Most theorize that Prien and U47 was lost on or around the 7/3/1941 although now by unknown cause as HMS Wolverines attack was on UA.
I ran across this in the uboat archive in the KTB war diary section

"9.3.
U 74 reports having lost contact. U 74 and U 99 receive operations areas between 59 and 620 with U 37, while UA is to be withdrawn further to the west. For the time being therefore the operations in the north have come to a standstill. U 70 and U 47 were again requested to send a situation report in the evening, but without result. Both boats are causing great anxiety, as they have not yet replied to the request made on the evening of the 7th. There is still hope that this silence is only due to W/T breakdown, especially in the case of U 47, which had already experienced a breakdown on this trip.

10.3.

An incomplete short signal giving position was picked up, but not repeated on request. This could possibly have been originated by U 70, more likely by U 47.

Now we know that U70 was lost before the 19th but there seems to be a possibility that U47 may of been lost sometime on return to port."

Does anybody know if this message was looked into further and what it contained ? as it seems U47's W/T was having problems by the looks of it from the war diary's entry on the 9/3.

Kind Regards