Ordered in 1811 and launched in 1819, Isis began as a 50-gun two-deck Salisbury-class design but was cut down during construction into a spar-deck frigate. Her later reduction to a 44-gun frigate, armed chiefly with 32-pounders and supplemented by 8-inch shell guns, shows how the traditional fourth-rate category was being absorbed into new large-frigate forms. The closing comparison turns to Type 22 frigates and Second World War Tribal-class destroyers, relatively scarce specialist-generalists able to step upward or downward when a fleet needed flexibility. Their value came from filling several fleet gaps in small numbers rather than becoming the mass-produced middle tier that the phrase general-purpose might suggest.
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HMS Isis marked the fourth rate's transition into large frigates
The last Royal Navy design officially ordered as a fourth rate was altered into a large frigate, anticipating later general-purpose ships that could step between roles.