[6] The Powerfuls had proved costly and demanding in manpower, and the new class had to represent a more economical solution, that could be built and operated in greater numbers.

[5][7] Although the powerplant used Belleville boilers, which were of the then-new water-tube type, most of the ships enjoyed the reputation of good steamers; the exception was Niobe, whose engines gave persistent trouble.

The first four ships were troubled by vibration; to remedy this, the engine rooms of the last four were rearranged; the new configuration also provided a 1,500 hp (1,100 kW) increase in power.

In general the Diadems served most of their careers in home waters, although there were some deployments to China Station, the Mediterranean and the Caribbean.

However, with the outbreak of the First World War, several of them were reactivated, initially for patrol duties with the 9th Cruiser Squadron in the Eastern Atlantic.

By the end of the war the survivors had again been withdrawn from active service, and all of them had been sold for scrap by 1932, with the exception of Andromeda, which served as a boys’ training ship until 1956.