The Imperieuse class carried substantial protection on paper, including a ten-inch waterline belt, nine-inch bulkheads, eight-inch barbettes, and a two-to-four-inch deck. Gun shields were two inches thick, and the conning tower received nine inches of armour. Excess displacement left much of the nominal waterline belt below the surface, where it could not protect the exposed side as intended. In practice, the armoured deck did more of the protective work. The result resembled a protected cruiser in some respects, despite the formal armoured-cruiser classification.
Chapter · 7:10-7:47
When an armoured belt sits below the waterline
Excess displacement submerged much of the ten-inch waterline belt, so the protected deck carried more of the defensive burden.