- 02.12.09
- 11:56 AM
The country that gave us Volvos, Saabs and ABBA has developed what it claims is the world’s first fully operational stealth warship that is essentially invisible to radar.
The two Visby-class corvettes will enter service by the end of the year. They are made from composite materials and use Rolls-Royce water jets to make them electronically undetectable at more than eight miles in rough seas and more than 14 in calm waters. The ship’s acoustic and optical signatures are lowered by its non-magnetic hull that, like the F-117 Nighthawk, features large, flat surfaces and sharp angles. The water jets are 10 to 15 decibels quieter than propellers.
"It’s very hard for a submarine to detect a water jet vessel," Patric Hjorth, technical manager of the Swedish Defense Materiel Administration, told Naval-Technology.com. "It has a very different signature from a propeller-driven craft as it fades into the background."
Sweden has maintained military neutrality since the end of the Napoleonic Wars, but it saw foreign submarines make a spate of incursions into its territorial waters during the 1980s. That led the government to call for the development of a anti-submarine and mine-hunting vessel that could patrol coastal areas. The Swedish defense firm Kockums got the job and decided invisibility would be better than invincibility.
"A warship’s survivability can be built on one of two premises: invincibility or invisibility,’" the company says in a statement (pdf). "For nations with deep pockets and imposing military budgets, invincibility is the chosen high-ticket objective. For countries with more limited material resources, the more affordable choice must be invisibility, to which stealth is the obvious path."
That’s not to say the Visby corvettes lack muscle. Each is armed with eight anti-ship missiles, three torpedo tubes, multiple grenade launchers, depth charges, submarine homing torpedoes and a fully automatic 57mm "general purpose" gun.
They’re nimble, too. The plastic and carbon fiber hull displaces 600 tons of water, about half that of conventional, steel-hulled ships of a similar size. "The need for agility and a high top speed meant that a light weight was an essential factor," Hjorth says. "You actually need waterjets for these vessels, as they’re more efficient than propellers at high speeds."
The vessels are capable of speeds exceeding 35 knots. Propulsion comes from two diesel engines and four gas turbines that power a pair of water jets. Water jets are about 10 to 15 decibels quieter than props, and to further minimize noise the Visbys use impellers with seven blades instead of five. Some of the components are made of bronze instead of stainless steel to further reduce their magnetic signature.
The original plan called for six Visbys in two classes — one for surface combat and the others for submarine hunting and mine detection — but cutbacks in the early 1990s by the Swedish government cut the fleet to five vessels. The first was launched in 2000; the vessels are being readied for active duty. The first two will be formally commissioned into active duty later this year. The U.S. Navy reportedly has expressed interest in them as well.
UPDATE 9:30 p.m. Eastern – several readers have noted that Lockheed built the Sea Shadow for the U.S. Navy in 1985 to test stealth technology. The U.S. Navy says the ship was for research purposes only, "was never intended to be mission-capable" and did was not designated as a "USS" vessel. It is listed in the Navy’s inventory as a "miscelaneous craft." We’ve corrected the headline.
Photos: Kockums.
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