That the US Navy only had 4 ships in the entire fleet that it own every bit of, and not leased in the 1980s. This was because the US kept 4 Destroyers that were sold to the Shah of Iran, but were not delivered in time before the students took our embassy personnel hostage for 444 days. So we kept the ships the Shah bought, as part of the package of payback.
Most ships have their reduction gears, the things that take the high speed of the engine down to the much slower speed of the propeller, on 99 year leases from companies such as Westinghouse.