The Rolls-Royce Exe, or Boreas, was a 24-cylinder air-cooled X block sleeve valve aircraft engine intended primarily for the new Fairey Fleet Air Arm aircraft, particularly the Fairey Barracuda. The Exe was under development in 1939, having been started in the 1930s, along with the Peregrine and Vulture. The Exe was named after the River Exe, although Rolls-Royce later transferred the using of river names to its gas turbine engines.