Welcome to the wacky world of subsim Shark Bait (good name by the way, hope it isn't too prophetic).
You've arrived at an interesting time too. Yup, SH4 has a few bugs in it, and some seem to be suffering more than others it seems (I appear to be one of the lucky ones, in that I've not had too many problems), but there probably isn't a better modding community around than the one for submarine sims, and the SH4 devs are on the case, so it will improve as the days go by.
That sub in BF42 was pretty good fun, about as accurate a rendition as the movie U571, i.e. not very, but it looked great and was fun all the same!
Be prepared to have an upset wife/girlfriend/pet dog etc as you become obsessed with making the perfect ambush on a convoy
