one more thing - i almost never try to torp a destroyer. It often misses, and they aren't worth any real tonnage, so it chalks up to a waste of a torp. In a typeVII, every torp counts. I only torp a destroyer if it's an emergency- if he detects me at <1km and he's going to rush me before i can turn/evade/get depth. Or, if it's a TF and there's like 3 DDs that are going to end up searching for me, I might try to kill/damage 1 or two to make my chances of survival a bit higher. In that case, kill the closest one, and then proceed to evade in the way I described.
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