Traveling underwater may be needed due to enemy air patrols, but it won't save you any fuel in the long run since you use more fuel after surfacing while recharging your batteries.
As mookiemookie indicated, you should travel at the most fuel-efficient speed, around ten knots (you can set a speed directly by clicking under the telegraph to go from 1/3, 2/3, etc. display to an actual knotmeter). Miles-per-gallon drop off quickly as you go faster or slower.
If that's not acceptable than you will have to play with limited fuel off.
|