The ships began as improved Lion-type battlecruisers but underwent reconstruction extensive enough to increase displacement and change their length, beam, propulsion, and protection. Their machinery evolved from British-built coal boilers with oil spraying through later Japanese systems to full oil firing. Their speed now helped them accompany and protect the Japanese carrier force rather than evade stronger opponents. The doctrinal change was clear: speed now served to close and fight instead of escaping direct confrontation.
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How the Kongō class became fast battleships
Major increases in displacement, dimensions, machinery, and armor transformed the Japanese Kongō-class battlecruisers into fast battleships suited to carrier operations.