Typical, sloppy, sentimental
It is a textbook torpedoing! The torpedo is set to run nice and deep, and as it passes underneath the ship, the detonation circuit senses a change in the magnetic field and triggers the explosive, neatly snapping the ship's keel, breaking its back, and sending it down with incredible speed. For the next five or ten minutes, bales of brown stuff erupt from the water, released from the cargo holds as the ship plummets towards the bottom. It gives the whole scene an unexpectedly festive air.
Some U-boat skippers would not be above machine-gunning the survivors, at this point, just to let off a little steam.
But the commander, Kapitänleutnant Günter Bischoff, is not yet a card-carrying member of the Nazi Party and probably never will be.
On the other hand, Bischoff is wrapped in a straightjacket and blasted half out of his mind on drugs.
So he doesn't. The Negroes are jumping out of the lifeboats, swimming to the bales, and clinging to them with just their heads out of the water, realizing it would take forever to hunt them all down. OL Beck knows the Liberators and the Catalinas are already airborne and vectored towards him, so he has to get the hell out of there. Since he has plenty of fuel, he decides to head south for a while, planning to double back north in a day or two, when the coast might be a bit clearer. It is the kind of thing that KL Bischoff would do if he had not gone crazy, and everyone on the boat has unlimited respect for the old man.
They run on the surface, as they always do when they are not making a positive effort to sink a convoy, so they can send and receive radio messages. Beck gives one to Oberfunkmaat Huffer, explaining what has just happened, and Huffer gives it to one of his Funkmaats, who sits down in front of U-691 's Enigma machine and encrypts it using the key for the day, then taps it out on the radio.
An hour later, they get a message back, straight from U-boat Command at Wilhelmshaven, and when the Funkmaat runs it through the Enigma, what he comes up with is: CAPTURE SURVIVING OFFICERS.
It's a classic example of military commandsmanship: if the order had come in a more timely fashion it would have been easy to obey, but now that they are an hour away it will be extremely difficult and dangerous. The order doesn't make any sense, and no effort is made to clarify it.
Given the time lag, Beck figures he can get away with giving this one a half-assed try. He really should swing round and approach the wreck on the surface, which would get him there faster, but which would be nearly suicidal. So instead, he closes the hatches and descends to periscope depth as he draws closer. This cuts the U-boat's speed to a crawling seven knots, so it takes them about three hours to get back to the atoll of bobbing brown bales that marks the site.
A damn good thing, too, because another fucking submarine is there, picking up survivors. It is a Royal Navy submarine.
This is so weird it makes the hairs on the back of Beck's neck stand up--and there's a lot of hair there, because like most submariners, Beck hasn't shaved in weeks. There's nothing weird, though, that can't be settled with a single well-placed torpedo. Seconds later the submarine explodes like a bomb; the torpedo must have touched off her munitions. Her crew, and most of the rescued Negroes, are trapped within, and don't have a chance of getting out even if they survived the explosions. The submarine drops off the surface of the ocean like the wreckage of the Hindenberg tumbling down on New Jersey.