well, they have travelled most of the time on surface - the VIIc is much slower submerged and it would drain the batteries unnecessarily. U-boat commanders put as well a great deal on saving Diesel to stay for longer on a patrol - so they run the diesel just at half speed to make enough distance over ground while conserving the Diesel. When I'm playing in the later years, I tend to run submerged during the day at very slow speed in the patrol area - to conserve Diesel, but as well to be able to use passive sonar to detect a convoy or single runner - it protects as well somewhat from being air raided.
Once I have detected a target I surface to get quickly to 2 other locations to triangulate the position of the target - submerging again to do that by sonar, because that ship is normally far away - figure out it's possible course and speed and then plan where a good location is to intercept it - then I surface and run AK to the interception location and wait there submerged - checking the target's course and if I did my job right, the target will appear soon on the horizon and I can start to make the final calculation of it's course and speed, and when it is near enough, figure out what kind of vessel that is, then make exact measures of distance to target, bearing to target, it's speed - then setting up the torpedos - open the tubes - and wait until I can see it's flag - never sink a neutral ship - and once all is ok, I wait for the right moment that my torpedos can run in a straight line to the target - once I've fired my torpedos I just hope for the best, that they don't fail - material quality was not the best - and that I'll hit it in a critical spot to sink it quickly.
This kind of hunting down a target was not presented in the movie - but it is the preferred method - it is rare that one just gets a target location to go to - one has to figure out targets and where to intercept them by oneself most of the time - I like this hunting without to actually see the target for most of the time - and it is a good feeling when it finally appears on the horizon, running directly into the trap.