Zigzagging isn't such a big deal. The ships average course doesn't change, they just zag along that course. So your TDC solution is basically still correct. They do slow down a little bit I think (like half a knot), so you might adjust this in the TDC. However, the big thing with zagging is that the torpedo will probably not hit on a 90 degree angle anymore, so it might bounce off. In this case I set my torps to 1m under the keeldepth, going for the magnetic pistol keelbuster shot. That is, unless i'm so close that I know the torp will hit at approx 90.
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