At 01.03 hours on 29 May 1942 the unescorted Norman Prince (Master William Richard Harries) was hit on the starboard side amidships in the engine room by one G7a torpedo from U-156 while steaming at 8.5 knots in fine weather about 70 miles west of Martinique. The U-boat had first spotted the ship eight hours earlier and immediately tried to attack submerged, but came too close and had to overtake it on the surface to get into a favorable attack position again. The explosion threw up a tremendous amount of water and debris, blowing the starboard lifeboat away and covering the motor boat with wreckage including one of the davits from the starboard boat. The main steam pipes were shattered and the ship settled by the stern on an even keel after the engine room and stokehold were flooded. The crew aft was under the impression that they were sinking quickly and one of the gunners (the ship was armed with one 4in, one Bofors and four machine guns) lowered the forward fall of the port lifeboat, the only remaining boat, without orders. An able seaman reacted quickly and released the after fall, which prevented the boat from being swamped when it reached the water. Only the third officer and ten of the crew were able to get in it before it had to cast off owing to the heavy swell. The men still on the ship tried to move the motor boat but to no avail, so they launched all rafts except the one on the stern. No distress signal could be sent as the main dynamo and radio shack were wrecked, but the emergency set was put on a raft together with the second radio officer. At 01.37 hours, U-156 fired a coup de grâce at the vessel before leaving the area without questioning the survivors. Its track was seen and reported by the purser before the torpedo struck on the port side between #2 and #3 hold when the men were still struggling to get the lifeboats and rafts into the water. The flooding of these holds caused the Norman Prince to sink very quickly almost vertically by the bow, followed by a boiler explosion. Many survivors still on deck were thrown forward when the stern rose into the air and only a few managed to jump overboard before the ship disappeared. The master, 13 crew members and two gunners were lost.

The chief officer had jumped from the poop and injured a knee when hitting some floating debris, but still managed to hand a lifebuoy to the master and found the second officer with a skull fracture clinging to some wreckage. They drifted away from each other in very rough sea and about one hour after the sinking the chief officer climbed onto a raft with the third engineer on board. By paddling around they eventually picked up six other crew members, while other red lights from life vests could be seen but not reached owing to the weather. At 03.00 hours, all men were transferred from the raft to the lifeboat in charge of the third officer which then held 25 crew members and seven gunners in very crowded conditions as it was designed to hold 24 persons. The raft was tied up astern and used as sea anchor to drift with the heavy swell until daylight. The boat searched the area in vain for survivors and then set sail, shipping much water and unsuccessfully trying to attract a US patrol aircraft by burning a smoke flare shortly thereafter. However, the survivors were picked up by the Vichy French steam merchant Angoulême after having sailed 40 miles in 26 hours in position 14°53N/62°55W and attracting her attention by morse signals and flares when spotting the lights of the neutral ship. They asked to be put ashore at Dominica or St. Lucia when learning that she was bound for Martinique, but they were landed there in the evening of 3 June. The chief officer, second officer, the second steward and one gunner were immediately taken to a hospital, while the remainder were transported to the military camp Fort de Balata on Mount Didier and interned for four months by the Vichy French authorities. While in the hospital the survivors met an injured German officer who had been put ashore on Martinique by a U-boat in February 1942. Unknown to them this was Lt.z.S. Dietrich von dem Borne, the former second watch officer of U-156, the same U-boat that had sunk the Norman Prince.

The only survivor not ending up in internment was the first radio officer Hubert John Tanner who had jumped overboard and was only about 30 yards away when the ship sank. He was swimming in the water with the master, the chief engineer and two stewards when the lifeboat was seen nearby and Tanner told them he would swim to the boat to get help. He swam half an hour to get closer and shouted but was obviously not heard, because it rowed away when he was within 15 yards. The radio officer hollered and cursed at them, however did not have the strength to swim back to the other survivors so he clung to a door, but rough sea and moderate wind made it tiresome to stay on it. About noon the next day he sighted three abandoned rafts in some distance and decided to swim to the largest one as he was thirsty and could no longer stand getting knocked off the door. He dove into the water and swam for 45 minutes despite being very scared of several sharks, but they did not bother him. Tanner discovered that someone had been on it as the stores were open and the flashlight taken out and he spent three days and three nights on the raft until attracting USCGC Unalga (WPG 53) by waving a piece of canvas after igniting two smoke flares failed in the morning of 1 June. The vessel had just received orders to discontinue her search for survivors when his raft was spotted in position 14°40N/62°15W. The radio officer was taken to Victoria Hospital in Port Castries, St. Lucia, to recover as his skin was scuffed off his arms, legs and backside.