Looking at the new tech tree, it seems frigates are enabled with Chemistry. There's also a naval unit enabled with Steel (the advance following Chemistry). I hope this naval unit not to be an Ironclad... One of my big rants in Civ series is Frigates getting obsolete too soon...
...or maybe instead of Ironclad the unit is a privateer
...or maybe instead of Ironclad the unit is a privateer
