Pirates changed their ships as easily as they changed their captains.
In the early days of the Age of Piracy, the changes were numerous and rapid. The outlaws began with a canoe or rowing boat, and used it to take a larger boat: with this they took a small ship, and with the small ship they took a big ship.
They kept this ship until they happened to take a better one.
These pirates sailed under flags of their own design. Interestingly, the pirates' usual black flag and death's head was called a Roger.