It doesn't matter if you're playing MP or SP, land or naval, if you're using only melee units, or only ranged units, you have a lot more problems that dealing with an enemy keeping their naval units out of reach of your Turtle Ships.
No, it's not "always better to attack with frigates". Frigates can't capture enemy ships like the privateer can. Frigate's can't capture the city like Turtle ships and Privateers can. Actually it's always better to use the correct unit for the job.
You made some good points Monthar but they aren't enough to convince me.
The biggest advantage of Frigates is they can be accessible right when Navigation is researched thanks to the upgrading path. I don't repeat enough in all my posts about warfare how much speed is so important in these kind of games.
I do build privateers and other units to ''mix up'' the whole thing(a good warmonger will prepare the right combination of units to make maximum damage in the shortest time possible) but most importantly is you don't want to lose units before they gain more.
Turtle ships will only serve if i really need to defend against a horde of privateers/frigates but again it will be for mp only(you know like me that the IA is too dumb to make a well coordonated attack) and at deity it's still useless because you can reach Navigation before any AIs anyway.
I will spend my stock of iron for frigates before starting privateers. I'm not talking about 2-3 ships, i'm talking about carpet of ships where the defender is so stumped that he just can't do anything about it.
I played 3 naval games so far and they were a walk in the park for all of them.
1-find iron
2-settle them
3-build trireme/compass ship(dont remember the name)
4-upgrade
5-build privateers when all iron is used
Strongest and fastest way to win naval games into ren. era. Privateers will be engulfed by all these frigates attacking from nowhere
Edit : War Chariot is dead(updated the OP)