The USS Yorktown was a U.S. Navy escort airplane carrier built and launched during the Second Great War. She was a converted freighter whose superstructure had been removed and replaced with a flight deck and small starboard island. She carried 30 airplanes including divebombers, torpedo carriers and fighters. As such, her complement was about half that of a fleet carrier. Given her origins as a freighter, she couldn't do more than 18 knots as compared to a regular warship's 30.
In 1943 the Yorktown and her sister ship, the Tripoli, joined two other escort carriers already at Pearl Harbor in preparation to drive the Japanese from Midway. Shortly after, the fleet sailed and discovered that the Japanese had already abandoned Midway.