HMS Royal Oak was one of five Revenge-class battleships built for the Royal Navy during the First World War. Completed in 1916, the ship first saw combat at the Battle of Jutland as part of the Grand Fleet. In peacetime, she served in the Atlantic, Home and Mediterranean fleets, more than once coming under accidental attack. Royal Oak drew worldwide attention in 1928 when her senior officers were controversially court-martialled, an event that brought considerable embarrassment to what was then the world's largest navy. Attempts to modernise Royal Oak throughout her 25-year career could not fix her fundamental lack of speed and, by the start of the Second World War, she was no longer suitable for front-line duty.
On 14 October 1939, Royal Oak was anchored at Scapa Flow in Orkney, Scotland, when she was torpedoed by the German submarine U-47. Of Royal Oak's complement of 1,234 men and boys, 835 were killed that night or died later of their wounds. The loss of the outdated ship—the first of five Royal Navy battleships and battlecruisers sunk in the Second World War—did little to affect the numerical superiority enjoyed by the British navy and its Allies, but it had a considerable effect on wartime morale. The raid made an immediate celebrity and war hero out of the U-boat commander, Günther Prien, who became the first German submarine officer to be awarded the Knight's Cross of the Iron Cross. Before the sinking of Royal Oak, the Royal Navy had considered the naval base at Scapa Flow impregnable to submarine attack, but U-47's raid demonstrated that the German navy was capable of bringing the war to British home waters. The shock resulted in rapid changes to dockland security and the construction of the Churchill Barriers around Scapa Flow. wiki