You are better off going to manual. That three-dot system for automatic firing is pure bullsh!t. You can be in manual and still use what is basically an automatic system (the way it should be).

When in manual and locking on a target, you will get a little set of commands for the XO. One of them is "Turn Off TDC." Click on this. You will have to do this only once for each time you load a mission. Click on "Never Mind" to make the commands go away. When I first load a mission, I usually go straight to a periscope to set this up and get it out of the way so I don't have to worry about it later.

Lock up a target (Spacebar). You will see the little red, yellow, or green indicator under the target. Leave that there until shortly before you fire.

Adjust your torpedo options: running depth, trigger, speed, etc to whatever you want. Leaving them at default is okay until you feel like experimenting at a later time.

Assuming you have preceeded all of these steps by properly positioning your boat to an optimal ambush position, wait until your target's little triangular indicator turns green and then hit the Spacebar again to unlock the target just before hitting the Fire button . . . don't ask why, but this makes it work. If you leave it locked while firing you are more likely to miss.

Hope this helps.