furthermore,
if you use underwater propulsion, you can cover only 38 kilometers over 4 hours, 55 mins, at flank speed. What will happen afterwards, is that you are stuck with 2 % of power and cant move. so,
[SPOILER] this is a cheat. You CAN surface passively by blowing ballast, stop the engine, and stand for 2 hrs and 19 mins, this should recharge your battery without affecting diesel fuel , although unrealistic, there you go.
Although this is a bug, yet, you have to endure a tonne of pain to reach home base, but for 150 kilometers or less, I would do it for a rookie sub skipper.[\SPOILER] Cheers