you could just be a poor player. I dont mean to offend, but it is a possibility.
Could you discribe your tactics in detail?
For example IMHO if the escorts saw you at 4000m you should have compleatly aborted your attack for at least 6 hours and you should never approach a convoy from behind under water, thats asking for trubble. Allways get ahead of the convoy and wait for them to come to you. That way you dont make any noise untill things start blowing up. To chase the convoy under water your E-motors must have been running very loudly
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