When the game first opens mark all the targets where they initially are, then after a few minutes and mark them again. Depending on where you are try and get a 3rd marking so you'll be able to get a more accurate picture of where each ship is heading to in case they go out of visible range. You can stay submerged for the first few attacks if you like, after you reload then surface using your markings to head in the right direction to intercept them and either submerge or attack them on surface. Even if one gets 4000 or 5000m away moving at flank on the surface you'll catch up with them pretty quickly.
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