Let me guess - it's perfectly calm sea? In these conditions, depth doesn't help you. They can find you with active sonar. You need to get distance. I'd stay at PD, or damn close (~40m). This way you can see your attacker (or at least his keel), and you can make the critical evasion move at the right time. Then flank to escape. You need a good flank run and then drop silent, with a short silhouette (show him your baffles) and you might just get away. Use decoys during the flank run if you have them.
Part 2 - is it >1943? If so, you might not want to get close to the convoy. This is what FaTs are for. You launch them in from a safe distance, and the pattern-running makes up for your crappy Target Motion Analysis and accumulated error over distance. In 44 I wouldn't go into a convoy in still water - you'll be busted for sure. In that case I stay at the periphery and fire in all 6 in pattern running, or acoustic homing, etc. Just fire and forget, and get away alive. DiD.
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