The "Implacable"-class ships had a flight deck protected by of armour. The sides of the hangars were either or . The ends of the hangars were protected by 2-inch bulkheads and the armour of the hangar deck ranged from in thickness. The waterline armour belt was thick, but only covered the central portion of the ship. The belt was closed by 1.5 to 2-inch transverse bulkheads fore and aft. The underwater defence system was a layered system of liquid- and air-filled compartments as used in the "Illustrious" class. The magazines for the 4.5-inch guns lay outside the armoured citadel and were protected by 2 to 3-inch roofs, 4.5-inch sides and 1.5 to 2-inch ends.