Unicorn's maintenance role and the treaty's auxiliary category

The Second London Treaty restricted auxiliaries to 28 knots, barred guns larger than 6.1 inches, and permitted no more than eight guns larger than 3 inches. Its definition also excluded torpedo armament, armor protection, and ships designed primarily to operate aircraft at sea. Unicorn's original design included a structure in the middle of the flight deck, consistent with her designation as a forward aviation maintenance ship. Her 24-knot speed, eight 4-inch guns, and lack of torpedoes fitted those particular auxiliary limits. Construction changes removed the central obstruction, and the completed ship carried limited armor on the flight deck and around the magazines, so the original classification needs to be distinguished from the ship as completed. Unicorn came from the same design study as Illustrious and became a template for later light fleet carriers.