As "runways at sea", aircraft carriers have a flat-top flight deck, which launches and recovers aircraft. Aircraft launch forward, into the wind, and are recovered from astern. The flight deck is where the most notable differences between a carrier and a land runway are found. Creating such a surface at sea poses constraints on the carrier – for example, the fact that it is a ship means that a full-length runway would be costly to construct and maintain. This affects take-off procedure, as a shorter runway length of the deck requires that aircraft accelerate more quickly to gain lift. This either requires a thrust boost, a vertical component to its velocity, or a reduced take-off load (to lower mass). The differing types of deck configuration, as above, influence the structure of the flight deck. The form of launch assistance a carrier provides is strongly related to the types of aircraft embarked and the design of the carrier itself.
What design feature do aircraft carriers feature to hinder launching and recovering aircraft?