She was powered by four-cylinder triple-expansion steam engines, driving two shafts, which developed a total of 23,650 indicated horsepower (17,640 kW) and gave a maximum speed of 23.3 knots (43.2 km/h; 26.8 mph).

[3] Because of the extra topweight of the turrets in comparison to their half-sisters their stability was reduced which made them very good seaboats and steady gun platforms because they did not roll as much.

[9] Achilles, accompanied by her sister Cochrane, and three other armoured cruisers were sent to reinforce the defences of the Shetland Islands on 2 August 1914, days before the start of the First World War.

The Germans planned to bombard the port of Sunderland on 19 August, with extensive reconnaissance provided by airships and submarines.

The Germans broke off their planned attack to pursue a lone British battle squadron reported by an airship, which was in fact the Harwich Force under Commodore Tyrwhitt.

[12] On 16 March 1917, Achilles and the armed boarding steamer Dundee were patrolling north of the Shetland Islands when they encountered the disguised German auxiliary cruiser Leopard.

Dundee retaliated by raking Leopard's stern, badly damaging the German ship and then Achilles opened fire herself.

[13] Achilles was transferred to the North America and West Indies Station in August 1917 for convoy escort duties,[14] but returned to Britain for a refit between February and December 1918.