Paeanblack said:Carriers.
Their only use is obtaining a beachhead on a totally unfriendly continent. Once you have a beachhead and rebase your airforce, carriers are just eye-candy.
The problem is that building enough carriers+fighters to crack that first city is way more expensive than building the requisite number of disposable marines/artillery/barrage-tanks to do the same job.
You need to be trying to establish beachheads on many separate, unfriendly continents that have no friendly cities within bomber range for the construction of a carrier task force to be worthwhile. Maybe it's just how I play, but I usually keep a few friends around.


Carriers (with their fighters) are the queen of the ocean and allow power projection and force multiplication that no other unit can bring to the game. If there are enemy coastal cities then a carrier task force is usually the quickest way to take them out. Wars should be short and use overwhelming force.
Force Multiplication
You don't need battleships to counter the AI fleets although a few do help. Your battleship versus his battleship is pretty much a 50 / 50 gamble and the same for destroyer versus destroyer. But strike the enemy battleship with 3 or 4 fighters from your carriers and the odds are pretty much 100 / 0 and even a destroyer can usually beat the battered battleship at say 99 / 1 odds.
When you want to take a coastal city just embark your Marines on the transports, send the destroyers to lower the defence and stike at the defenders down to half strength with the carrier fighters, attack with the marines from the transports... the city is yours and you are unlikely to have lost a unit. Land other defencive units to hold the city.
Next turn sail, to next coastal city and repeat. Three turns after I declare war I have 3 enemy coastal cities in my control (or razed if I can't hold them). That is what a carrier force can do for you. On the other hand, land forces in the age of the carrier that you use, are slowed down by the enemy culture and the need to rebase bombers.
The key advantage of the force multiplication is that my units will survive the combat and hence gain experience and produce lower war weariness, while your units doing the same tasks are gambling at bad odds and take casualties. That means they don't get better unless very lucky and you gain a lot of war weariness. War weariness is very expensive in lost gold and beakers if you have to raise the culture slider to counter it. The cost of your "expendable unit" strategy can be more than just the lost hammers.
Hammer cost and logistics
Simple comparison of the total hammer costs is misleading anyway. The carrier is a mobile platform that should never really be fighting itself. It is its fighters and jet fighters that do the work. Unlike a battleship that has to fight and once damaged is at risk. Unlike tanks and marines fighters don't get promotions so it doesn't matter where they're built or what civics you run.
I build my fighters in cities that can't build anything else useful like small fishing villages or captured enemy cities with low production and I use the whip to hurry them

If you lose a battleship, a tank or marine its replacement has to be transported to the frontline and that takes several turns even using airports. A fighter lost from a carrier is replaced this turn and ready to go again next. For me the carrier is the most flexible and useful of all naval forces since it is the only one that can (through its fighters) weaken enemy forces and shift the combat odds in my favour.
Maybe you'll look at carriers a little differently in future. At least give them a go at how they enable coastal cities to be taken as I described above.