Dual-fuel machinery embodied a conservative design compromise

The class displaced 3,350 long tons and carried six 4-inch guns, four 3-pounders, and two submerged 21-inch torpedo tubes. The ships measured 123.4 metres overall, with a 12.6-metre beam and 4.3-metre draught, and carried 317 officers and sailors. Twelve Yarrow boilers supplied two Parsons turbine sets driving four shafts, which produced 25 knots and a range of 4,260 nautical miles at 10 knots. Protection consisted of a curved deck about half an inch thick on the flat and 25 millimetres on the slopes, plus a four-inch conning tower of uncertain practical value. During the First World War the ships gained four more 4-inch guns and a 3-inch anti-aircraft weapon. The anti-aircraft gun was replaced by another 4-inch gun in 1918, although sources disagree over the precise final fit. Their mixed coal-and-oil fuel supply guarded against oil shortages, but it also left an opportunity to test an all-oil design and higher-pressure boilers unused. Conservative procurement protected lives and public money, yet a two-ship class offered room to accept more technical risk.