I'm not clear on submarine stealth (N.Korean/S. Korean incident)?

Sonars work in one of 2 ways. Active sonar acts a lot like radar - you send out a sound wave and listen for it to bounce back. Active sonar can detect a submarine that is making no noise at all. This works very well in deep water but in shallow water the sound waves bounce off the bottom, off rocks, old wrecks and so on, so you end up with a lot of false targets. There is also the problem of varying salinity and temperature in coastal waters with warmer fresh water rivers dumping into the colder salt water ocean. This creates layers at different depths which the sonar waves can bounce off and a good sub skipper will use that to his advantage, hiding under those layers.

Passive sonar is like a big underwater microphone - it listens for noise. Passive sonars in the open ocean can achieve incredible ranges on noisy targets - hundreds or even thousands of miles. In shallow water they can be nearly useless however with all the varying currents and noise sources from nearby land.

The Cheonan was sunk near land in 80 meters of water.

To give you an idea of what is involved: In 1982 the British Royal Navy fought a war against Argentina for control of the Falkland Islands. This was the biggest naval war the world had seen since World War 2 and the Royal Navy was a specialized anti-submarine force whose primary Cold War task was to keep the shipping lanes from the U.S. to Europe safe from Soviet subs. Argentina had just 2 operational submarines - a World War 2 vintage relic which was knocked out of action while on the surface early in the conflict and a relatively new but not ultra-sophisticated German-built Type 209 export submarine the San Luis. The Royal Navy while hunting this one sub expended around 250 depth charges, Limbo bombs and anti-submarine torpedo's and were in fact in danger of running out of ASW ordnance. This came as a surprise to the commander of the San Luis who in interviews after the war stated his boat never came under direct attack. The RN - the best ASW force in the world - expended millions of Pounds worth of ordnance on whales and phantom noises.

So, an inherently quiet diesel-electric sub, laying silent in ambush in shallow water could certainly put a fish in a small, unsophisticated corvette that has little ASW capability to begin with.

One advantage of the old diesel electric designs is when they are running on batteries and going slow, they make very little noise. So if you are using passive sonar, they are almost impossible to hear. Yes, you can ping for them, but with each ping you tell the sub exactly where the ship is that is doing the pinging. By comparison a nuclear sub makes quite a bit of noise.

Also, a sub running on batteries runs "cool." Vs a nuclear sub that has a pretty good heat signature.

And as with any object, the smaller it is, the more "stealthy" it is. There is a minimum size that you can make a nuclear sub and still have it safe and efficient. Whereas you can make very small diesel electric subs, such as the ones the Japanese tried to use on Pearl Harbor in 41.

And it only takes one torpedo in the right place to sink a ship, so all the mini-sub needs is one.

Many nations have diesel subs. We get together are 'play' wargames routinely. Diesels have some abilities which nukes do not have, and nukes are their abilities.

Is a pistol better than a rifle? Well both are good, but with each you must change how you fight. The pistol is better for in buldings or dense forests, the rifle is better for open fields.

Diesels can operate in shallower water and can take advantage of bergs, coastal shoals and surface ships to hide behind.

Nuke subs like deep blue water and can detect things from hundreds of miles away. But they need deep channels before they can go into a harbor.

Some of NKs subs are used as manned missiles,which is how I suggested they did it before the investigation proved it to be a missile.These guys are like underwater kamikazes,they probably were detected but to late to save the ship.What I believe happened is that NK was incurring came upon the ship.and it became a target of opportunity,i.e they lucked out,then decided to destroy it. That is my theory anyway.Do not believe the Nks have stealth technology for subs.

Stoned

Not so easy in shallow waters or under certain oceanic conditions. Modern military tech is not as foolproof as the Pentagon would have you believe.

go online and research what subject you interested in.im pretty sure you're get your answer..