 The waterline belt of the Lexington @-@ class ships tapered 7 – 5 inches ( 178 – 127 mm ) in thickness from top to bottom and was angled 11 ° outwards at the top to increase the armor 's relative thickness to horizontal , close @-@ range fire ( a concept taken from HMS Hood ) . The down side to this practice was a reduction of the relative height of the belt , which increased the chance of plunging shellfire going over or under it . The belt covered the middle 530 feet ( 161 @.@ 5 m ) of the ships . Forward , the belt ended in a bulkhead also tapered from seven to five inches in thickness . Aft , it terminated at a seven @-@ inch bulkhead . This belt had a height of 9 feet 4 inches ( 2 @.@ 8 m ) . The upper deck was 2 @.@ 25 inches ( 57 mm ) thick in two layers . The third deck over the ships ' machinery and magazine was armored with two layers of Special treatment steel ( STS ) totaling 2 inches ( 51 mm ) in thickness . The steering gear , however , was protected by two layers of STS that totaled 3 inches ( 76 mm ) on the slope and 4 @.@ 5 inches ( 114 mm ) on the slope . 
