Charley - actually the 'horrible old Leopard' was a 50gun ship, a small two decker, not a frigate. These ships were used on detached duty, or on convoy escort, or sometimes as a flagship on a detached station where there were only small frigates or unrated vessels used.
Originally this class of vessel was part of the line-of-battle, but for many years they had been too small. The last of them built for the Royal Navy were built during the American Revolution (as we like to call it!) as they could work closer inshore than a 74 and it was unlikely that they would have anything bigger to fight. Which was true until the rest of Europe joined in!
I agree with you entirely about the Aubrey/Maturin books. They are the finest of their kind. Little Hawk, I like the conversations because of the flavour of the language used, which seems to fit the period better than any other books on the same subject. O'Brian's command of the subject was alao without equal.
I'm sorry to go on about all this, but the Napoleonic Royal Navy is my special interest, as by groaning bookshelves will bear witness.**BG**