Airships get thier lift from a enchanted piece of star metal in thier keel. The metal really just wants to go back up to the stars and slightly resist any form of movement other than up, so the ships need some other means of propulsion to move forward. Currently the Thousand Kingdoms use sail power.
There are rumors that the Feathered Empire also now has flying ships, but no one really believes it.
The current map of the plays ship looks like this:
(select for full size)