For the historical aspect look at
www.uboatarchive.net. They have a few KTB (Kriegstagebucher). Look at the one for the U-156 (Laconia incedent). About half way down the page you will read that the U-156 met up with U-125. The commander of the U-125 came across in a dingy and transferred 5 bridging sets (metal straps for connecting batteries) to the U-156. And that is out of their logbooks. So between fuel transfers and parts, the U-156 was going nowhere fast. So the option on time can be somewhat mapped out from them. And if wolfpacks are already in-game as a variable, couldn't you just drop all the u-boats but one? You would already have some parameters for the meeting set up by the game. Pardon the explainations, just doing it in case any new guys are following along.