Hi Lorenzo

In WW1 at least four German U-boats were sunk by merchants:
UC-36 was rammed by French steamer MOLIÊRE on 21 May 1917
U-49 was rammed and sunk by gunfire from British steamer BRITISH TRANSPORT on 11 Sep 1917
UB-119 was rammed by British steamer GREEN ISLAND on 5 May 1918
U-103 was rammed by British passenger ship OLYMPIC (sistership of TITANIC) on 12 May 1918

In WW2 only one German U-boat loss is credited to a merchant ship alone:
U-633 was rammed by the British steamer SCORTON on 10 Mar 1943

However, U-576 was sunk by aircraft after being damaged by gunfire from the US freighter UNICOI on 15 Jul 1942 and U-70 was sunk by warships after being rammed by the Dutch tanker MIJDRECHT on 7 Mar 1941.

Best regards
Rainer

Crew member of uboat.net