Schpeedy is right. It's 1939 so the DDs aren't as godly as they start to be around 41. A long range attack would be your best bet. Also if you can identify the escorts see if they are faster than you or not. Many escorts used in the earlier part of the war were slower than a surfaced U-boat. If you can likely outrun them then get back at least 5000 at night and fire a good spread at the convoy then turn and use your aft torps too and then gun it for home. If the DDs don't even get close to chasing you you could try it again. In 39 they have no radar and you should be able to evade easily on the surface and since you don't need batts for that you aren't entirely screwed if you get into trouble.
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