Gunther Prien was lost with all of the crew of U-47 during March 1941. The German U-Boat ace had been responsible for sinking the Royal Oak and the Arandora Star. Post war research suggested that he had been sunk by HMS Wolverine on 7th March. Later research has suggested that HMS Wolverine sunk another U-Boat that day and the exact circumstances of the loss of U-47 may never be known. One possibility is that U-47 was sunk by H.M.S. Arbutus and H.M.S. Camellia during an attack that was not claimed as a confirmed kill.
H.M.S. Arbutus and H.M.S. Camellia did make a confirmed kill on 7th March, U-70 was the subject of sustained depth charging in a combined attack. Naval Intelligence was able to piece together a detailed sequence of events from subsequent interrogations of prisoners:
At 0843 on 7th March, 1941, H.M.S. “Arbutus” received a signal from H.M.S. “Camellia” to the effect that the latter had sighted a U-boat on the surface, but that her Asdic had failed. “Camellia” carried out an attack by eye when the U-boat dived. “Arbutus” closed the position and obtained an echo at 0925.
At 0927 she fired a pattern of six depth charges, settings 500, 250, 350 – 500 – 350, 250. At 0937 “Arbutus” fired a second pattern with the same settings and dropped a calcium flare. At 0945 “Camellia” carried out an attack. According to the Germans, the earlier depth charges were some distance away and did no damage.
At 1123, 1134 and 1146 “Arbutus” carried out further attacks with the same depth charge settings as previously. According to prisoners the U-boat maintained a depth of 80 metres (262 ft.). Her crew thought that “U 70″ had an air leak, and that bubbles of air rising to the surface were giving away their position to the British. The depth charge attacks caused more and more damage, water began to enter the U-boat, and when this became serious four men were detailed to work the hand pump; they pumped for a while whenever depth charges were dropped, and not during the intervals, as they feared that the noise of the pumping might be heard by the hunting craft.
In order to save electricity all lights were extinguished except those vitally necessary. Prisoners denied that the electric motors or batteries suffered damage. The U-boat went to 90 metres (295 ft.), and then to 100 metres (328 ft.). More and more water entered. It was realised that to escape by increasing speed was out of the question, and an attempt was made to lie motionless until evening, when there might be a chance of escaping in the dark on the surface. After rising to 80 metres (262 ft.), the U-boat found herself 10 minutes later as deep as 120 metres (393 ft.), where she managed to remain for some time. She then apparently got out of control.
She sank deeper and deeper, the indicator hand of the depth gauge showed an increasing depth down to 200 metres (656 ft.), the deepest limit marked, and then went on until stopped by the check. Ominous cracking sounds were heard, the paint flaked off the sides, and the crew thought that the hull would be crushed by the tremendous pressure of water. The supply of compressed air was by that time reduced to 25 kilograms; this was used in a last attempt to rise; the U-boat had gone down by the stern, and was now at an angle of 45 degrees. Most of the instruments had been smashed and the damage was described as amounting to almost complete wreckage of the interior of the U-boat.
A considerable quantity of water had also entered through the hull abaft the conning tower, where the U-boat had been damaged by the ramming. The crew were huddled together in the forward compartment. The U-boat came slowly on to an even keel. A desperate attempt was made to hold her level, both electric motors were set at full speed ahead, and all tanks were blown. At 1244 “Arbutus” made her seventh attack with her four remaining depth charges, settings 350-500-350, 500. “U 70″ was then trying to rise and the depth charges exploded above her; she succeeded, however, in surfacing.
The Captain opened the conning tower hatch and six men were said to have been thrown out by the tremendous excess pressure inside the U-boat.
H.M.S. “Arbutus” opened fire and claimed to have hit the base of the conning tower with 4-in. H.E. Well-directed pom-pom fire prevented any attempt on the part of the Germans to man their gun. Prisoners denied that “Arbutus” fire hit “U 70.” “Arbutus” turned to ram the U-boat, but found ramming unnecessary, as the Germans were jumping overboard. She just missed “U 70,” dropping two Carley floats near the crew, and closed the U-boat which was still proceeding with her forward hydroplanes up.
The Petty Officer Telegraphist admitted that it was not possible to send any signal announcing their fate. He added that the German Naval Authorities would assume from the cessation of signals that something had happened to “U 70.” The Germans maintained that they made sure of the sinking of their U-boat by opening the vents before they abandoned ship. “U 70″ sank at 1318; she still had five torpedoes, having fired seven of her total of 12.
“Arbutus” picked up 26 survivors who included the Captain and three other officers; two officers and 18 men were lost. Prisoners stated that “U 70″ was to have ended her cruise at St. Nazaire.