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Stealth on the Water PUBLIC ACCESS

The Swedish Navy's Visby Corvette is Designed to be Virtually Invisible in Pursuit of Hostile Submarines and Underwater Mines.

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Mechanical Engineering 123(01), 56-61 (Jan 01, 2001) (6 pages) doi:10.1115/1.2001-JAN-4

The Swedish Navy’s Visby corvette is designed to be virtually invisible in pursuit of hostile submarines and underwater mines. The Visby is designed to be difficult to detect by an enemy using radar, infrared, hydro-acoustic monitoring or any other sensor system. The craft’s success could change naval warfare as profoundly as did the ironclad ships in the 19th century. Sweden’s YS2000 class corvette, the first known production naval stealth vessel, takes shape at Kockums’ shipyard on Karlskrona Island. The Kockums shipwrights designed the Visby to be electronically undetectable at a range greater than 13 km in rough seas, and at more than 22 km in calm seas, without electronic jamming. With the assistance of jamming, the Visby is invisible at more than 8 km in rough waters and 11 km in a calm sea. Computing Devices Canada also designed a monitoring system so that the Visby can perform a background check on its own vibration levels to ensure that the ship’s sound is within acceptable levels.

On June 8th, his majesty King Carl XVI Gustaf of Sweden christened the latest addition to his country's navy, the Vis by corvette. Some 450 guests from 22 nations attended a ceremony that may have been as historic as the launching of the Merrimack during the American Civil War, because the Visby is the first known production model stealth vessel.

The Vis by is designed to be difficult to detect by an enemy using radar, infrared, hydro-acoustic monitor The Vis by is designed to be difficult to detect by an enemy using radar, infrared, hydro-acoustic monitoring, or any other sensor system. The craft 's success could change naval warfare as profoundly as did the ironclad ships in the 19th century.

The background to the launch was laid in the middle and late 1980s, when small submarines of unknown origin continuously violated Swedish national waters, seemingly with impunity. The Nordic country initiated a program in 1988 to design a stealth corvette that could defend its waters against submarines and detect undersea mines.

Sweden's YS2000 class corvette, the first known production naval stealth vessel, takes shape at Kockums' shipyard on karlskrona Island.

Grahic Jump LocationSweden's YS2000 class corvette, the first known production naval stealth vessel, takes shape at Kockums' shipyard on karlskrona Island.

Originally, Sweden's admirals wanted two different classes of corvette: YSM, the abbreviation of Ytstridsfartyg Mindre, or Surface Combatant/Small in military English, and YSS, for Ytstridsfartyg Storre, or Surface Combatant/Large.

The navy commissioned Kockums AB of Malmo, Sweden, to build a test platform craft to develop its stealth corvettes. Kockums built the HMS Smyge surface ship at its Karlskronavarvet shipyard on Karlskrona Island, where the company launched a naval ship, the Blekinge, in 1682.

The fiberglass and Kevlar Smyge was too small to be a true prototype of a combat vessel, measuring 30.4 meters long and 11.4 meters at the beam. Rather, it served as test rig for various systems. For example, it carried a 40-mm Trinity gun, two RES 14 anti-ship missiles, and a complement of torpedoes and mines. The HMS Smyge also successfully tested stealth technology, sensors, communications systems, and navigation equipment during its sea trials, which also gave Swedish crews experience manning a stealth vessel.

The Kockums shipwrights reduced the radar, infrared, magnetic, visual, acoustic, and hydro-acoustic signatures of the Smyge by making all the vessel's structures angled flat surfaces, and equipping all of the ship 's inlets and outlets with radar-absorbent material. They also equipped the Smyge with a quiet propulsion system consisting of two water jets made by KaMeWa Group of Kristinehamn, Sweden. KaMeWa's jets are used on fast ferries, catamarans, and other high-speed marine craft.

An intake on the bottom of the Smyge's hull pulls water into the KaMeWajet system. The pilot moves the jet nozzles from the bridge to direct the jet stream to port or starboard to turn the ship. He lowers a gate across the water jets to put the Smyge in neutral or to reverse the craft.

The water jets are powered by two 2,040-kilowatt MeR diesel engines. The ship can make more than 40 knots in calm weather.

A pair of 460-kW DSI 14 diesel engines made by Scania of Sodertalje, Sweden, powers lift fans to create an air cushion that can support more than 80 percent of the Smyge's 140 metric tons. Only 0.7 meter of the hull projects into the water when it sails, as opposed to 1.9 meters when the craft is at rest. Riding on the air cushion reduces the vessel's pressure signature, making it harder to detect.

By 1993, budgetary considerations led the Navy to combine the YSM and YSS corvettes into a single class of corvette, the YS2000, or Surface Combatant 2000, which fulfills a variety of roles, including mine clearing and laying, submarine hunting, surface combat, surveillance, and control operations, in Swedish and international waters. Kockums partnered with the Swedish Navy, Defense Material Administration, and the Royal Institute of Technology to develop the new vessel, named the Visby in honor of a Swedish coastal city.

Kockums developed a sandwich-type composite for the Visby consisting of vinyl ester resin layers surrounding a polyvinyl chloride core containing carbon fibers.

Grahic Jump LocationKockums developed a sandwich-type composite for the Visby consisting of vinyl ester resin layers surrounding a polyvinyl chloride core containing carbon fibers.

The Visby design team had to devise a composite material that would provide a very low hull weight and required stealth properties, at an acceptable cost. They devised a sandwich-type composite consisting of vinyl ester resin laminating a core of polyvinyl chloride that contained carbon fibers. The combination was affordable, durable, lightweight, shock-resistant, and completely nonmagnetic.

The Kockums shipwrights first vacuum-injection molded flat, composite panels. They joined the panels into the fore, aft, center, and deckhouse sections of the corvette, then joined the sections. The resulting hull has large flat surfaces and sharp edges to reduce its acoustic and optical signature on the horizon. To enhance that lower profile, engineers designed all possible components to be concealed within the corvette or under hatches.

The large, flat surfaces and sharp edges that are built into the Visby's hull reduce the vessel's acoustic and optical surface on the horizon

Grahic Jump LocationThe large, flat surfaces and sharp edges that are built into the Visby's hull reduce the vessel's acoustic and optical surface on the horizon

Kockums designed a combined diesel or gas-CODOG-propulsion system for the new corvettes. This consists of a pair of 16V 2000 M90 diesel engines made by Motorenund Turbinen-Union GmbH in Friedrichshafen, Germany, and four TFSO aeroderivative marine gas turbines designed by Vericor Power Systems in Atlanta, along with Honeywell Engines and Systems in Phoenix. The diesels are used for low-speed operations and the turbines for medium- to high-speed maneuvers, which exceed 35 knots. Both the diesels and turbines are connected to a pair of gearboxes that run two KaMeWa water jet propulsors.

The MTU naval engines are compact and relatively lightweight, designed to operate over a wide power and speed range. Each M90 diesel on the Visby is 115.3 inches long, 55.1 inches wide, weighs 7,991 pounds, and generates 1,801 horsepower at 2,300 revolutions per minute. The MTU diesels have low acoustic and infrared signatures. MTU designed its 2000 and 4000 series engines jointly with Detroit Diesel Corp.

The TFSO Vericor gas turbines were designed as compact power plants suited to naval configurations like the Visby, which represents their first installation, where two turbines are coupled to a single shaft. The TF50s measure 64 inches long, 39 inches high, 33 inches wide, and weigh 1,565 pounds each. When running at 16,000 rpm, burning jet fuel, diesel, or kerosene, each turbine provides 5,346 hp.

Vericor engineers designed the TF50 to be easily maintained. The turbines' modular construction enables machinists to remove the combustor turbine module to periodically inspect the nozzles, blades, and combustor liner for wear.

The Kockums shipwrights designed the Visby to be electronically undetectable at a range greater than 13 km in rough seas, and at more than 22 km in calm seas, without electronic jamming. With the assistance of jamming, the Visby is invisible at more than 8 km in rough waters and 11 km in a calm sea.

The Visby is 72.8 meters long, 10.4 meters at the beam, and displaces 600 metric tons when fully equipped. It will be crewed by 18 officers and 25 enlisted men.

The stealth corvette is prepared to carry Bofors Rb23 BAMSE surface-to-air missiles to fight enemy helicopters. BAMSE is a joint venture formed by Saab Bofors Dynamics AB and Ericsson Microwave Systems. The missiles can fly up to 12 kn1 in altitude at ranges of 15 km.

The YS2000's artillery includes a Bofors 57-nun 70 SAK Mark III general-purpose gun. This third-generation Bofors naval gun is equipped with an integrated fire control computer that calculates aim, trajectories, and ballistics. Bofors designed a cupola to shield the Visby's Bofors gun. Its barrel is exposed for only a few seconds when it is fired.

The vessel carries a suite of anti-submarine warfare, 127-mm, rocket-powered grenade launchers, depth charges, and torpedoes. The latter are launched through three fixed, 400-mm torpedo tubes. The anti-submarine arsenal, except for the torpedoes, will be deployed by the Alecto multimission launcher, which was designed by Saab Dynamics. The Alecto also will take countermeasures against enemy torpedoes, and send out chaff and infrared decoy rounds to confuse an opponent's detection systems.

Computing Devices Canada in Nepean, Ontario, designed an integrated, multiple sonar suite for the Visby corvettes that can be used for mine countermeasures and anti-submarine warfare under the auspices of the Swedish Defense Material Administration's Hydra program, named after the many-headed serpent of Greek lore.

"Computing Devices designed the concepts and infrastructure for the multisensor sonar system," said Fred Cotaras, the lead Hydra system engineer and chief engineer at Computing Devices Canada. "We worked with subcontractors to refine existing sensor array designs to serve the Visby in its shallow littoral water defense role."

For example, the active sonar mounted on the Visby's hull is used to search for mines and submarines, as does its counterpart on other vessels. "A unique feature of the Visby hull sonar is its narrow, steerable vertical beam and narrow horizontal beams, which reduce reverberation caused in the shallow water environment of Swedish coastal waters," Cotaras said.

During anti-submarine maneuvers, the Visby's crew can deploy a towed array sonar, basically, a series of pressure sensors with preamplification and digitizing electronics embedded in an oil-filled hose about two inches in diameter. The passive system is rolled off a winch in the back of the corvette. Unlike most towed sonars, .the Visby's is towed behind a neutrally buoyant cable that is attached to a traditional heavy cable, which puts the sensors more than a mile from the stealth corvette's water jet propulsors, a much greater distance than in typical installations.

Cotaras worked with his colleagues and subcontractors to devise a variable-depth sonar that is deployed from the aft end of the Visby, concurrently with the towed array sonar. One of the major innovations on the variable depth sonar is its mechanical handling system, which is designed to work in a confined space and to compensate for the ship's motion. One meter wide and four meters long, the tow body is gripped by a saddle that an operator manipulates, lowering the sonar between the Visby's water jets and into the water to the chosen depth.

"The variable-depth sonar enhances the hull-mounted sonar's anti-submarine capabilities by operating beneath the warm layer of water near the surface," Cotaras noted.

When the towed array sonar cannot be deployed during anti-submarine situations, or when the Visby is mine hunting, the stealth vessel deploys a line of sonobuoys. Each sonobuoy is a commercially available, free-floating hydrophone, which enables the Visby to detect an enemy vessel that might attack it during anti-sub and antinune exercises. The sonobuoy is also used to track hostile torpedoes so that the corvette can avoid them.

A Kockums shipwright joins the stealth corvette's composite panels together in the same shipyard where his ancestors may have served the Swedish crown 300 years ago.

Grahic Jump LocationA Kockums shipwright joins the stealth corvette's composite panels together in the same shipyard where his ancestors may have served the Swedish crown 300 years ago.

Computing Devices Canada also designed a monitoring system so that the Visby can perform a background check on its own vibration levels to ensure that the ship's sound is within acceptable levels. This system consists of a series of accelerometers mounted throughout the ship.

Together with the Defense Materials Administration and Bofors, the Canadian company adapted the Bofors Double Eagle Mark II, the remotely operated vehicle the Visby uses to search for mines. The ROV operates at a distance in front of the corvette, and is equipped with three separate sonar systems on a rotating mechanical arm. Two systems spot mines, and the third sonar classifies the mines according to size and shape. An on-board video camera lets an operator aboard ship identify their color.

The Bofors Double Eagle Mark II works in tandem with an STN Atlas Electronik Seafox ROV. The Seafox remotely operated vehicle is equipped with a sonar system and a camera to confirm that the object is a mine, then is detonated to destroy itself and the mine.

The major challenge facing the Canadian engineers was integrating the multiple sensor systems. "We made use of commercially available computer networks and architecture to control and take data from each sensor system. This let us process, refine, and display the data at any of three consoles, and interact with the ship's command system," said Cotaras.

Embedded within the integrated sonar system network is a wide variety of data bases compiled by the Swedish Navy on sea bottom types and sound-speed profiles. The Visby uses this information during anti-submarine and mine hunting exercises to examine sonar noise it detects in the context of its environment, and sends results to the sonar system's tracking subsystem.

The Visby's engineers worked to conceal all of the stealth craft's components within its hull or under hatches to make it harder to see the vessel.

Grahic Jump LocationThe Visby's engineers worked to conceal all of the stealth craft's components within its hull or under hatches to make it harder to see the vessel.

During mine countermeasures, the sonar suite draws upon charts of historically established mines and mine-like objects that the Swedish Navy gained during mine clearing operations in the Baltic States during its Campaign for Peace after the end of the Soviet Union.

The Swedish Defense Material Administration selected the 9LV Mark3E combat management system developed by CelsiusTech, now part of Sa ab Systems, to provide the command, control , and communications functions on the YS2000 corvette. More than 50 9LV series combat management systems are in service on vessels of the Australian, Danish, Finnish, New Zealand, Pakistani, and Swedish navies.

The 9LV is based on open system architecture and uses the Windows NT operating system. Saab engineers customize the communications system to support duties ranging from anti-submarine warfare to data communication, and to interface a wide variety of weapons and sensors, including surveillance and target indication radars, and surface-to-surface missiles.

Kockums prepares to launch the first of five Visby corvettes ordered by Sweden's Navy to detect and fight enemy submarines and clear their mines.

Grahic Jump LocationKockums prepares to launch the first of five Visby corvettes ordered by Sweden's Navy to detect and fight enemy submarines and clear their mines.

The Visby's combat management system is equipped with a high-capacity digital communications switch developed by Infocom in Denmark and Kockums in Karlskrona, which interconnects the corvette's voice and data communications channels. This provides the Visby with internal communications or open conference lines as well as access to external communications with a variety of radio links and landbased networks.

Operators of the combat management system use the new CeCots multifunction console developed by CelsiusTech in collaboration with Hitec AS of Norway, a company that typically designs computerized communications systems for offshore mining and exploration, to run the Windows NT operating system. The

personal computer workstation-based console consists of three large, flat screens that display tactical data and maps, as well as radar, optronics, sonar, and early warning readings.

In addition, the Visby combat management system incorporates a new subsystem that tracks the corvette's own radar cross-section and other stealth properties relative to the ship's target and displays them graphically. Thus, the ship's officers are informed how visible the corvette is, and can adjust their tactics accordingly. During anti-submarine maneuvers, the CeCots displays three-dimensional images of the underwater landscape beneath the corvette, which can be viewed at different angles in real time.

His Majesty King Carl XVI Gustaf of Sweden signs his name on the Visby during its christening ceremony, which was held last June 8.

Grahic Jump LocationHis Majesty King Carl XVI Gustaf of Sweden signs his name on the Visby during its christening ceremony, which was held last June 8.

Kockums will build three more Vis by-class corvettes for the Swedish Navy by 2004. Like the first vessel, they will be used primarily for mine countermeasures and anti-submarine warfare.

Two more Visby corvettes are on order, and these will be designed for attacking enemy surface craft. Toward that end, Kockums will install a hangar on the ship for a helicopter that can land, take off, and refuel on the upper deck.

Copyright © 2001 by ASME
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