I tried this attack technique last night, from 3500 yards, thus being able to use the high speed torpedo setting and also not being very close to the escorts. It works like a marvel

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10 torpedoes, 5 kills (each ship was attacked by 2 torpedoes, one set to minimal depth, one to keel depth, the former set to impact and the latter to magnetic).
One large freighter was stubborn enough not to sink, but his engines were a wreck, so late in the night I was able to surface and finish him with the deck gun.