When oil and small-tube boilers became credible fleet choices

When did all-oil firing and small-tube boilers become realistic choices for Queen Elizabeth successors?

Oil became sensible once Britain funded the required stockpile and infrastructure. Agincourt appears to have been a later opportunity for small-tube boilers, while the Revenge class and Renowns retained older machinery.

Before Britain invested in oil stocks and import infrastructure, Welsh coal was the dependable domestic fuel supply. By the time oil became easier to justify, the Queen Elizabeth design was too far advanced to recover all the space and structure originally devoted to coal. Roughly 900 tons could consequently have been used more efficiently. Agincourt appears to have incorporated small-tube boilers, while the Revenge class did not and Renown and Repulse retained that limitation. Many, though not all, of the boilers required for Courageous and Glorious were nevertheless available.