The need for monitors for shelling enemy positions from the English Channel had become apparent only at the start of the war and they were designed with some haste.

For machinery the two monitors received diesel engines, which were then a novelty – the majority of ships being steam powered.

The use of diesels meant that they had no need of boiler rooms which went well with a low draught, nor of large funnels which reduced the amount of superstructure.

These engines were originally designed for much smaller freighters and therefore they proved particularly slow and unreliable.

The turret was on multi-sided barbette made of individual flat plates, cutting down on the build time.