The fastest speed ever recorded for an aircraft carrier while underway at sea was achieved by the USS Enterprise (CVN-65). The USS Enterprise was a nuclear-powered aircraft carrier in service with the United States Navy. During the carrier's sea trials and speed runs in 1961, it reached a top speed of more than 40 knots (46 miles per hour or 74 kilometers per hour).
It's important to note that such high speeds are typically attained during special sea trials and are not sustained during regular operations. In normal operational conditions, aircraft carriers usually cruise at lower speeds to optimize fuel efficiency and extend the ship's range and endurance.