We are used to seeing movies of U-Boats heading out to sea; attacking ships etc. but we were not aware that the Type VII Frontboote did not have the range for these normal operations.  They had to be refueled – all of them, and this book lists more than 1,000 operations of refueling combat U-Boats on the high seas.  Learn more about this unknown part of the War at Sea at https://sharkhunters.com/books/ and for information about Sharkhunters, the official history of the U-Boats, go to https://sharkhunters.com/
This large 8½ x 11 inch book lists all the refueling of combat boats on the high seas with hundreds of photos.  This a facet of the U-Boat War no one has touched on and there is no other list like this anywhere in the world.