U-553 returning to Saint-Nazaire, Loire-Atlantique, France from its second combat patrol on the 19th of July 1941, she came home with two combat pennants and a badly-damaged attack periscope.
During her 2nd Patrol on the 12th of June 1941 and after five days at sea, Kapitänleutnant Karl Thurmann came upon stragglers from the westbound convoy 'OG-64' north of the Azores. At 01.22 hours Thurmann sank the British steamer "Susan Maersk" (2,355 tons), which went down in 90 seconds with a loss of all of her 24 crew. Because of the darkness and the speed with which the ship sank, only a rough tonnage estimate was possible, thus the gross registered tonnage on the sinking pennant is incorrect. Later that same day, at 15.05 hours U-553 attacked the Norwegian tanker "Ranella" (5,590 tons). A torpedo struck the ship, but it didn't sink, a further shot was attempted at 1536 but also failed. After that attempt, Thurmann turned away in the direction of the steamer, passing under the bow of the "Ranella", which was now stopped. The periscope, which was not fully lowered, struck the tanker and and the top two metres were bent backwards. Finally, at 1635 hours, a third torpedo struck its target, causing the ship to break in two. It still didn't sink, however 100 rounds from the 88-mm deck gun were needed to finally sink the two parts of the wreck. The 29-man Norwegian crew had already left the ship and safely reached the Azores, 300 miles away after 12 days at sea.
U-553's tenth and final sortie began with her departure from La Pallice, La Rochelle on the 16th of January 1943. On the 20th, she sent a radio message: "Sehrohr unklar" (periscope unready for action), and was never heard from again. She had suffered no casualties to her crew until this date but all hands were lost. She most probably sank because of technical problems and was officially declared missing on 28 January 1943.
(article copied from the net)