 Yorktown was powered by two triple @-@ expansion steam engines which each drove one of the pair of 10 @.@ 5 @-@ foot ( 3 @.@ 2 m ) , three @-@ bladed screw propellers . The cylinders of each engine were 22 , 31 , and 51 inches ( 56 , 79 , and 130 cm ) in diameter and had a 30 @-@ inch ( 76 cm ) stroke . Each engine was rated at 3 @,@ 400 indicated horsepower ( 2 @,@ 500 kW ) and together were designed to move the ship at 16 knots ( 30 km / h ) , though the ship exceeded that in her trials . 
