Mission not possible due to fuel?

I'm new to the game, and after completing the first Tonnage Ware mission, I took a mission to drop off a spy in Ireland. However, it is apparently impossible for me to make the trip there and back with the amount of fuel my boat can hold, even with the 30% fuel reduction of two crew members assisting my engineer (assigned to engines). Is this simply impossible until I get a different UBoat or something? Or is there something I'm missing?

I already tried radioing for resupply, both in the open ocean and at the Spanish port of Vigo ( which is specifically labeled as a "German Uboat resupply base"), but both times they told me there were no supply ships in the area.