When completed, in December 1906, HMS Dreadnought was the most powerful battleship in the world and revolutionised naval power: The first all-big-gun battleship to enter service, and the first battleship to be powered by turbines. She was two and a half knots faster than her rivals and carried twice the firepower of earlier battleships. The generation of ships she replaced became the “pre-dreadnoughts”. The father of Dreadnought was Admiral Sir John Fisher, First Sea Lord, arguably the greatest man to fill that role. He ordered design studies for a battleship armed solely with 12-inch (305 mm) guns and a speed of 21 knots (39 km/h; 24 mph). Dreadnought was the first battleship to have a uniform main battery. Her launch sparked a naval arms race. Other navies, particularly the German Imperial Navy, rushed to match it as WW I approached. On 18th March 1915 she rammed and sank German submarine SM U-29, becoming the only battleship to sink a submarine. She played no further hostile part in the War and was reduced to coastal defence in the English Channel in 1916. She re-joined the Grand Fleet in 1918 only to be reduced to reserve in 1919 and sold for scrap in 1921.