Well by doing it that way you are sort of blind firing your fish, they are designed to steer to a preset gyro angle to "intercept" the target you've sent them after.
If i had to guess why your fish are missing 100% of the time in the given scenario its because the TDC has them launching after a ghost of a target.
I will also mention that by leading the target without the use of the TDC your torpedoes may be all hitting the target at too great an angle and glancing off.
It really is best to set in the TDC settings and then fire.
additionally, double check the draft of the target ship by reference to the ID manual in game... if this is early war, it is possible that all of your fish are running a good meter or two deeper than what you've set due to design flaws.
its best to set them to about one or two meters depth.
try formulating an actual solution to the target and locking that data into the TDC... also set your running depth shallower.
press H to go to the hydrphone station and set bearing to 000 to listen to the torpedo, you can hear if it is tracking straight or turning or whatever... if it is hitting and glancing off you will hear the "clang" sound instead of the boom
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