All you need is..
Acoustic Homing torpedos or use the Fat in a ladder search - how hardest thing to ascertain is range. You can get that by watching the length of passive contact - get a feel for the transition oa usable solution. Next you need to watch the bearing drift - or which way the target is heading by how the bearings change. you need to drive the boat such that you can control that rate or drift - you will come up with a speed and a direction when sonar indicates the merchie is on constant range. your speed is close and your heading is close zig zag is no problem if you look at the big scheme of where he is going and if it makes sense - make the appropriate offsets and size for the FAT and shoot the weapon.
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