The book is an Americanization of a British story, where the Captain is RNVR and the DE is a Captain Class, so that would place the Enemy Below story of the captain hunting a U Boat in the time frame 43 to 45. Considering that he already had lost another DE under his command, it is more probable that the story takes place in the latter part of that period, perhaps the last part of 44 or early 45.
Now as to the question of is it likely that you would attack any ship, loaded or not. The answer surely must be yes, because what is not loaded today will certainly be loaded tomorrow with things that can hurt you. It is the very same principle involved in firing at a tank that is retreating away from you. They might not be able to hurt you today, but there is always tomorrow.
The general idea is kill what is the enemy's, wherever you find him, unless there is some overriding factor, such as the security of your mission, that prevents it.
: This may seem kind of arcane, but...
: In The Enemy Below the captain (Mitchum)
: said his wife was killed when his ship was
: torpedoed heading WEST to the US and the
: implication is it was late(r) in the war.
: Because I apparently have to much time on my
: hands I started to think why would a U boat
: risk attacking a WEST bound ship let alone
: an entire convoy of empty ships that might
: be carrying German PoWs.
: I would think the emphasis would be on
: loaded ships, and I realized all of what I
: have read talks about attacks on east bound
: Yes I realize it's a nit to pick and it is a
: movie, albeit a good one.
: So did U Boats attack empty convoys headed
: west? Seems like an equal risk for much less
: gain and loaded ships heading east seem much
: more valuable given a choice.