Why side-by-side replenishment overtook astern methods

When did the Royal Navy move from astern refuelling to side-by-side replenishment?

Both methods were tested before the war; alongside systems matured through late-1930s experiments and the urgent Pacific oiler build-up.

Astern and alongside methods both appeared in interwar experiments, including attempts to service several ships at once. The Royal Navy had explored both arrangements before operational demand settled the issue. European amphibious requirements converted some oilers into landing ships and reduced the pool available for fleet support. Indian Ocean and Pacific campaigns then exposed the shortage and forced a crash programme to rebuild oiler capacity and alongside techniques.