The ships' underwater protection system was based on the system used in the "Dunkerque" class. The void between the armor belt and the outer hull plating was filled with a rubber-based compound referred to as "ébonite mousse"; the material was used to absorb the impact of an explosion and prevent water from flooding uncontrollably. Behind this compartment there was a bulkhead that was thick; the compartment created between this bulkhead and the "ébonite mousse" was used to store fuel oil in peacetime, but was kept empty in combat conditions. Further inboard, a torpedo bulkhead would contain the blast effects of a torpedo or mine detonation. This bulkhead was increased to abreast of the secondary magazines and the forward main battery turret, as the narrowing hull reduced the total width of the underwater protection system. Additional "ébonite mousse" was placed between the torpedo bulkhead and the fuel tanks for additional flooding control. The compound was also used in compartments on either end of transverse bulkheads to ensure that the central citadel remained watertight.