That depends on a year, you are in. From 39 to late 41 you are able to attack conwoys on the surface, from their flank, at night ofcorse. Thats what I do, and I am pretty succesfull at this. In 42, when the escorts get radar, the best strategy is the one, described btaft.
Priority targets are definately T3 and T2 tankers, the next ones are C3 or Liberty and Victory ships (all have about the same tonnage). Also, if you see a passanger liner in the conwoy, thats the priority, at least for me (it has 24.000 tons). Otherwise, big tankers are the main ones.
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