A commander of a Type IIA U-Boat, with early rubbish batteries, is ordered to sail from Kiel and patrol AF84 for 24 hours and then sail back again. He 'porpoises' all the way there to maximise his range, he sits just within the patrol square for 24 hours to qualify for 'patrolling' it and then he heads home.
Only to run out of fuel.
How did they manage it? There's barely enough fuel to return to Wilhelmshaven.
[Note: Using GWX2.0]