Try the following books:
Vom Original zum Modell: Uboottyp VIIC page 18.
U-boats by Robert C. Stern page 150.
U-boats in Action, Squadron/Signal page 34.
U-boat War by Timothy J. Kutta page 45.
Good Luck and Good Hunting,
Americus, Georgia C.S.A.
"Every man dies, not every man really lives." Braveheart