Back in 2005, after years of negotiations, India signed a deal to buy six French Scorpene class diesel-electric submarines. But now problems have developed. The first Scorpene was supposed to enter service by 2012, at a cost of about $500 million. But political and management problems have delayed that date by two years, and raised the price per boat by over $100 million. The French have raised the prices for some key components, and India has had some problems in getting production going on their end. The first Scorpene was to be built in France, with the other five built in India.
The Scorpenes are similar to the Agosta 90B subs (also French) that Pakistan recently bought. The first of the Agosta's was built in France, but the other two were built in Pakistan. The third Pakistani Agosta was delayed over a year because Islamic terrorists had killed some of the French engineers working on the project. The Scorpene purchase was seen as a response to the Pakistani Agostas.
The two designs are similar, with the Scorpene being more recent (and the result of cooperation between a French and a Spanish firm.) The Agosta is a 1,500 ton (surface displacement) diesel-electric sub with a 36 man crew and four 21 inch torpedo tubes (with 20 torpedoes and/or anti-ship missiles carried.) The Scorpene is a little heavier (1700 tons), has a smaller crew (32) and is a little faster. It has six 21 inch torpedo tubes, and carries 18 torpedoes and/or missiles.
Both models can be equipped with an AIP (air independent propulsion) system. This enables the sub to stay under longer, thus making the sub harder to find. AIP allows the sub to travel under water for more than a week, at low speed (5-10 kilometers an hour). The Pakistanis have an option to retrofit AIP in their current two Agostas.
Both of these modern subs are very lethal weapons against surface warships. With well trained crews, Agostas and Scorpenes can get close to just about any surface ship, no matter how good the defenders anti-submarine defenses are. But it's the AIP boats that are the real killers. Without AIP, subs spend most of their time just below surface, using their diesel engines (via a snorkel device that breaks the surface to take in air, and get rid of the engine exhaust.) Snorkels can be spotted by modern maritime patrol aircraft, and both nations are getting more of these. The introduction of the Agostas and Scorpenes was seen as an escalation in the naval arms race between Pakistan and India.
While India was largely concerned with the Pakistani navy when the Scorpene contract was negotiated and signed, China is now seen as the primary adversary. The Chinese subs are not as effective as the Pakistani boats, both because of less advanced technology, and less well trained crews. India could use their Scorpenes to confront any Chinese attempt to expand their naval presence into the Indian ocean. Thus the delays and cost overruns with the Scorpenes are causing quite a lot of commotion in India.