A monitor designed around two 15-inch guns

At about 8,000 tons standard and 8,500 tons fully loaded, the monitors combined a shallow draft with a very broad hull. Each ship was about 123 metres long and 27 metres in beam, with a draft of roughly 3.5 metres. Four oil-fired boilers and reciprocating engines drove two shafts, usually moving the ships at about eight knots and at most twelve. Their decisive feature was the pair of 15-inch Mark I guns around which the hull was arranged. That trade-off made sense because the guns could outrange most land artillery and gave the ships a specialised fire-support role rather than a fleet-combat role.