Swedish corvette is engineered for stealth

By Edward Lundquist at December 22, 2010 05:18
Filed Under: Navy insights

Swedish corvette is engineered for stealth

By Edward Lundquist

The Swedish corvette Visby is stealthily to the core, right down to the all-composite shaft. 

From the carbon fiber hull and radar absor­bent coating to the enclosed gun mount, flush antennas and telescoping mast, this ship has extremely low signatures.  The camouflaged appearance is hard to see, and the engine exhaust being ducted into the water jets reduces the infrared signature (the engine exhaust mixed with the water does create a visible vapor “exhaust”).  The machinery sits on vibration isolation mounts, and non-magnetic materials are used throughout.

Despite this unique approach to warship design, Lt. Cmdr. Bjorn Spangberg of the Royal Swedish Navy’s Third Naval Warfare Flotilla doesn’t see a downside. 

Spangberg is head of Trials Unit Visby technical division with the Third Naval Warfare Flotilla, and before that was aboard the Visby class as chief engineer.

 “Regarding the composite shafts I can’t come up with any cons at all,” Spangberg says.  “They are almost maintenance free.   We only have to perform visual inspections of the shaft surface, flexing elements and bulkhead seals.  Existing bolts are checked regarding tightness once a year.”

The composites weigh about half as much as steel. If the ship was built with tradition construction materials it would displace 1,300 tons instead of 650.  And the com­posites contain the heat and smoke, so compartment fires do not spread before the installed firefighting system can extinguish it.  Every thing aboard is fire retardant, even the bed sheets.

Five ships of the class have been built or are building.  The lead ship was commissioned in 2000. 

HSwMS Härnösand and sister-ship HSwMS Helsingborg were commissioned Dec. 16, 2009. They are the two newest Visby-class corvettes, says Cmdr. Patrik Norberg, who commands the Härnösand.

Visby-class corvettes HSwMS Härnösand (K 33) and sister-ship HSwMS Helsingborg (K 32) at the Swedish Navy Base at Karlskrona. (Photo: E. H. Lundquist)

Ac­cording to Royal Swedish Navy Capt. Anders Olovsson, 3rd Naval Warfare Flotilla com­mander at the Karlskrona naval base, Visby has “lean manning,” with 27 officers and 16 ratings, for a total of 43.

With the two Detroit/MTU diesels she can make 15 knots, and with the diesels and with the four Vericor TF50A gas turbines, the 238-footVisby can move out smartly at 35 knots.

I accompanied Härnösand to sea in the Baltic for a gunnery exercise.  The steerable KaMeWa waterjets make Härnösand very maneuverable during mooring evolutions, and the bow thruster makes it even easier.  Helm controls are operated by one person.  The ship comes up to speed rapidly.  While underway at 35 kts., I watched from the bridge as the ship made a 180-degree turn inside its own wake.

Gas turbines exhausts gases are cooled with seawater, Spangberg says.  “We also cool gases produced from boiler, auxiliary machinery and low speed machinery in order to keep the heat signature low.  The internal seawater cooling systems are used for cooling everything but the exhaust gases from the gas turbines.  When the main engines are running, their exhaust gases are cooled by seawater drained from the waterjet units via a regulating system for optimal cooling effect due to specific power used.”

The cooling system for the gas turbines are built by Mecmar, a Norwegian company.  “They have built systems like ours for high-speed ferries around the world.  One unique thing is that we have two gas turbines that come together in one exhaust system on each side (port and starboard). The exhaust system leads from the engine room aft and up to the cargo deck, and then turns down through a horizontal surface in front of the transom.  The advantages include reduced heating signature, reduced gas volume, reduced size of exhaust piping, and reduced weight,” Spangberg says.

“The Visby-class corvettes for Sweden have been in the limelight. They took a long time. They cost a lot. But we are leaving the conscript navy and this flotilla is rapidly evolving into a standing operational force,” Olovsson says. “These ships are warfighters. They can fight in all dimensions.”

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Captain Edward Lundquist, U.S. Navy (Ret.) is a principal science writer with MCR Federal LLC. 

 

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