Conserving Fuel - Fuel Management

Hello!

I play SH4 on 100% realism (it's a sim, right?). My objectives are pretty much always to travel to Japan (or thereabouts), sink some merchants and return to Pearl Harbor.
However, it seems like I barely have enough fuel to make it there and back, let alone conduct any maneuvers or hunt at any range once I'm at the target area.

I'm on a porpose class sub. I've resorted to travelling at 1/3 speed, submerged, resurfacing only to recharge the batteries. Is this what I should be doing?

Please let me know what you think. Thanks!

Ipecac