1940 should more or less be a cake walk, even against a black swan frigate.
something isnt right, either you are moving faster than 2 knots or you are waiting it out right in front of the convoy and the frigate is detecting you as he passes over. In my most recent run through a career in SH3 i didnt really start having problems with escorts in til early 1943, for the most part, earlier years they are pretty inexperienced.
i suggest approaching the convoy from ahead and from the port or starboard side about 1500 meters abeam the lead escort ship.
If you spend a lot of time watching the lead escort you will notice he is sailing a zig zag course ahead of the convoy searching for u-boats. but if you pay close attention - you will notice that he will periodically come to a dead stop. when he does this, his hydrophone receivers are functioning as close to 100% as possible.
if you are close when he stops, or turning more than a knot or two... he WILL find you. the further from him you are the better off you will be. but i would say 1500 to 2000 meters should be plenty of space. once he has passed by and you are in his baffles or nearly so... you can proceed with your attack against the convoy.
I recommend attacking the 2-3 largest ships you can and then diving deep when the fish leave the tubes. big tankers always are sent a spread of two from my boat. but if i am low on torpedoes then i will not target the big guys, i will go for the small merchants that are more likely to sink when they are hit with one torpedo.
good luck out there
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