One of my favorite authors when I was a teenager was Alistair MacLean. (He wrote "Force 10 from Navarone" and "Where Eagles Dare" among many others.) There was a book of his short stories published in the 80's named "The Lonely Sea". One of the stories was "Arandora Star". It is a tragedy about a ship that carried German POW's that was sailing from Europe to Canada. In that story the author talked about how the ship was brightly lit to identify it as a POW ship and how the German Command was notified of its passage. Well it was sunk by a U-Boat and most of the POW’s died as a result. There was some controversy over it because the Germans claimed they were never notified. Also the area where the prisoners were held was overcrowded and barb wired in order to separate the prisoners from the crew. Apparently that incident led to changes in how POW’s were transported and how the Germans were notified.

No answer here, but why would the u-boats want to sink ships headed West? They would have been empty, and the targets would be headed East?