I just recently started playing SHIII and I'm already addicted to it

I play the game on the 'normal' setting, and although this isn't 100% realistic (actually, it's only 29% realistic

), I would like to do the patrols in a realistic way.
The mission are always "patrol grid XX for 24 hours". So what I do is; go to that grid, plot a course through the grid, similair to the first preset search pattern and sink a ship or two. However, in my first patrol I didn't come across any ship after 24 hours, so I went to another grid to find a C2 cargo wich I sunk.
But, is this realistic..? How were the real patrols in WWII? Did the U-boats patrol a certain area for a certain time, and then immediately return to base? Or did they patrol als long as they wanted?
(by the way; If I'd turn fuel limit on, what would happen if I run out of fuel? Is it a mission fail or is it possible to get refuelled on sea, by friendly units?)