Water armor and treaty displacement

The Royal Navy used seawater in torpedo bulges as a working part of underwater protection rather than merely as ballast. Water spread an explosion's force across a larger bulkhead area and could be carried in the bulges of battleships and carriers. Because it was not counted like permanent armor under standard displacement, the arrangement also conserved treaty-accounted tonnage. British testing near the end of the First World War helped establish the practice before the later treaty definition was negotiated. The result joined survivability engineering to an accounting rule that British designers understood how to exploit.