Why six third rates would not replace the 1812 frigates

What if the early US Navy had built six third-rate ships of the line instead of its large 24-pounder frigates?

The third rates could challenge local blockaders but would be slower and easier to counter with larger British squadrons; the United States needed both heavy ships for cover and frigates fast enough for commerce raiding.

Six ships of the line would have posed a more serious local threat, but the Royal Navy could respond by concentrating more and larger ships. American frigates were valuable because they could escape blockades, use shallower waters, and raid commerce; third rates could not perform that mission as freely. A balanced force might use heavy ships to cover frigate breakouts, but Congress was unwilling to fund the scale and upkeep needed to contest British sea control.