Why water counted as cheap treaty protection

The scheme relied on air, seawater, and space between the hull and protective bulkhead. Air added no displacement, and seawater could be taken aboard only when war made it necessary. Under the Washington Treaty's standard-displacement definition, that water did not count as permanent ship weight. Nelson could therefore gain a substantial wartime torpedo-defence layer without declaring it in the treaty figure. The idea combined naval architecture with careful use of the accounting rules, though the crew still had to manage flooding and pumping in service.