Action of 4 April 1941 The Action of 4 April 1941 was a naval battle fought during the Atlantic Campaign of the Second World War . A German commerce raider encountered a British auxiliary cruiser and sank her with heavy losses after an hour of fighting . Background . The " Kriegsmarine " auxiliary cruiser was raiding in the mid Atlantic in early 1941 . On that cruise , the Germans engaged two other British armed merchant cruisers ( HMS " Alcantara " and HMS " Carnavon Castle " ) in surface battles but they ended indecisively . So when " Thor " encountered , her crew were already battle tested and anxious to sink an enemy combatant . " Thor " was long and weighed , she was armed with six , two and four naval gun s along with four torpedo tube s . The raider also carried an Arado Ar 196 A - 1 floatplane for reconnaissance and had a complement of 349 officers and crewmen . HMS " Voltaire " was larger than the raider — displacing — but with a smaller crew of 269 men and officers . She had eight and two naval guns , including at least one anti - aircraft mount . " Thor " was returning to Germany when she found " Voltaire " heading to Freetown about southwest of the Cape Verde Islands . Action . It was about 06 : 15 on 4 April when crewmen of " Thor " spotted smoke on the horizon . Captain Otto Kähler assumed the vessel to be a coal burning ship so he altered course into the direction of the smoke . When the Germans were able to make visual contact with HMS " Voltaire " , they suspected she was a neutral ocean liner as she did not attempt to escape . The British — under Captain J . A . Blackburn — sighted the approaching Germans coming head on , so they fired a burst of anti - aircraft fire as signal for identification . A signal from " Thor " was not returned , but the British soon discovered the identity of the approaching ship at about 06 : 45 , when it replaced the flag of Greece with a German naval ensign and fired a shot across " Voltaire " s bow . The British responded by manning their guns and firing a broadside with their mixed armament , but to no avail ; they were outgunned and outranged . After only four minutes of dueling at around away , the Germans began striking " Voltaire " with their guns . The first shots entered the radio room and the generator room of " Voltaire " , heavily damaging the vessel , knocking out communications and steering gear and putting her into a list . Heavy fires also broke out and nearly covered the entire deck of the British ship . Despite the fire the Royal Navy gunners continued fighting for nearly an hour . For the next several minutes , the two sides fired ; only one British shot managed to hit " Thor " , and it caused no casualties , the shot tearing off some radio equipment attached to the main mast . By 07 : 15 , only two of the British guns were in action , while " Thor " circled around " Voltaire " , firing rapidly . At 08 : 00 , the German ' s guns overheated so Captain Kähler decided on a torpedo attack to end the engagement . But just as " Thor " was lining up to fire a spread , a white flag was observed aboard " Voltaire " and so the firing ceased . Captain Blackburn — having lost 72 men killed in action — gave the order to abandon ship and for the next five hours the Germans rescued 197 survivors , two of whom died later on and the rest became prisoners of war . Kähler also recorded that half of the rescued sailors were wounded . After the battle , " Thor " continued on to Germany to refit for a second raiding voyage . She had fired 724 rounds in a 55 minute battle , more than 50 percent of her ammunition .