 The forecastle deck was 25 mm thick while the upper deck was 155 mm ( 6 @.@ 1 in ) over the citadel . Below it , the 50 mm middle deck acted as a splinter deck . The upper deck was 100 mm thick above the 220 mm waterline belt extension . The bottom edge of the forward splinter belt met with a 65 mm ( 2 @.@ 6 in ) arched deck . Another arched deck of the same thickness covered the stern aft of the rear transverse bulkhead . 
