First, second, and third rates were all battleships of their age

What steam-and-steel warship types correspond to the age-of-sail rating system?

First through third rates were all battleships in function, while frigates carried out the cruising role and fourth rates sat between those categories.

A one-to-one analogy in which only first rates count as battleships does not work. First, second, and third rates were all ships of the line built for fleet battle, with first rates representing the prestige peak. Fourth rates occupy a less tidy heavy-cruiser or battlecruiser-like space, while fifth rates and smaller frigates performed the cruising role. Mission translates across eras more reliably than gun counts.