From u-boat.net:
Günther Prien went aboard for his last patrol on 20 February, 1941.
This incident is now being questioned, as the traditional credit for
U-47's sinking has always been attributed to the British destroyer
HMS Wolverine, but new data suggests that the destroyer was actually attacking Eckermann's
UA which had to withdraw from the battle with heavy damage.
It is now speculated that
U-47 was hit by one of her own circling torpedoes. (Two US submarines in the Pacific are also believed to have been lost to the same sort of equipment failure.)