At 20.48 hours on 16 March 1943, U-172 fired a spread of four torpedoes at convoy UGS-6 and heard a detonations after 2 minutes 50 seconds, 3 minutes 10 seconds and two after 3 minutes 20 seconds. At 20.50 hours, a stern spread was fired and two hits were observed after 56 seconds and 58 seconds, both torpedoes probably hit the Benjamin Harrison in station #73. The other detonations must have been depth charges dropped by the American escorts.

The Benjamin Harrison (Master Goerge Hunter Sterne) was hit by two torpedoes on the starboard side in the #5 hold, began to settle slowly and appeared to be in no great danger of sinking quickly. The master did not order the eight officers, 35 men, 27 armed guards (the ship was armed with one 4in, one 3in and four 20mm guns) and one passenger to abandon ship. Some men shouted this order, creating a great confusion and they began leaving the ship in a chaotic manner. The explosion had damaged one of the lifeboats and in their haste two others were improperly launched, causing the occupants to fall into the sea. Only one boat was launched effectively with 16 men clung to the full boat and 15 had to swim with their life preservers until they were picked up by the Panamanian motor merchant Alan-A-Dale and were landed at Oran, Algeria on 24 March. Three other survivors were picked up by USS Rowan (DD 405), which sank the wreck with gunfire at 21.30 hours about 150 miles east-northeast of Terceira, Azores and were landed at Casablanca. Two officers and one armed guard were lost.