Quiet but deadly: diesel-electric submarines, the U.S. Navy's latest annoyance.

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The Navy in recent months has had to contend with several provoking episodes at sea--Iranian small boats speeding at its cruisers, destroyers and frigates; Russian bombers flying over its carriers; and Chinese subs shadowing its warships.

Hard-to-detect submarines--such as quiet, diesel-electric boats--are particularly vexing, Navy officials say. They contend that an undersea arms race already has begun in the western Pacific.

Nations there in recent years have begun to acquire stealthy diesel-electric submarines. Some of those nations, say Navy officials, could one day threaten U.S. access to strategic coastal areas of the world or interrupt the flow of commerce around the globe.

Although the Navy has the world's most technologically advanced fleet--including state-of-the-art nuclear attack submarines--officials acknowledge that these comparatively low-tech diesel-electric boats could give an enemy an asymmetric advantage.

"The beauty about a diesel submarine is that it has the potential to be far quieter than a nuclear submarine," says Guy Stitt, president of AMI International, a Bremerton, Wash.-based company specializing in naval market analysis. Diesel boats are propelled by batteries when submerged and move through the water by diesel engines when on the surface.

Once they have powered up their batteries, the submarines can sail to the bottom of coastal waters and remain undetected for days. Though they can't travel long distances or sail very quickly, advancements in technologies, such as air-independent propulsion and fuel cells, have allowed diesel submarines to extend theft operational ranges underwater.

But perhaps their best selling point is their relatively inexpensive price tags. The Russians have sold diesel submarines for as little as $200 million and the French have exported their Scorpene submarines for $300 million.

"It is within the scope of many, many countries to be able to afford them. They don't need a lot of them. They don't need to sail them very far, and they don't have to be particularly proficient with them," says Vice Adm. Samuel Locklear, commander of the Navy's Third Fleet, which prepares strike groups to deploy to the Pacific and the Middle East.

More than 39 nations possess diesel submarines. One of the latest tallies indicates a total of 377 ships in the world, says Richard Dorn, an analyst at AMI International. And there could be an uptick in the next few years.

With China continuing to increase the size of its navy, a number of neighboring nations also have begun to develop their undersea capabilities.

"There's a push on in Asia that really seems to be driven by China," says Stitt. Singapore, Malaysia and Indonesia all have closed deals on diesel submarines, and now Thailand is following suit.

Driving the market in part is Russia, which during the past 18 months has been aggressively selling ships, including its Kilo-class diesels.

"We've seen a huge increase in the number of sales that they're booking for Kilos, primarily motivated by the need for funds to strengthen their second tier shipbuilding groups," says Stitt.

Since the end of the Cold War, Russia has lost many of its secondary shipyard suppliers--the engine, pump and valve manufacturers, piping companies and the like. But Russia is attempting to revitalize those small companies.

"They're going out and making all these deals to sell submarines and ships and using those funds to reinvigorate the industry, which in turn will also benefit them in building up the Russian fleet," says Stitt.

Russia has exported 30 Kilos around the globe and 26 are still in active service. It will deliver two more submarines to Algeria by 2010, five to Venezuela by 2020, and six to Indonesia by 2018. China received its 12th and final Kilo last year.

The number of Kilos that are being sold is particularly concerning because many of the submarines are equipped with Klub anti-ship cruise missiles.

Some nations have a desire for regional hegemony and want to strengthen their influence in an area. That's most definitely the reason for President Hugo Chavez buying subs for Venezuela, says Stitt.

But for other nations, the reasons are less clear.

"There's a wide array of military assets you can buy, so why would you buy a diesel-electric submarine? As far as I know, it's not to protect your own port," says Locldear in an interview at Third Fleet headquarters perched atop Point Loma in San Diego.

That China's submarines are surfacing boldly near U.S. warships is a telltale sign of newer advanced technologies, such as acoustic tiles and cavitation-reducing propellers, that are being employed on the submarines, says Stitt.

China's new Song-class diesel submarines have tracked U.S. Navy ships operating in the seas near Japan and Taiwan. Last November, after China denied the USS Kitty Hawk's port call in Hong Kong at the last minute, a Chinese submarine shadowed the carrier as it entered the Taiwan Straits on its return voyage to Yokosuka, Japan. In the late fall of 2006, a Song-class submarine surfaced within torpedo range of the Kitty Hawk off the coast of Okinawa, Japan.

Despite the tensions, those episodes and the topic of submarines did not come up directly in conversations with Chinese officials in January, when the commander of Pacific Command, Adm. Timothy Keating, visited the nation.

"We watch them carefully. It's an area of warfare at which they're stretching a little bit," he told reporters during a breakfast meeting in Washington, D.C. "Their numbers of submarines are increasing. The capabilities resident in those submarines are not unimpressive. They're pretty good--we're better."

China's fleet of nuclear and diesel submarines includes 10 Song class, 12 Kilo class, one Yuan class and 32 Romeo class.

"We know that they are continually expanding their reach in what they view as their own areas of interest, and that their submarine force is vital to expanding that reach," says Locklear.

The proliferation of diesel submarines in the Pacific is one of the major factors behind the Navy's decision to move six submarines from the Atlantic Fleet to the Pacific Fleet, says Rear Adm. Joseph Walsh, commander of the Pacific Submarine Force. Because more than 140 diesel subs are within reach of critical "choke points" in the area, anti-submarine warfare is Pacific Fleet's top war-fighting priority, he adds.

The Navy saw its anti-submarine warfare skills diminish after the end of the Cold War. In those days, enemy Soviet nuclear submarines were noisy, and could be detected with passive sonar.

But modern-day diesel submarines are not as easily heard, particularly in regions of the seas where biological life and merchant shipping can camouflage their acoustic signatures. It is there, in the noisy waters of the littorals, where detecting submarines can be a cat-and-mouse game, Navy officials say.

Rear Adm. John Waickwicz, who was the head of the Naval Mine and Anti-Submarine Warfare Command until he retired in January, says the Navy is looking at anti-submarine warfare in new ways.

"When you talk about countries that have 30, 40, or 50 submarines, you can't wait until they're around you, because they're going to overwhelm you," he says.

Potential enemies have figured that to defeat the U.S. Navy, they must "go out and buy submarines, and buy mines," he says.

The mine and anti-submarine warfare command is calling for the deployment of a network of sonobuoys over a wide expanse of ocean to detect enemy submarines. But the project has been marred by technological and funding problems. The most significant hitch is that the data collected by the sensors takes too long to analyze, says Waickwicz. "You need to do it in real time to take action on it."

False alarm rates on many of the fleet's current detection technologies are too high, Waickwicz adds. That forces commanders to waste resources on non-existent threats.

Officials insist that the Navy's anti-submarine warfare capabilities are the best in the business, but they acknowledge that it will take some time to hone the skills to combat stealthy diesel submarines. Waickwicz says that training has improved in recent years, but some individual units are not adequately prepared for at-sea operations.

For example, some units have demonstrated sonar operator proficiency on simulations that are not sophisticated enough to replicate the real environment, which puts the sailors at a disadvantage when they conduct operations at sea, says Rear Adm. Frank Drennan, the new commander of the Naval Mine and Anti-Submarine Warfare Command.

"The requirements are still the same--they just have to work on them in a challenging environment so that operators are truly proficient when they go to sea," he says.

Hunting for quiet diesel submarines in the shallow waters of the littorals is akin to trying to identify the sound of a single car engine in the din of a major city, he says.

There are variations in the underwater topography, with sand bars, coral reefs and channels. Different depths of water and changing salinity and temperatures alter how sounds propagate. Marine life and merchant shipping also complicate the search by generating ambient noise.

The only technology that the Navy considers suitable for detecting and tracking diesel submarines is active sonar. It disperses signals out into the water where they bounce off of objects. Those echoes are captured by hydrophones and interpreted by sonar technicians.