At 11.49 hours on 28 Sep 1942, an armed guard on the unescorted Alcoa Mariner (Master John Luther Martino) spotted the wake of a torpedo from U-175 20 miles off the Orinoco River, Venezuela. With hard right rudder, the torpedo passed 15 feet astern, but three minutes later a second torpedo struck on the port side just forward of the poop deck. The explosion opened a crack three feet wide across the deck, destroyed the interior compartments and flooded the engine room. The stern dropped 30°, apparently held in place by the shaft. The eight officers, 33 men and 13 armed guards (the ship was armed with one 4in, four 20mm and two .30cal guns) abandoned ship in four lifeboats and by jumping overboard. One hour later, the U-boat hit the ship with another torpedo aft of the engine room on the port side, causing the ship to sink rapidly by the stern at 13.05 hours. The lifeboats were sighted by a US Army patrol aircraft and six hours later the Canadian motor merchant Turret Cape rescued all hands and landed them in Georgetown.

The master, John Luther Martino, commanded earlier the Alcoa Cadet, which was mined and lost near Murmansk on 21 June 1942.