 The ship 's armor was constructed with nickel steel . The main belt was 460 mm ( 18 in ) thick amidships , and tapered down to 250 mm ( 9 @.@ 8 in ) at the lower edge . On either end of the central citadel , the belt was reduced to 305 mm ( 12 @.@ 0 in ) at the waterline and 250 mm on the lower edge ; the belt extended for the entire length of the hull . Above the belt was 101 mm ( 4 @.@ 0 in ) thick side armor . The main battery guns were protected with 380 mm ( 15 in ) of armor , and the secondary turrets had 101 mm thick sides . The main armored deck was 69 mm ( 2 @.@ 7 in ) thick . The conning tower had 230 mm ( 9 @.@ 1 in ) thick sides . 
