Why do aircraft carriers float?

Any object placed in water is pushed upward by a force equal to the weight of the water it displaces. A ship floats when it can displace its own weight before it is fully submerged, which is simply a matter of average density: steel is about eight times denser than seawater, but a hull shaped as a hollow box filled with hangars, fuel tanks, magazines and accommodation for 5,000 people has an average density well under one tonne per cubic metre. A Nimitz-class carrier weighs about 100,000 t fully loaded, so it must push aside 100,000 t of seawater, roughly 97,000 cubic metres. With a waterline length of about 317 m and a beam of 41 m, that volume is reached at a draught of around 11 m (37 ft).

Floating is easy; staying upright is the harder engineering problem. The flight deck and island sit high above the water, so designers keep the centre of gravity low with heavy machinery, reactors, aviation fuel and ballast deep in the hull, and the wide beam provides a restoring moment when the ship rolls. Carriers also have a bulbous bow and fine underwater lines despite the overhanging deck; the hull below the waterline is far narrower than the 77 m (252 ft) flight deck suggests. Trim is managed with ballast as fuel and stores are consumed so that stability stays within limits.

The hull is divided into hundreds of watertight compartments, so flooding is confined and the ship can lose a large part of its buoyancy and still float. Historical losses such as USS Yorktown in 1942 came from multiple torpedo and bomb hits combined with fires and progressive flooding, not from a single breach of the hull.