Beginning in March 1943, one visually very significant change was authorized for ships then in the early stages of construction. This involved lengthening the bow above the waterline into a "clipper" form. The increased rake and flare provided deck space for two quadruple 40mm mounts; these units also had the flight deck slightly shortened forward to provide better arcs of fire. Of the Essex-class ships laid down after 1942, only Bon Homme Richard followed the original "short bow" design. The later ships have been variously referred to as the "long-bow units", the "long-hull group", or the "Ticonderoga class". However, the U.S. Navy never maintained any institutional distinction between the long-hull and short-hull members of the Essex class, and postwar refits and upgrades were applied to both groups equally. Less immediately visible aspects of the March 1943 design modification included a safer ventilation system, the Combat Information Center moved below the armored deck, two flight-deck catapults and the elimination of that on the hangar deck, and a third Mk 37 fire-control director; some of these changes were also made to short-bow ships nearing completion or as they returned to the yards.
Modifications were made throughout the Essex building program. The number of 20mm and 40mm anti-aircraft guns was greatly increased, new and improved radars were added, the original hangar deck catapult was removed, the ventilation system was substantially revised, details of protection were altered, and hundreds of other large and small changes were executed. In the meantime, earlier ships were continually modified as they returned to the yard for repair and overhaul. For example, Intrepid, one of the first to be commissioned, by the end of the war had received two H-4B flight deck catapults on place of her original single H-4A; three quad 40mm mounts below the island to starboard, three more on the port side and one additional on both the starboard quarter and the stern; 21 additional 20mm mounts; SM fighter-control radar; FD Mk 4 radar replaced with Mk 12/22; and an enlarged flag bridge. In fact, to the skilled observer, no two ships of the class looked exactly the same.
Read more about this topic: Essex Class Aircraft Carrier