I dont know if it is gonna help anyone but i noticed that tonnage bar fills up if im not loading saved game.
For example im turning game in off line mode - im starting in bunker - playing
If im loading game when im during patrol then bar isnt filling.
If destroyers or anything else are gonna destroy my u-boot - im simply turning off the game and again turning it on, loading save
My point is - do not load saved game after death. Just turn off and turn on game and play