Just curious to find out. As I have seen, many people play at full realism except manual targetting, and to me atleast that's understandable.
I don't use manual targetting because it takes a lot of time when done each and every time and it asks me to churn my few working brain cells with very simple yet highly repetitive and boring geometry and newton's laws of motion numericals. It's the stuff taught in schools, nothing extraordinary.
And doing all that is not bad at all. What I find tedious is the requirement to constantly alt+tab out of the game and use some add-on or program to solve the data you have observed from the periscope with (or without) your knowledge of geometry and physics.
Alternatively, some people just fire at extremely close range with fast speed torpedo setting and a bit of experience which also works well, but both these methods are not really something to brag about imho.
If I do use manual targetting, since surveying with and without optical instruments is a part of my professional expertise (as an engineer), I can figure out torpedo solutions on the fly but am I really going to be arsed enough to do that? Nopes! I leave all that stuff on my desk at work, and certainly don't like to use it in a game where I'm engrossed with other stuff like role playing, overall strategies, daring / experiment / exploration / gloating over my victim's misfortune.
Sorry but in my book, a game is for entertainment!
So is there some aspect of it that I've overlooked?