Was it really Churchill that personally switched the Royal Navy from coal to oil?
Or was it simply inevitable after Rudolph Diesel invented his engine? Diesels were smaller, more powerful, safer, and more versatile than steam engines, so if England wanted to keep up with Germany, the choice was a natural one.
The engines that replaced the coal engines weren’t diesel engines. They were oil fired steam turbines.
Oil replaced coal in steamships long before significant numbers of large ships were powered by Diesel engines. Even supertankers used steam because they had plenty of space for boiler rooms.
The RN used oil to generate steam. Diesels were not powerful enough to move 35K T ships. The first successful big ship use of diesels were th Graf Spee class armored cruisers of the late 20s. They were used because of the range offered by the diesel over steam.
With the advent of the diesel engine, the change was inevitable. But a good politician will get in front of the parade and lead it. Maybe he did push it along, but it was just a matter of time before the change happened.