 It was a resounding commercial success; there were 21 direct copies, and another six near-sisters, plus six near-copies (see List of gunboat and gunvessel classes of the Royal Navy).Known as the "flatiron" gunboats, these vessels had a single large gun kept behind hinged shields, rather than a complex disappearing mount.The simplified mounts were intended as much to lower the center of mass as to afford protection, and resembled a "lift battery."