The point of Battle of the Atlantic for the German side is to destroy the convoy. However, doing that provides VERY little score. You can sink one destroyer or 10 cargo ships and get the same score.
Killing cargo ships requires using stealth to avoid the planes and player-controlled destroyers, yet yields few rewards. Please either increase the score for completing the objective or change the objective.
Submarine warfare isn’t about going 1 on 1 with destroyers at close range. It’s stupid and suicidal.