 By early 1939 it was clear that the first two Lion @-@ class battleships could not be delivered before 1943 at the earliest and that further battleship construction would be necessary to match the German and Japanese battleships already under construction . The main constraint on the construction of any new battleships was the limited available capacity and the time required to build large @-@ calibre guns and their gun turrets . Using four existing twin 15 @-@ inch mountings offered the possibility of bypassing this bottleneck and allowed the construction of a single fast battleship more quickly than building more Lion @-@ class ships . The turrets were originally built for the battlecruisers Courageous and Glorious during World War I and were removed during the conversions of these ships to aircraft carriers in the 1920s . To save time , the Lion design was modified to accommodate the four turrets , and preliminary design work began in July 1939 . The square or transom stern was retained as it was estimated to improve speed at full power by .33 knots ( 0 @.@ 61 km / h ; 0 @.@ 38 mph ) . This made Vanguard the only British battleship built with a transom stern as the Lions were never finished . 
