Save fuel travel submerged/on surface 1/3 or slow ahead. TypeII coastal subs have small fuel tanks too so no long patrols with those. VII class in the future will get you much further and with IX you can travel "the world". Milkcows are awailable sa mods and also included with Rub and IuB mods .
Prost!
EDIT: To add...yeah no refuelling in 1939 that is correct not even with current mods available
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