In June 2017 the South Korean Navy received its second locally designed and built minelaying ship; the 4,200 ton Nampo. This is to be the first of a class of four or five minelayers. Normally no one builds that many minelayer ships but the Nampo is unique in that its design is based on the recently introduced South Korean FFX Frigate. The first of these entered service in in 2013. The FFXs are 3,200 ton ships armed with a 127mm gun, eight anti-ship or cruise missiles, three torpedo tubes, a RAM anti-missile launcher, and a Phalanx anti-missile gun system. There is space aft for two helicopters. The ships are highly automated, requiring a crew of only 140. Top speed is 61 kilometers an hour. Range is 8,000 kilometers. Most of the equipment (including electronics) and weapons are locally built. South Korea in building at least fifteen of these ships. South Korea subsequently introduced an upgraded FFX 2 frigate design. South Korea hopes to export the FFX to many navies who want a quality, low cost, warship. Meanwhile, South Korea has also built larger warships and is getting more into submarine production.
The Nampo lacks the anti-ship missiles but does carry the helicopter and anti-submarine torpedoes (and sonars) for finding and fighting submarines.
In the late 1990s South Korea planned to build two Wonsan class minelayers but only one was built and it entered service in 1998 as South Korea decided to try reducing the North Korean threat with lots of foreign aid. That failed spectacularly and in 2010 a North Korean sub torpedoed and sunk a South Korean warship (and denied it). Nampo joins the older Wonsan, a 3,300 ton ship that can carry 500 naval mines and deploy them quickly to protect South Korean ports and shipping lanes from North Korean warships and submarines. The Wonsan was also armed with a 76mm cannon, two 40mm autocannon and six 324mm (12.8 inch) torpedo tubes.
Only a few countries still build ships that specialize in minelaying and all of them are countries (South Korea, Poland, Sweden and Finland) with long coastlines with lots of shallow (60 meters or less depth) waters. Having a large threatening neighbor with a big fleet is another incentive. South Korea feels it needs the ability to put a lot of mines in the water quickly to foil a North Korean attack on key ports and naval bases. A minelayer that is actually armed like a modern warship seemed a good solution to this problem. For that reason South Korea will spend the rest of 2017 putting the Nampo through extensive sea trails and tests to see how well this new idea works with an actual ship. If it does not seem to work the Nampo design can be easily converted into a FFX type ship. Moreover in many navies coastal patrol ships like the FFX are often designed to lay some mines in wartime. The Nampo is different in that it can lay a lot of mines very quickly and deal with armed opposition if need be.
South Korea is secretive about its naval mine design and capabilities but it is believed the Nampo can carry and quickly lay (drop into coastal waters) at least 500 modern bottom (shallow water) mines. These are probably based on the American Quickstrike design with sensors and control electronics customized and manufactured by South Korean firms. It is important that South Korea keeps details of how these Koreanized Quickstrikes work in order to have a maximum impact on attackers.
U.S. forces, mainly the air force currently use the Mk-62 "Quickstrike" naval mine. This is basically a 227 kg (500 pound) bomb with a sensor package attached to the rear. There are three different sensor packages, each providing a different set of sensors to detonate the mine. The Mk-62 is a bottom mine which is dropped in shallow water and then detects a ship passing above using pressure (of the ship on the water), magnetism (of the metal in the ship's hull), or vibration (or a combination of those). The sensor also comes with a computer, to enable the mine to follow certain instructions (like only detonate for ships that meet a certain criteria). These mines also become useless over time because the most effective ones require battery power to operate their sensors. Data on how that works is obviously kept secret but it appears that South Korea has bottom mine designs that will reliably disarm after a certain period and have capabilities for South Korea to safely check that status.
The United States mainly uses aircraft to deliver Quickstrike mines. The B-52 and B-1B drop the mines at an altitude of about 300 meters (1,000 feet), while moving at 500-600 kilometers an hour. The mines are usually dropped in known shipping lanes, especially those that serve as approaches to a major port. During World War II air dropped mines proved devastating to Japanese shipping. Same thing when they were used against North Vietnam during the Vietnam War.
In another irony Korean waters were, over a century ago, modern naval mines were widely used for the first time during the Russo-Japanese war (1904- 1905). These were contact mines, floating in shallow water and kept in place with an anchor and chain. When the tide was right they would be just below the surface, ready to explode whenever struck by a ship. Some 2,000 of these mines were used to destroy sixteen ships during the Russo-Japanese war. That's one ship lost for every 125 mines used. Nearly fifty years later this type of mine was again used in Korean waters. In the late 1940s the Soviets provided North Korea with 3,000 naval mines, many of 1904 vintage. These were used to defend Wonson harbor in 1950. It took several weeks for UN forces to clear these, at a loss of a dozen ships hit. Half of these ships were destroyed.
North Korea is believed to have stockpiled more than 50,000 naval mines, most of them of the bottom mine variety although it is unclear how well maintained that mine arsenal is. That is a small arsenal compared to Russia (over 200,000) and China (over 100,000). Iran has a few thousand naval mines which is believed to be smaller than South Korean stockpile. It is generally agreed that all these mines are a serious danger. While often ignored, naval mines are a formidable weapon. But these passive weapons just don't get any respect. The historical record indicates otherwise.