In all reality the boats could cover about 11000 miles fully topped off. Sometimes one tank used for surfacing was filled with fuel and used up in the transit over to the war zone. Unfortunate we do not have that option so 10 kts is the best most efficient speed. Eventually you will find resupply ships that you can use to refit. If you use ahead slow when you are hunting in th area assigned helps save fuel and you attract less attention. Once in the war zone I only go flank at night to end rounds. Other than that I set it at 5-8 kts while hunting. If you play your cards right after topping off at Midway, transit at 10 kts, upon reaching the east coast of Japan you should have just over 3/4 fuel to use.

I remember the same question with Silent Hunter III

From what I understand it is a question of computing power. It is far more complex for a computer to calculate movement across a 3D sphere then on a 2D surface - so developers choose the latter approach and give the illusion of a round planet by limiting the horizon through haze.

Saying that, I'm supprised there isn't a way to implement the need for Parabolic courses etc even on a flat map. Or am I just being a bit too hardcore?

I've found out what happens. My batteries only recharge up to 98%, so I'm burning a lot of diesel I should be saving. Once I turn battery recharging off, I can get back to refuel.