[Beery]

U-boat losses really started to mount as early as July 1942, with losses in double digits for almost every month after that. February 1943 saw 19 U-boats destroyed. The numbers of U-boats destroyed in the last 6 months of 1942 (75) are not a great deal lower than the losses encountered in any of the following 6 month periods - Jan to June 1943 (113), July to December 1943 (124), January to June 1944 (129), and the last 6 months of 1944 (112), yet they are vastly more than the losses in the first 6 months of 1942 (21). Clearly, late 1942 and early 1943 saw a huge difference in the Allies' ability to kill U-boats. Perhaps in reality, U-boats were not travelling submerged during the day in late 1942 and early '43, but if they weren't, they certainly should have been.

According to Uboat.net, 1942 was the year aircraft became a significant killer of U-boats, and aircraft accounted for 31 of the 96 U-boats sunk in that year, so I think it's a mistake to travel surfaced during the day even in 1942. It may have been a mistake that real U-boat crews made, but it's a mistake nonetheless.

The other thing that argues for travelling submerged in early 1943 is the fact that BdU initiated the flak boat program in early 1943. Clearly, air attack was recognized as a major danger by this time, which means that air attacks were having great success. In light of this, it has to be a mistake to treat aircraft lightly, even in 1942 and '43.