After years of delay, India's navy is preparing to take delivery of one of the world’s stealthiest and most deadly fighting tools: the INS Kalvari , an attack submarine named after a deep-sea tiger shark.The commissioning later this month of the Scorpene class submarine is a milestone in India’s effort to rebuild its badly depleted underwater fighting force, and the first of six on order. It comes as China 's military expands its fleet to nearly 60 submarines -- compared to India's 15 -- and increases its forays into the Indian Ocean in what New Delhi strategists see as a national security challenge.A Chinese Yuan-class diesel-powered submarine entered the Indian ocean in May and is still lurking, according to an Indian naval officer who asked not to be identified, citing policy. It's an unwelcome reminder of China's rapidly expanding naval strength at a time when Indian and Chinese soldiers are engaged in a border dispute stand-off in Bhutan. China’s defense ministry didn’t respond to a faxed request for comment.The official opening in July of China’s first naval base at Djibouti at the western end of the Indian Ocean, recent submarine sales to Pakistan and Bangladesh and a visit last year of a Chinese nuclear-powered submarine to Karachi, have also exposed how unprepared India's navy is to meet underwater challenges."The lack of long-term planning and procurement commitment in defence acquisition plans can be considered tantamount to negligence" by the Indian government, said Pushan Das, a research fellow at the New Delhi-based Observer Research Foundation's National Security Program. India needs to "counter increasing PLA-N activities in the region," he said, referring to the People's Liberation Army Navy.Ministry of Defence spokesman Nitin Wakankar would not comment on the Indian Navy ’s submarine fleet plan.Since 1996, India's attack submarine fleet has dwindled to 13 diesel-electric vessels from 21 as the navy failed to replace retired boats. The entire fleet -- a mixture of Russian-origin Kilo class vessels and German HDW submarines -- is at least 20 years old. All have been refitted to extend their operational lives until at least 2025.In contrast, China’s underwater fleet boasts five nuclear-powered attack submarines and 54 diesel-powered attack submarines. By 2020, the force will likely grow to between 69 and 78 submarines, according to the Pentagon’s latest report on China's military.Still, analysts say it will be years before China can pose a credible threat to India in the Indian Ocean."Simple geography gives India a huge strategic advantage in the Indian Ocean," said David Brewster, a senior research fellow with the National Security College at the Australian National University in Canberra. "And although China has been sending in submarines, you have to understand they are probably decades away from being able to seriously challenge India there, especially while the United States is present."China's navy needs to enter the Indian Ocean through narrow choke points like the Malacca Strait that runs between Indonesia and Malaysia. Indian surveillance planes deployed to Andaman & Nicobar Islands patrol the area, and one spotted the Chinese submarine in May.In the meantime, India is slowly upgrading its underwater fleet.The INS Kalvari is the first of six French-made Scorpene submarines on order in a 236 billion rupee ($3.7 billion) project awarded in 2005 to the state-owned defense shipyard Mazagon Dock Shipbuilders Ltd. and France’s Naval Group, formerly known as DCNS Group. Junior defense minister Subhash Bhamre said in July that the first of these would be delivered in August.In February 2015 India approved the construction of six nuclear-powered attack submarines. Few details have been released about the 600 billion rupee program.And on July 21, India initiated another program to build six more diesel submarines. It sent information requests to six manufacturers -- Germany's ThyssenKrupp Marine Systems GmbH, Naval Group of France, Madrid-based Navantia SA, Sweden's Saab AB, a Russia-Italian joint venture called Russian Rubin Design Bureau and a consortium between Mitsubishi Heavy Industries Ltd. and Kawasaki Heavy Industries Ltd. The project is worth about 500 billion rupees.As well as its attack submarines, India is developing an underwater nuclear deterrence. The first nuclear-powered submarine that can launch ballistic missiles was commissioned in 2016, part of a program to build at least three. The navy is using a Russian nuclear-powered submarine it leased for 10 years in 2012 to train the crew. China has four nuclear-powered ballistic-missile submarines.Even with the announced programs, India isn't likely to meet its 2030 deadline for shoring up its submarine fleet. To deter both China and Pakistan, planners reckon the fleet needs at least 18 diesel, six nuclear and four nuclear-armed submarines."While the operational urgency cannot be undermined, there is a need for the Indian Navy to fight its wars with Indian-made submarines," said KV Kuber, a Delhi-based independent defense analyst who previously served on government-appointed committees that reviewed defense industrial policies. "Even if we go for a global tender to meet the urgent requirements of the Indian Navy, we would still be years away from acquiring them. Yet, this is the fastest route."