The G3-derived system put an outer airspace and an inner floodable buoyancy space ahead of a protective bulkhead. About 2,870 tons of seawater could be admitted in wartime, increasing draft by 23 inches and reducing speed by about one-third of a knot. Nelson lacked the G3's compressed-air system for blowing the water out, so pumps and flooding arrangements had to do the work. Light framing and venting details were meant to manage the blast before it reached the inner hull. Keeping the spaces dry in peacetime avoided a permanent draft, speed, and fuel penalty.
Notable Point · 41:57-43:12
Nelson's water-filled underwater protection
Nelson used internal air and buoyancy spaces that could be flooded in wartime to absorb torpedo blast without carrying the speed and fuel penalty continuously.