300 m minimum arming range for torpedoes.
Acoustic torpedoes from 400 m onwards home in acoustically, until then they run straight
Against destroyers? Well, in the later years of the war, outfit your sub in port with as many acoustic torpedoes you can lay your hands on. I always have the rear tube and at least one forward tube of my boat outfitted with them, in case a destroyer comes too close and I can't shake him off.
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