A two-ship force cannot guarantee one ready hull simply because the other is in refit. A ship leaving dockyard hands still needs its crew restored, its systems checked, and time to work up. An accident can also remove the ship counted as available. Reliable coverage may therefore require three hulls for each continuously deployable unit. The interwar Royal Navy had many cruisers, but worldwide commitments kept them at sea when major modernization demanded long dockyard periods. Wartime yards could turn out small Flower-class corvettes and Hunt-class destroyers faster than complex cruisers. New cruisers did not ease the pressure until roughly 1942, so immediate D-class service kept winning out over the proposed anti-aircraft conversion.
Chapter · 18:32-21:34
Why useful cruisers missed their overhaul
Worldwide commitments kept the D class in service, and even a two-ship force could not guarantee one fully ready hull while the other was in refit.