Greetings Archer! I also use the donation Narwhal, and I find I have an enormous range. The most economical transit speed is 2/3s on the surface, and it should be over 11k. Running submerged is your problem. Recharging batterues uses fuel like you were running at full speed.
I recommend only running submerged during the day if air contacts are so frequent diving becomes irritating. At 2/3s submerged speed you should cruise at 4 knots for 14 or 15 hours. At night always stay surfaced, unless the weather is so bad you fear blundering into something in the fog (pre radar).
Good luck, and share some war-stories!
