Do your traveling at ~ 9 kts. On the surface.
Submerge to attack or survive. Moving submerged is fuel intensive. Avoid it when possible.
If you've sustained any form of damage, follow closely the recharghing of your batteries. It is possible that they wont recharge 100%. In such a case stop auto recharging. Recharhing batteries costs diesel.
Yes most of your fuel will be spent to reach your mission objective and return to base. If you allocate 10-20% of your fuel for reserve/contingency purposes you'll find that the actual availability of fuel for combat/patrol purposes maybe lower than 20%. Not unrealistic mind you. Fleet boats would routinely travel 30-40 days to and back from their patrol stations where they remained for less than a week. The Pacific is a BIG pond!
As the war progresses and more "forward" bases are established fuel becomes a lesser concern but always remember nobody if going to tug you home if you run dry!
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