German submarine U-171

German submarine U-171 was a Type IXC U-boat of the Nazi German Kriegsmarine built for service during World War II. The submarine was laid down on 1 December 1940 at the AG Weser yard at Bremen as 'werk' 1011; it was launched on 22 July 1941 and commissioned on 25 October 1941 under the command of Kapitänleutnant Günther Pfeffer.

After training with the 4th U-boat Flotilla, U-171 was transferred to the 10th U-boat Flotilla for front-line service on 1 July 1942. It was sent to patrol in the Gulf of Mexico and was sunk by a naval mine in the Bay of Biscay 115 days into its first, and only patrol whilst returning to Lorient in occupied France, with the loss of 22 of its complement of 54. For many years it was believed that GS U-166 (1941) had been sunk by an American aircraft in what had in reality been an unsuccessful attack on this submarine in the Gulf of Mexico.

Patrol
U-171 departed Kiel on 17 June 1942, at the end of which it was to return to Lorient, where it would be based for future sorties. It negotiated the 'gap' between Iceland and the Faroe Islands and crossed the Atlantic Ocean, entering the Gulf of Mexico.

The submarine sank a 4,351 ton Mexican general cargo merchant ship, the SS Oaxaca on 26 July 1942, at 28.38333°N, -96.13333°W off Corpus Christi, Texas. The ship was at the time en route from New Orleans to Tampico via Veracruz. The first spread of two torpedoes missed the ship, but a second spread, also of two eels (German U-Boat slang for torpedoes), was successful when one hit the port side near the fore-part of the ship. Six of the crew of 45 died.

On 1 August 1942, also in the Gulf of Mexico, U-171 came under attack from a U.S. Coast Guard J4F-1 Widgeon aircraft, causing it little damage. However, for decades it was believed that the U-boat attacked that day was GS U-166 (1941); the credit for sinking U-166 went to that aircraft.

The wreckage of U-166 was discovered in 2001, just a short distance from its last victim, the Robert E. Lee, meaning that the credit for the sinking of U-166 should have gone to the U.S. Navy patrol craft, PC-566, which had reported that they believed they were successful in their depth charge attack on the submarine following the U-boat's successful torpedoing of the Robert E. Lee, but PC-566 were believed by investigating officials to have missed.

Having not been sunk, as had been believed by the Allies, U-171 continued its patrol. On 13 August 1942 it sank the 6,779 ton US tanker R. M. Parker Jr. at 28.83333°N, -90.7°W which is about 25 mi south of Isles Dernieres, Louisiana. The ship, which was carrying water ballast, was struck by two torpedoes; the submarine then surfaced and fired five rounds from its deck gun into the wreck. The whole crew of 44 survived, being picked up eight hours later by the United States Coast Guard auxiliary USS Pioneer.

On 4 September 1942, the submarine had its final success, the Mexican tanker Amatlan, 6,511 tons; again in ballast, at 23.45°N, -97.5°W. This ship had evaded three spreads of two torpedoes each, before being hit by a torpedo fired from U-171's stern tube. There were 10 dead and 24 survivors.

Loss
U-171 was sunk at 13:00 hours on 9 October 1942 in the Bay of Biscay near Lorient, in position 47.65°N, -3.56667°W, by a mine. Twenty-two men died, thirty survived. Captain Günther Pfeffer (1914–1966), was one of the lucky ones.

The wrecked submarine was classified as a "military cemetery" in 1999 by the French authorities: divers are then warned that going inside the boat is strictly forbidden.