Oh the Humanity?

[DicheBach]

General question about WWII era submarine warfare: how "standard" was it to "allow" the crew of merchant ships to escape?

I'm reminded of that scene in Das Boot where they torpedo the thing and then give it several hours to abandon, and then when they torp it again, they realize there are still crew on board and it is a dramatic moment where one of the crew of the uboat says something about rescuing them and the skipper points out that is impossible.

And then I also noted this little comment in the wiki page for U-178 (one of the boats which served in the Monsun Gruppe).

In the early hours of 14 July, the American Liberty ship Robert Bacon was torpedoed about 35 miles off the Mozambique Light. The crew of 44 and 27 Armed Guards (the ship was armed with two 3-inch and eight 20 mm guns), abandoned ship before U-178 finished her off with two more torpedoes. The U-boat surfaced and questioned the survivors in one of the boats, giving them directions to land and wishing them good luck before leaving.

Given the savagery in so many other aspects of WWII, it is almost a bit unbelievable that submarines would indulge the risk of trying to assist the crews of their victim merchant ships to survive.

Was it doctrine that subs would allow merchant men to disembark before striking a coup de grace under conditions where it was tenable? Or was it just an unwritten convention?