Consult this book to know more about the German submarine U-48. 

Ref.: Franz Kurowski,  U-48: The Most Successful U-boat of the Second World War. Frontline Books, 2012.

From the book cover:

In August 1939, U-48 left Germany, commanded by 'Vaddi' Schultze, to take up a waiting position around England. It scored its first success on 5 September, when it torpedoed the British freighter Royal Sceptre, then the Winkleigh on 8 September. On both occasions - the first of many - Schultze showed himself to be a notable humanitarian: he addressed signals to Churchill giving positions of the sinkings so that crews could be saved. By 1 August 1941, U-48, the most successful boat of the Second World War, had sunk 56 merchant ships of 322,478 gross tons and one corvette. She was then transferred to the Baltic as a training boat. Schultze became commander of operation 3 U-Flotilla and later was appointed commander, II/Naval College Schleswig. He died in 1987 at the age of 78. U-48 was scuttled on 3 May 1945.

AUTHOR: Frank Kurowski (born in Dortmund, 1923) fought with the Frika Korps as a Luftwaffe paratrooper. He has been a freelance journalist since 1958 and is the author of more than 100 books on World War II, including the acclaimed series of Panzer Aces: German Tank Commanders in World War II. SELLING POINTS: - A complete history of a key U-boat and notorious commander - Detailed analysis of the strategic situation and political developments - Full sailing details, records of conversations and tactical analysis ILLUSTRATIONS: 16 page plate section *