Israel recently revealed that one of its Dolphin class submarines (that entered service in 1998-2000) had secretly spent nearly two years in an Israeli shipyard. The sub was partially disassembled and its engine and plumbing was cleaned and upgraded. Hull cracks were repaired and various other items were fixed. The boat, which entered service in 1999, is now expected to remain in service until 2030.
The three older boats have all been earlier upgraded to include larger fuel capacity, converting more torpedo tubes to the larger 650mm size, and installing new electronics. The fuel and torpedo tube mods appear to have something to do with stationing the subs off the coast of Iran. Larger torpedo tubes allow the subs to carry longer range missiles.
The larger fuel capacity makes it easier to move Dolphins from the Mediterranean to the Indian Ocean. Although Israel has a naval base on the Red Sea, Egypt, until recently, had not allowed Israeli subs to use the Suez Canal. So the Dolphins were modified to go around Africa, if they had to. But now the Egyptians, who are also feuding with Iran, frequently allow Israeli subs to use the canal. Larger fuel capacity also allows the subs to spend more time on station off the Iranian coast. Currently the Dolphins can stay at sea for about 40 days (moving at about 14 kilometers an hour, on the surface, for up to 8,000 kilometers). Larger fuel capacity extends range to over 10,000 kilometers and endurance to about 50 days.
Israel equipped its new Dolphin class submarines with nuclear cruise missiles in 2002. Israel also fitted their 135 kilometer range Harpoon missiles with nuclear warheads. These missiles are fired from the sub's torpedo tubes. The 1,625 ton Dolphins can carry 16 torpedoes or missiles and have ten forward torpedo tubes (four of them the larger 650mm -26 inch- size). The Dolphins are considered the most modern non-nuclear subs in the world. The first three cost $320 million each. All have a crew of 35 and can dive to a depth of more than 200 meters (660 feet). The Dolphin design is based on the German 209 class subs but has been so heavily modified that it is considered a different class.
The Israelis have developed a cruise missile, which has a range of 1,500 kilometers and carries a 200 kiloton nuclear warhead. The objective of deploying nukes on subs is to further enhance deterrence to any nation launching a nuclear strike against Israel. If one of the Dolphins is always at sea even a first strike against Israel would not prevent a nuclear strike by submarine launched nukes.
Meanwhile, Germany has agreed to pay 20 percent of the cost of a sixth Dolphin class submarine for Israel, which was ordered earlier this year. Two more are under construction and will arrive in the next two years. The sixth one should arrive in 2015. These new Dolphins cost about $650 million each.
The three Dolphins under construction have a fuel cell based (AIP, or Air Independent Propulsion) system which enables them to stay under water for over a week at a time. The Dolphins are also very quiet, and very difficult to hunt down and destroy. The first three Dolphins didn't have the AIP system.