because the earth is spherical in reality and the SHIII map is flat in the game the distances involved are all screwed up so without modification the historical recreation of the trip is not possible.
I believe the GWX mod "fixed" this by dramatically increasing the range of some of those u-boats in the game. so they would have range enough to make the trip non stop
but its been a long time since i've had the old girl off the shelf.
in stock... you'd run out of diesel - if i remember correctly the range on the u-boats was programmed to be historically accurate, but because of the flat earth, the north atlantic is substantially larger in SHIII than in real life as the distances are stretched a bit particularly in the northern most areas where we would spend a great deal of our patrol time.
The next subsim to roll out HAS to have a spherical map like the flight simulator seris - some way - some how.
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