Well, the best I have been able to get the batteries, despite a lot of additional attempts, is still between 27 and 30 hours (from 100% to 10%) at 2 knots. That's still about 15 hours too short, but I haven't been able to find anything to tweak using either minitweaker or S3D that makes any real difference. Even tried playing around with the submerged drag, hoping the boat might go farther with the same propulsion if the drag was reduced, but no such luck
I did notice that if you drop to 1 knot you can stretch your batteries out to at least 43+ hours (oxygen hit 10% at that point, with batteries still at about 25%) - but of course you'll only go half as far in that time, which could be material if you're trying to extricate yourself from certain enemy waters before you surface.
I've about run out of ideas (and time) again, so I'm gonna have to shelve this effort and get back to it later unless (hopefully) somebody beats me to a fix.
One last possibility I may look into if I can find any old posts about it - I noticed that my green bar wasn't quite 100% in the engine room (it was close though, maybe 90%, so I'm not sure how much improvement could be expected) - I'm wondering if maybe there's some way to give the engine room crew a quality/experience/efficiency/morale or other such boost to improve submerged range like you can improve deck gun loading time? Anybody tried that already?