 In some ways the armour protection given to the Indefatigables was weaker than that of their predecessors as the armour was reduced in thickness in a number of places , but it was spread out to a greater extent . The main belt ran from stem to stern ; it was 6 inches ( 152 mm ) thick for the middle 298 feet ( 91 m ) of the ship , but was reduced to 4 inches ( 102 mm ) abreast the end 12 @-@ inch gun barbettes and magazines and thinned still further to 2 @.@ 5 inches ( 64 mm ) at the ends of the ship . A 4 @.@ 5 inches ( 114 mm ) bulkhead met the barbette of X turret while the forward bulkhead was 3 – 4 inches ( 76 – 102 mm ) in thickness . The gun turrets and barbettes were protected by 7 in ( 178 mm ) of armour , except for the turret roofs which used 3 in ( 76 mm ) of Krupp non @-@ cemented armour ( KNC ) . The supporting beams for the turret roofs were reinforced over those of the Invincibles based on lessons learned in firing trials conducted during 1907 . The thickness of the main deck was generally 1 in ( 25 mm ) of nickel steel , but increased to 2 inches ( 51 mm ) around the base of the barbettes . The lower deck armour of nickel steel was 1 @.@ 5 in ( 38 mm ) on the flat and 2 inches thick on the slope , but increased to two inches at the ship 's ends . The sides of the forward conning tower were 10 in ( 254 mm ) thick while the spotting tower had four inches . The roof and floor of both towers were KNC armour 3 inches thick while the conning tower 's communication tube was four inches of KNC . The torpedo director tower 1 inch of nickel steel all around . Nickel steel torpedo bulkheads of 2 @.@ 5 @-@ inch thickness were fitted abreast the magazines and shell rooms . The funnel uptakes were protected with 1 @.@ 5 inches of nickel steel on the sides and 1 inch on the ends . Krupp cemented armour was used throughout , unless otherwise mentioned . 
