[magic452]

To zero the TDC you can set speed to zero and send to TDC.
Set AoB to zero and send and last turn the periscope to zero and pull the range triangle down as far as it will go and send range/bearing to TDC.
This will clear out any data in the TDC and any torpedo shot will go straight out the bow at 0 bearing.

One other thing to do is check the Gyro offset dial and make sure it's also at zero.

It's not necessary to do this if you just loaded from a save as default is zeros.

It's best to start from zero for any zero gyro angle shot, O'Kane, Cromwell or vector analysis method. I like vector analysis myself but O'Kane is also very good. For a VA shot it's mandatory to zero the TDC.

Magic