Huntington Ingalls Industries has proposed two "patrol frigates" for naval use, based on the NSC hull. Patrol Frigate 4501 is very similar to the NSC, the main differences being a modified stern ramp[21] and a knuckleboom crane replacing the overhead crane.[22] The crew is increased to 148,[21] and it was offered to the US Navy as a replacement for the Littoral Combat Ship;[22] the FY13 cost of an LCS was $446.3m compared to $735m for an NSC.[23]
Patrol Frigate 4921 is a more radical redesign with a crew of 141,[21] adding weapons and sensors at the expense of reducing range from 12,000 nautical miles (22,000 km) to 8,000 nautical miles (15,000 km).[22] It adds a 12-cell Mk56 VLS launcher for ESSM air-defense missiles, just behind the main gun which is upgraded from 57mm to a 76 mm Super Rapid.[21] Two quad launchers for Harpoon (missile) anti-ship missiles and a triple launcher for torpedoes are added to the stern.[22] It retains the SeaRAM/Phalanx CIWS and 6 machine guns of other NSC variants.[21] The stern is closed in and houses a towed-array sonar;[22] there is a hull sonar for mine countermeasures and an ESM suite.[22] The original "National Patrol Frigate" concept had an AN/SPY-1F air-defense radar[24] but by 2012 the PF4921 was being shown with an Australian CEAFAR radar.[21] This coincided with HII promoting an NSC variant for the Royal Australian Navy's upcoming Offshore Combatant Vessel project.[25] Brazil, Saudi Arabia and Germany have also shown interest in NSC derivatives.[21]
