Wow, great answers -- both the easy answer, of "Can't do it in SH3" and apparent "undocumented features" that allow charging on the surface with no fuel

Lots more discussion than I anticipated, and interesting to see some background. Steve's mention of hydrodynamics and the link made me wonder, since it's related: for surface running, since these boats weren't designed to do any planing, will setting the depth such that the longest possible section of the boat is at the waterline result in the greatest possible speed (at least before the disproportionate amount of engine power needed to overcome the design hull speed)?
I'm really enjoying seeing how the game dynamics both mimic and diverge from real-life boat ops, too.
Thanks, all.
Zygoma
May the number of your surface operations exceed your number of dives by one, at the end of your career.