These were large carriers with good armour, but to achieve that and keep within the 27,000 tons, the ships were only one hangar deck with 60 aircraft, compared to the two hangar decks on the Athena and Goliath classes. Of the three Poseidon class carriers, only the Poseidon received the major refurbishment planned for all three. With Hyperion and Demeter only receiving essential upgrades to keep them at sea as long as possible they were paired with either the Poseidon or one of the Goliath Class which had been fully upgraded.
The upgrade removed all of the original armament which was then replaced with 8 twin 4", 4 quad 40mm, 6 twin 40mm, and finally 2 twin 25.4mm. Radar, new homing equipment for the aircraft, catapults and many other items that had been invented for use on aircraft carriers since these ships were designed.
As Completed
Displacement: 27,000 tons normal, 34,500 tons full load.
Dimensions: 847 x 104 x 27 feet
Machinery: 4 shaft, geared turbines, 120,000shp
Speed: 31 knots
Endurance: 8,000 miles at 15 knots
Armour: 3" belt, 3" deck.
Armament:
8 x 5" (8x1)
8 x 75mm (8x1)
Aircraft: 60
Crew: