RANGE - Can't Make It There and Back

[bsalyers]

Did you toggle between your diesels on surface and electrics below

I'm not sure what you mean; if you're on the surface, you're running diesel. If you're submerged, you're on electrics. The game does this for you automatically. The only exception is when you are using the snorkel, which allows use of diesels while submerged.
This was my 30th patrol; it's been awhile since I was a n00b. I don't think I made any navigation errors. I used the slowest speed available, regardless of propulsion.