Wilhelm Zahn commanded U-56 and almost changed the war. The episode is documented in the dissertation called "Wolves Without Teeth, The German Torpedo Crisis in World War Two" by David Habersham Wright (Georgia Southern University, 2010)
http://digitalcommons.georgiasouther...99&context=etd
It's a fascinating read. The U-56 story is on p. 63
Salute,
AD