At 09.13 hours on 24 Oct 1943 the unescorted Siranger (Master Amund Utne) was hit on the starboard side at #1 hatch by a Gnat from U-155 and sank by the bow after 30 minutes. The ship had been spotted at 03.50 hours and missed by two torpedoes at 09.10 hours. The 41 crew members and four passengers abandoned ship in three lifeboats, one of which had to row back to rescue a British passenger who had jumped overboard. The U-boat then surfaced and asked two men in one of the boats to come aboard. An engineer was later allowed to return to the boat, but the wounded third mate Otto Friis Hansen was kept as prisoner of war. A tendon in his wounded forearm was shortly thereafter operated by Stabsarzt Dr. Franzen on board of the U-boat. He was landed at Lorient on 1 Jan 1944 and sent to the POW camp Milag Nord near Bremen.

On the morning of 27 October, the three lifeboats were separated. The boat of the master sighted a lighthouse in the evening and the next morning met a fishing vessel, which directed them to Nazare, just north of Cururupu, Brazil. By 30 October, the survivors from all three boats arrived at Cururupu.