 The barbettes , gun turrets and the front of the casemates were all 6 inches thick while the sides and rear of the casemates were protected by 51 millimeters ( 2 @.@ 0 in ) of armor . The deck was 63 millimeters ( 2 @.@ 5 in ) thick and the armor protecting the conning tower was 356 millimeters ( 14 @.@ 0 in ) in thickness . The ships had 30 watertight compartments in their double bottom and an additional 136 or 137 between the bottom and the upper deck . 
