Without a 'lock' on the target, the torpedo will follow the course of the last periscope/TBT bearing. RADAR is very helpful for lining up on a target in the fog, but if you don't have RADAR, set the bearing on the periscope/TBT to '0' (zero), and take your best shot.
Also, torpedoes don't seem to 'arm' at less than 400 yds., so the problem becomes even more difficult.
MAY THE FORCE BE WITH YOU