Trouble beneath the waves: India’s submarine fleet is ageing and replacements are years away

India’s conventional submarine fleet is ageing just as the Navy needs more underwater vessels. Older submarines require retirement, but replacement programmes have fallen behind schedule. This can reduce the number of boats available for patrols, training, and combat missions. The Navy’s planned fleet expansion by 2030 is therefore becoming harder to achieve.

The warning signs are clear. Four Sindhughosh-class submarines have retired, including INS Sindhughosh, the class leader. Only six Kalvari-class Scorpene submarines have entered service. Shishumar-class boats have received mid-life upgrades, but they average around 35 years old and still have capability gaps.

More retirements are expected before new boats arrive. Project 75I’s first submarine may come about seven years after its contract is signed. Until then, the Navy could have fewer conventional submarines and less operational flexibility, weakening India’s underwater combat strength.

India currently operates 19 submarines in the categories described: 16 conventional diesel-electric boats and three strategic submarines. The conventional fleet spans the Kalvari, Sindhughosh, and Shishumar classes. Strategic submarines support the country’s nuclear deterrent and form a different part of naval planning.

The Navy’s stated goal is to induct 24 conventional submarines by 2030. That target is intended to build a larger and more capable underwater force. It reflects the 30-year submarine-building plan drawn up in 1998, which envisaged parallel foreign and domestic construction programmes.

Progress has been slower than planned. Four Sindhughosh-class boats have retired, while only six Kalvari-class Scorpene submarines have been inducted. Older Shishumar-class vessels remain in service after upgrades, but their age and capability gaps make the 24-boat target increasingly difficult to meet.

A conventional diesel-electric submarine uses diesel engines and batteries to move underwater. Because its batteries need recharging, it must periodically surface or snorkel. A strategic submarine has a nuclear deterrence role and carries ballistic missiles, making it part of a country’s nuclear forces rather than only its conventional fleet.

India’s Kalvari, Sindhughosh, and Shishumar boats are conventional classes. Its Arihant-class strategic submarines carry nuclear-capable missiles. INS Aridaman, the third Arihant-class boat, is designed to carry up to eight K-4 missiles, each with a range of 3,500 kilometres.

The distinction matters because the missions differ. Conventional boats are used for underwater combat and surveillance. Strategic submarines provide a concealed nuclear retaliation option. The article does not detail the propulsion system of India’s strategic submarines, but it highlights their ballistic-missile role and contribution to the nuclear triad.

Submarines are difficult to detect because they can operate concealed deep underwater. A submerged boat may avoid visual detection and remain hidden from surface vessels and aircraft. This concealment makes underwater forces valuable in both conventional combat and nuclear deterrence.

The article highlights a reported example involving a US submarine and the Iranian frigate IRIS Dena off Sri Lanka. The submarine reportedly tracked and sank the frigate without the warship detecting the underwater threat. The episode showed how an unseen submarine can gain the first-move advantage against a surface vessel.

For India, the same principle supports maritime operations and strategic security. A hidden conventional submarine can threaten surface ships while limiting its own exposure. A hidden strategic submarine is even more important because it can preserve the ability to retaliate after an attack on land-based assets. This makes concealment central to India’s deterrent posture.

Air-independent propulsion, or AIP, allows a conventional submarine to remain underwater for two to three weeks without surfacing or snorkelling to recharge its batteries. That gives the boat longer submerged endurance and reduces the need to expose equipment or raise its vulnerability near the surface.

Without AIP, a submarine must snorkel more frequently. Snorkelling helps recharge batteries but can increase the chance of detection by aircraft and surface vessels. A boat equipped with AIP can spend more time hidden, making it harder for opponents to locate and track during patrols or combat operations.

The technology is therefore a major capability issue for India’s ageing fleet. Only the six Kalvari-class submarines are slated to receive a DRDO-developed AIP system. Project 75I also involves submarines equipped with AIP, while Project 76 is expected to use a modified DRDO-developed system.

When older submarines retire faster than new boats arrive, the fleet shrinks. Fewer operational submarines must cover the same maritime responsibilities. This can reduce patrol frequency, training opportunities, and the Navy’s ability to respond simultaneously in different areas. It also leaves less room for maintenance and repairs.

India is already facing this pressure. Four Sindhughosh-class submarines have retired, while only six Kalvari-class Scorpene boats have been inducted. Shishumar-class submarines have received upgrades, but they average around 35 years old. More older submarines are due to retire in the coming years.

New programmes cannot immediately close the gap. Project 75I’s first submarine is expected about seven years after contract signing. Project 76 and nuclear-powered attack submarines under Project 77 will follow later. Unless deliveries accelerate, India’s underwater combat capacity may decline before planned expansion takes effect.

A nuclear triad combines nuclear forces based on land, at sea, and in the air. Submarines are especially important because they can remain concealed deep underwater. An opponent may struggle to find and destroy them before they can respond, making the nuclear force harder to eliminate in one attack.

India’s Arihant-class ballistic missile submarines provide this hidden sea-based element. INS Aridaman, the third Arihant-class submarine, is designed to carry up to eight K-4 nuclear-capable missiles. Each missile has a stated range of 3,500 kilometres. A fourth ballistic missile submarine has also been launched at Visakhapatnam.

This survivability strengthens deterrence. If land-based nuclear assets come under attack, a concealed submarine can preserve the ability to retaliate. India plans larger S-5 submarines for the future, while the existing Arihant-class boats already reinforce the country’s nuclear triad alongside land-based missiles and aircraft.