U-541 Petersen 4-Apr-1944 14 days
Certainly Petersen was trying with gusto. On the 14th of May while offshore from the border between South Carolina and Georgia (east of Savannah), Petersen reported attacking an unescorted tanker with six torpedoes and observing only two detonations at the end of their runs (Wynn Vol.2, p.22).
SOURCES: Gudmundur Helgason, Rainer Kolbicz, www.uboat.net, 2013, Kenneth Wynn, U-boat Operations of the Second World War, Volume 1 and Volume 2, 1997, R. Busch, and H.-J. Röll, German U-boat Commanders of World War II, 1988, Franz Kurowski, Knights Cross Holders of the U-boat Service