That would be the TDC. The RuB package tries to make it as realistic as the game will let it. So something in the pack probably disables that stuff.
Try unchecking a few things and see what happens. Like God's eye mode or manual targeting.
Better yet play without it and after you fire a torpedo at that merchant with a destroyer bearing down on you so you have to go deep. Just sitting there and listen to the stopwatch tick, tick, tick away or wait to hear the explosion(s) as your torpedo hits and bulkheads start breaking as she slips under. All this while your maneuvering to get away from depthcharges and trying to sneak away undetected. It feels like an eternity. You'll forget about that little green line on the TDC real quick.
