Hi Jamson,

You might want to pick up a copy of:

White, John F. U-Boat Tankers 1941-45 Submarine Suppliers to Atlantic Wolf Packs. Annapolis, MD: Naval Institute Press, 1998. ISBN: 1-55750-861-5. Copyright: John F. White, 1998.

It goes into some detail and is essentially the standard reference on this subject due to a lack of other books on the subject. It’s a great read and even goes into the underwater refueling tests that were conducted.

Regards,

Ken Dunn