Crew of U-550 prepares to abandon ship on April 16, 1944. Photo taken from USS Joyce (DE-317). The twenty-one ships of Convoy CU-21 included SS Pan Pennsylvania, at the time the largest tanker in the world. CU convoys brought fuel to the United Kingdom after the North African landings had taken temporary priority. She carried 140,000 barrels of aviation fuel, fifty crew, and thirty-one armed guard. Board for the United Kingdom, CU-21 left New York on April 15, 1944. Her escort included five Destroyer Escorts of Task Group 21.5 of Escort Division 22 operated by United States Coast Guard crewmen, and one United States Navy Destroyer Escort. As the convoy formed up in the Atlantic Ocean off New York City, SS Pan Pennsylvania took her station in the convoy. U-550, commanded by Kapitanleutnant Klaus Hanert (February 1, 1918 - ????), had been lying in wait off New York for just such a target. She was one of only three U-boats to brave the long-range aircraft and hunter-killer task groups to attack Allied shipping. A single torpedo struck SS Pan Pennsylvania at 0800 Hours, and without waiting for the ship to stop, her crew began to launch lifeboats, which flipped over into the frigid water. USS Joyce (DE-317) and USS Peterson (DE-152) picked up fifty-six survivors, but twenty-five were lost. USS Gandy (DE-764) screened the warships during the rescue. Hanert attempted to avoid detection by holding a silent position near the sinking tanker. Unable to hear the Allied ships with the tanker capsizing near him, U-550 began to move away at 0950 and was immediately hit by thirteen depth charges from USS Joyce; eleven detonated. U-550's engineer later told interrogators, "We waited for your ship to leave; soon we could hear nothing so we thought the escort vessels had gone; but as soon as we started to move--bang!" U-550 surfaced 2,000 yards (1.8 kilometers) off Joyce's stern. Joyce and Peterson opened fire, killing many Germans as they tried to get the U-boat's guns into action. They only stopped firing when Gandy moved into position for ramming, blocking their fire. Gandy rammed U-550 near the conning tower. After Gandy was clear, all three escorts opened fire again; U-550 was able to fire a few rounds from their 20mm (.78 caliber) aft cannon on the conning tower. It's unclear if Gandy's fire set SS Pennsylvania Sun aflame or if the torpedo hit caused the gas to burn, but the tanker spewed black smoke to the horizon. It would be sunk by Allied forces two days later. Later disputed by the German survivors, Joyce sighted a flare and interpreted it as a surrender. She pulled alongside and prepared to board her. Thirteen crew members, including Hanert, were pulled from the sinking U-550, but not before they set scuttling charges. Joyce's crew heard an explosion; forty-four of her crew of fifty-six were killed in the gun battle or unable to get out. Thirteen survivors were pulled aboard Joyce but machinist Heinrich Wenz died of his wounds and Joyce's commanding officer, United States Coast Guard Lieutenant Commander Robert Wilcox, conducted his burial at sea. Lacking a German Naval Ensign an American flag was used. The twelve remaining survivors were taken to the United Kingdom for internment for the rest of the war. CU-21 proceeded across the Atlantic without further attacks.