The Aircraft Carrier is absolutely necessary for anyone intending to fight overseas. Early planes in particular do not have enough Movement to re-base across an ocean, so an Aircraft Carrier can dramatically boost the mobility of your air units.
Aircraft Carriers can attack, but only in melee. However, their Combat Strength is low compared to other naval units of the same era, so use it as it is designed: to carry aircraft across the oceans, and maybe once in a while, to deal the last hit to capture a city. Do not stick them in the middle of the action; always escort them with other ships, or just park them somewhere in the deep ocean and let your planes make quick work of everything in their massive range (and remember, if the Carrier is destroyed, all aircraft on board will be lost as well). Usually, the planes are much more important than the additional hit the Carrier itself is able to lend to the fight. Unfortunately, this means that the Aircraft Carrier will not have many chances to gain XP. What you can do is try to wear down enemy ships with your main fleet, then finish them off with the Carrier when safe.
Air fighter units can still deploy away from the base Aircraft Carrier like normal. Additionally, fighter units' deploy range will dynamically change to adapt to the position of their mother ship when the Carrier moves, granting these airplanes a drastic increase in mobility.
Luckily, despite having the lowest Sight of all late-game naval units and the fact that air fighter units cannot deploy into the fog of war, the Aircraft Carrier will adopt the highest Sight value among all airplanes on board. However, since the Sight of aircraft is always smaller than their Movement, it is helpful to bring along a "spotter" to help fighter units deploy farther away from their mother ship if needed.
It was likely inevitable that someone would have the insane idea of flinging an airplane off a moving ship at sea. That someone was Commander Charles Samson of the British Royal Navy in 1912 AD from the deck of the HMS Hibernia, following the example of the American Eugene Ely, who had been the first to takeoff from a stationary ship (USS Birmingham in 1910) and later land on one (USS Pennsylvania in 1911). Then, despite the nay-sayers, came the HMS Ark Royal, originally laid down as a merchant ship but converted to a “flat-top” for launching planes; she was commissioned in September 1914, just in time to serve throughout WW1. By the late 1930s, various navies – notably the Japanese and the American – had designed large aircraft carriers, meant to bring airpower to bear across the expanses of the Pacific against those behemoth battleships that ruled the waves. Soon enough, they issued in the end of the battleship, commencing with the Japanese attack on Pearl Harbor in 1941. These days, naval task forces are centered on the carriers, with every other ship dancing to their tune.
- The Aircraft Carrier's model is based on the Yorktown-class aircraft carrier.