both were complete failures.
On first attempt I managed to get into firing position and launch 6 torpedoes from 5km distance. Unfortunately I was detected soon after, and the convoy zig-zaged, which resulted with 3 torpedoes missing, 2 duds, and one hit, probably causing only a minor damage. I was hit with one depth charge but managed to escape.
On second attempt I was detected before I could even launch a torpedoes. Although the depth charges shook the boat, I escaped again.
At least I'm still alive.
Next time I'll equip myself with the steam torpedoes. Hopefuly their long range will help me to avoid detection.