must have been some corruption in your saved games
glad its working for you now
as to long range, there is a big difference in max range and max effective range. effective range is where the odds of the torpedo reaching its target before the ship deviates from its course and speed or the sea conditions change the torpedos tragectory.
in short the closer the better and anything over 3000m is very long range and 4000m-5000m means you just get lucky since ships dont sail in a straight line and will change course from time to time to avoid being easy targets for subs. the farther away you are the longer it takes the torp to reach the target and the likely hood the ship remains on a straight steady course that whole time is very very low.
now that being said, the game does have flaws/cheats and there are a lot of unrealistic things that can happen in the game so im not saying you cant do it, only that you shouldnt be able to do it with any sort of accuracy or regularly