Iran recently announced that it had put four more mini-submarines into service, for a total of eleven in the last five years. Over the last decade, Iran has, apparently with technical help from North Korea, been building mini-submarines for operations along its coasts, and throughout the Persian Gulf. The first two entered service about five years ago. The Iranians say they will use the mini-subs to lay mines or launch underwater commando attacks. While the North Koreans provided some technical assistance, the Iranian sub is a local design, smaller than most North Korean mini-subs, which is a reflection of the more turbulent seas found off the Korean coast.
The Iranian subs appear to be based on the North Korean M100D, a 76 ton, 19 meter (58 foot) long boat that has a crew of four and can carry eight divers and their equipment, or several naval mines, or a torpedo. The North Koreans got the idea for the M100D when they bought the plans for a 25 ton Yugoslav mini-sub in the 1980s. Only four of those were built, apparently as experiments to develop a larger North Korean design. There are believed to be over 30 M100Ds, in addition to eleven of the Iranian variation.
Building subs like this are not high tech. A drug gang in Ecuador was recently caught building a 30 meter/98 foot long submarine on a jungle river. This boat was three meters/nine feet in diameter and capable of submerging to about 30 meters. The locally built boat had a periscope, conning tower and was air conditioned. It was captured where it was being assembled, and a nearby camp, for the builders, appeared to house about fifty people. This was the first such sub to be completed, but not the first to be built. Nearly a decade ago, Russian naval architects and engineers were discovered among those designing and building a similar, but larger, boat. However, that effort did not last, as the Russian designs were too complex and expensive. It was found easier to build semi-submersible craft. But more and more of these are being caught at sea. The recently discovered sub was not military grade. It could travel submerged, but not dive deep. It was built using the same fiberglass material used for the semi-submersible craft, but was larger, and had berths for six crew. There was space for about ten tons of cocaine. It probably cost several million dollars to build and was weeks away from completion and sea trials. The drug sub was similar to the small subs being built since the 1970s for offshore oil operations and underwater tourism.
North Korea has developed several mini-sub designs, most of them available to anyone with the cash to pay. The largest is the 250 ton Sang-O, which is actually a coastal sub modified for special operations. There is a crew of 19, plus either six scuba swimmer commandos, or a dozen men who can go ashore in an inflatable boat. Some Sang-Os have two or four torpedo tubes. Over thirty were built, and one was captured by South Korea when it ran aground in 1996.
North Korea is believed to have fitted some of the Song-Os and M100Ds with acoustic tiles, to make them more difficult to detect by sonar. This technology was popular with the Russians, and that's where the North Koreans were believed to have got the technology.
The most novel North Korean design is a submersible speedboat. This 13 meter (40 foot) boat looks like a speedboat, displaces ten tons and can carry up to eight people. It only submerges to a depth of about ten feet. Using a schnorkel apparatus (a pipe type device to bring in air and expel diesel engine fumes), the boat can move underwater. In 1998, a South Korean destroyer sank one of these. A follow on class displaced only five tons, and could carry six people (including one or two to run the boat). At least eight of these were believed built.