Nilgiri-class frigate

The Nilgiri-class frigates, formally classified as the Project-17 Alpha frigates (P-17A), are a series of stealth guided-missile frigates currently being built by Mazagon Dock Shipbuilders (MDL) and Garden Reach Shipbuilders & Engineers (GRSE), for the Indian Navy. The seventh and final ship of the Project 17A frigates, named Mahendragiri, was launched on 1 September 2023 at the Mazagon Dock by Dr Sudesh Dhankhar, wife of Indian vice-president Jagdeep Dhankhar. It is expected to be commissioned in 2024.

The frigates were designed by the Warship Design Bureau (formerly Directorate of Naval Design) – an internal establishment within the Indian Navy responsible for designing warships; the organisation is also distinguished for having contributed to the designing of numerous Indian warships, including INS Vikrant – India's first indigenously-designed aircraft carrier, the Arihant-class submarines – India's first indigenously-designed nuclear-powered ballistic missile submarines and the Shivalik-class frigates – India's first indigenously designed frigates equipped with stealth technology.

The class' design was finalised by the DND in 2013; it was later unveiled to the public in April 2018 – when MDL displayed a scale model of the frigate at the «DefExpo 2018» defense exhibition, held at Chennai, Tamil Nadu, India.

The frigate's design incorporates a significant degree of stealth and low radar-observability – through the utility of composite materials, radar-absorbent coatings and low-observable/radar-transparent technologies; the usage of the aforementioned materials assists the vessel in maintaining a low radar cross-section (RCS).

The vessel's physical profile also features a substantial level of stealth through the application of different physical forms – including an enclosed mooring deck, flush deck-mounted weapon systems and a reduced number of antennae.

The frigate's infrared signature, most specifically emissions from its propulsion exhaust and power generation machinery, are reduced through the application of Venturi effect and fluid injection, which helps to reduce the plume and hot metal temperatures of exhaust. To maintain acoustic silence, the frigate is equipped with propellers designed to have onset of cavitation at higher speeds to reduce hydrodynamic noise. In addition to the propellers, the vessel's hull also features special acoustic enclosures for some of the machinery to reduce the emission air-borne noise.

The P-17A frigates are the first major class of Indian-designed warships to be built using the methodology of integrated modular construction – a manufacturing process in which multiple modules (or «blocks») of a vessel's hull are pre-assembled/pre-outfitted independently, before being aggregated for final assembly.

In December 2015, MDL contracted Fincantieri S.p.A. to provide technical assistance and essential expertise in the construction of the seven frigates using the «modular construction» methodology.

The P-17A frigates were named after the former-Nilgiri-class frigates, which served in the IN between 1972 and 2013; the first six ships of the series were allotted the names utilised by the older class, namely – Nilgiri, Himgiri, Taragiri, Udaygiri, Dunagiri, and Vindhyagiri. The seventh and final vessel of the P-17A series, which did not have a namesake from the older class, was given the new name of Mahendragiri.