there are times when your torpedo hits a bulkhead or support beam and doesnt do the same damage to the ship as you expect it to so angles and hit locations can make a big difference in your results but also here is something to think about that not everyone thinks of.
ships in the game will do some damage repair of sorts, so if you fire a torpedo and wait for the result then fire a second torpedo, you wont get the same damage from two torpedos spread over time as you would if you fired both torpedos close together.
a ship that sinks with 2 torps fired together may need 3 or 4 torps to sink if you wait to see the result of each torpedo before firing another one.
i learned this by trying not to waste torpedos and i would wait around to make sure the ship wouldnt sink before i fired another torpedo so i only used what i had to but i then noticed doing that caused most ships to need more torps to sink than when i wasnt trying to save torpedos.
over time you will get the feel for how many torpedos each ship needs and the key locations to target for best results.
