In Das Boot we see Der Alte planning to approach Gibraltar on the surface, maneuver between the British lookouts and only then dive. But why didn't he attempt to approach underwater, or at the periscope depth?
The Type VIIC U-boat has a range of 150 km while submerged. This seems enough to cross the strait:
In addition to that, Der Alte mentions that the surface current in the strait plays to their advantage, dragging the boat roughly in the desired direction. So why didn't he try for something like:
- Approach the perimeter of Gibraltar, well outside the range of the sentries.
- Submerge to the periscope depth and continue approach until the concentration of sentries becomes too great.
- Perform the final check of the course to make sure the boat was moving in the desired direction and submerge to the 150-200 meters depth (previously we were shown that the boat evaded the destroyer for some 6 hours while remaining on that depth so it should be reasonably safe) and tiptoe forward on electric motors. Then, judging by the hydrophone, determine whether the boat have passed the main line of defense, and...
- Surface and zoom away.
Later, when the boat is already spotted, we see it sprinting on the surface for several minutes before diving. I could understand that the dive was delayed to bring the wounded navigator into the boat, but why did the captain delayed the dive further?