Hmm... In the stock game using 1/3 power from Midway to Japanese waters even early Gatos should arrive with 65-70ish% fuel remaining. (1/3 power or ~9-10 kt is the best range power/speed). Not a lot of extra for creating havoc, but does allow a round trip with a bit of mayhem.
I have not (intentionally) run submerged more than is necessary to prosecute an attack or evade the DDs. Since engine power is required to recharge the batteries, I'd expect extended submerged ops to use more engine power and thereby fuel than the same run on the surface...
Hopefully other Kaleuns will add more info.
(Unrelated to this problem but FYI later in the game, crewmembers gain special engine skills that will allow arrival with 90% or so. Checking crew status between patrols allows their identification and reassignment to the engine room if necessary)
A related thread is at
https://www.subsim.com/radioroom/sho...d.php?t=245090
