U.S. Navy battle efficiency pennant, given to ships which win the annual fleetwide competition for sustained superior performance in an operational environment.  They also paint a white "E" on the ship superstructure.  It is displayed beneath other award flags, if any.  If the same ship wins a consecutive award, the pennant changes to the special battle efficiency pennant (and a gold "E").  For more information, see here, here, or U.S. Navy document NTP 13(B) [1], section 1605.
Own work
Carl Lindberg