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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:

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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:

  1. Approach the perimeter of Gibraltar, well outside the range of the sentries.
  2. Submerge to the periscope depth and continue approach until the concentration of sentries becomes too great.
  3. 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...
  4. 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?

  • The nickname of the Captain-Lt was Der Alte, not Der Alle. Der Alte is, in this case, translated to mean "The Old Man" ... the Capt was the oldest aboard, by far, at 30. – CGCampbell Feb 23 '17 at 0:46
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In the book it is explained that the captain delayed the dive, because he judges that the boat is so heavily damaged that once it dives, it will not be able to return up without some repair. So he speeds to shallower water, where the otherwise unstoppable dive is halted by the seafloor. In fact, after diving he tries to level the boat, but it does not react and descends further. Only after some major repair they manage to surface.

This is explained to the bewildered crew only when the boat is well under repair, with the comment that these are the situation in which inexperienced captains lose their boat.

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    So the captain actually expected the boat to survive the impact with the seabed? I thought this was one of the liberties that the film director took... It seems to me that collision puts so much strain on the hull that it could be fatal even on the lesser depth. – Silent Inky Feb 17 '17 at 22:42
  • Hoped might be a better word than expected. – Steve-O Feb 18 '17 at 4:50
  • I think that resting on the ground was a standard maneuver for submarines to avoid detection. Early sonar could not pick up the shape, and you can shut down all engines so you cannot be heard either. – Jan-Christoph Schlage-Puchta Feb 18 '17 at 11:07
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    I have some no-too-exact memory of Buchheim writing at some point something like "Different from ordinary ships, for submarines the seabottom is your friend." I am currently away from home, but in 10 days I should be able to check the quote. – Jan-Christoph Schlage-Puchta Feb 19 '17 at 16:39
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    Found the reference: "Among the greatest fears of a ship is grounding. To keep his ship free from the ground is the first plight of a sailor. For our boat grounding is only a danger, when it travels above water like any other ship. But in shallow waters touching the ground is a rehearsed procedure. A submerged submarine cannot be kept on the same depth without using its machine. ... But in waters, that are not deeper than the diving limits of the submarine, the submarine can rest on the bottom and shut down its machine." Taken from Buchheim, Jäger der Weltmeere, translated by myself. – Jan-Christoph Schlage-Puchta Mar 2 '17 at 15:17
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Not based on any written source, but tactical thinking.

  • 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.

Even the electric engines are noisy enough for a destroyer to pick up. (As we have seen in previous scenes, when the boat was hunted by convoy escorts for hours.)

That is why the plan of the Kaleun included shutting down the engines and having the boat carried through the strait by the current.

At periscope depth, especially at night, a boat is next to blind. The periscope is very close to the surface, so you have very limited range, very limited field of view, only one man able to see at all, and the various lenses further reduce what little light would be available -- and you want the night to be as dark as possible when you attempt a breakthrough like that.

So, once submerged, there would be no further "creeping up" on the sentries, only the current. And the current is not strong enough to carry the boat far before they have to resurface for air.

Resurfacing too close to the sentries at the far side of the strait would mean risk of detection, which -- this close to the naval base at Gibraltar and its effectively unlimited supply of ASW ships and resources -- would mean certain destruction of the boat.

So the idea was to get as close as possible to the strait -- on the surface -- before diving.

As for what happened once the boat was detected, I point to Jan-Christoph's answer.

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