Conventional submarines remain central to sea-denial strategy in the Indian Ocean. Examine their limitations and the role of AIP technology in overcoming them.

Sea denial means stopping an adversary from using a sea area without having to control it yourself. Diesel-electric submarines are hard to detect and carry torpedoes and mines [1], which makes them a cheap way to do this at the Indian Ocean's choke points. Their endurance and the state of India's fleet limit how well they can do it.

Limitations of conventional submarines

• Battery-bound endurance: diesels run on the surface and batteries drive the boat underwater [1]. Battery capacity therefore caps how long it can stay submerged.
• Exposure while recharging: the boat must come close to the surface to run its diesels. At that point aircraft, radar and satellites can find it most easily.
• Ageing fleet: the 1999 CCS 30-year plan aimed for 12 new boats by 2012, but none had joined the Navy by 2016-17 [2]. In 2017-18 the Navy had 14 conventional boats plus one leased boat, aged 17–31 years. The goal was 18 conventional and 6 nuclear submarines [3].
• Slow induction: the sixth Scorpene was commissioned only in January 2025, 13 years after the target [4]. Old boats spend longer in refit, so fewer are ready for sea at any time.
• Design dependence: the new INS Karanj was built at MDL in collaboration with France's Naval Group [5]. India now builds submarines at home but does not yet design them.

Role of AIP

• Longer time submerged: AIP produces power underwater without taking in air, so the boat surfaces less often and is harder to detect [6].
• Home-grown solution: DRDO's fuel-cell AIP makes its own hydrogen on board [7]. Its land-based prototype passed a key test in March 2021 [6].
• Upgrade path: the system is to be fitted to Kalvari-class boats during their refits [6]. P-75I boats are reported to carry AIP from the start [8].
• Cost-effective bridge: it brings conventional boats closer to nuclear-submarine stealth for coastal sea denial, at a much lower cost.

Remaining gaps

• Fitting AIP needs certification from Naval Group, because a new section must be cut into a hull India did not design [6].
• DRDO is still working on the energy modules and the design of this extra hull section [7].

Conventional submarines are still India's most affordable tool for sea denial, but battery limits and an ageing fleet weaken them. AIP retrofits done on time, a faster P-75I programme and a shift from building foreign designs to designing at home, in line with Aatmanirbhar Bharat, can give India a credible undersea presence. Together with nuclear submarines, this would support the MAHASAGAR vision for the Indian Ocean.