MS-29 Yono / IS-120 Ghadir Class Midget Sub - research quest

There is something of an ongoing quest by Open Source Intelligence enthusiasts to piece together a better understanding, and balanced view, of the North Korean 'Yono class' midget submarine, and its related types in service with other countries, particularly Iran where it is called the IS-120 Ghadir Class. Iran is deploying it in growing numbers within the Persian gulf, with at least 11 operational at this time.

Iranian IS-120. Image: FARS News

Another blog post covering it is here http://covertshores.blogspot.com/2010/06/north-korean-small-submarines-file.html

Here are the latest estimated views showing the internal layout as discerned from video and other evidence:

There have been many iterations of these sketches and the 3D model, but each has differed in minute detail that only the most enthusiastic observer might notice.

The original schematic/cutaway sketch was by the blogger Plane Man. We can now see that it was wrong in almost every detail, but correct in many ways too. It acted as the starting point of this project.

The first key error is that the small cylindrical item between the railings at the front of the fore deck is not a crew access. Instead the crew access in the main sail is in fact entering the 'torpedo room' - the video was misinterpreted. Another early sketch by Plane Man had similar errors:

Below it is a screen capture of our scale model from about the same angle, with many sections transparent. As you can see, the older sketch has forward hydroplanes which are mounted too high up and are much too large. Also evident is that the navigation lights on the sail (red on port, green on starboard) are misinterpreted as windows in the older sketch.

Although the original source is not clear (if you know please tell us so that we can give credit!), this South Korean sketch which we found during research turns out to be very good, although not exactly as per the IS-120 model:

(Red annotations added)

Another RoK illustration (author/source unknown to this author) of the related but smaller P-4 class submarine was also useful:



An earlier model of the MS-29 was created by TLAM Strike using SubSim game modelling approach.

Compared to the IS-120 model/sketches, the above MS-29 model has a more rounded nose and lacks the toothed edge between the upper deck cladding and the pressure hull. This is not incorrect, rather it shows some of the observed differences between the IS-120 and the early-production MS-29. The model shows externally mounted torpedoes which conforms to some DPRK defector accounts. This arrangement is either altogether wrong or more likely mixes up various DPRK submarines - whichever, the account of the defector are otherwise credible and extremely enlightening. William Magoffin was among the researchers contributing to this effort and has modelled many North Korean submarines, creating excellent visualisations.

The main break however came from several videos of the internal arrangement of the Iranian IS-120 shown on Iranian news channels and available in poor resolution on Youtube:

Birth, Death, and Rebirth of the Iraqi Navy

The Iran-Iraq War
In the early 1980s Iraq had built up a nucleus of a capable navy; it included a Frigate, 11 missile boats, 10 torpedo boats, plus a number of mine sweepers and landing craft. In November of 1980 it vanished. Following several skirmishes between Iraqi and Iranian patrol boats in the northern gulf the Iranian missile boat Paykan sank two Iraqi missile boats one a BGM-84 Harpoon missile and the other with its 76mm gun on November 5th.

Iranian Missile Boats similar to the Payken.
Photo: MEHR

In Operation Morvarid (November 28) Five Osa missile boats (some sources say seven, with two sunk by ship launched Harpoon missiles) and four Komar class missile boats (most sources incorrectly cite these as P-6 torpedo boats) were sunk by Iranian F-4 Phantoms. The Iranians lost the missile boat Paykan in return to a barrage of four SS-N-2 antiship missiles as the missile boat withdrew. The Iraqis also lost a number of smaller patrol ships and support vessels, approximately 80% of the Iraqi navy. This major failure of the Iraqi navy can be traced to two things; one the poor reliably of the SS-N-2 STYX missile. The Iraqi's expended prodigious numbers of N-2s in several engagements against the Paykan, the N-2s proved vulnerable to not just chaff and decoys but high speed maneuvering, gunfire and shoulder launched anti-aircraft missiles. Two the lack of effective air defense both from air support so close to shore and on-board air defense, many Iraqi ships were lost to Maverick missile firing F-4 Phantoms which could engage from far beyond the range of their anti-aircraft defenses. Iraqi air force fighters proved unable to defend their navy counterparts and lost 6 fighters in the battle.

On April 12 and May 2nd 1983 Iraq patrol boats attacked the Iranian oil rig in the Northern Gulf. In these attacks the Iraqis lost two Osa missile boats. This was the last attacks by the Iraqi navy in the war.

Osa I class missile boat
Photo: US Navy

A Fleet In Exile

The Mussa Ben Nussair impounded in La Spezia.
Photo: HarpoonHQ Database

When war with Iran broke out Iraq was in the process of purchasing several Italian naval vessels. The few vessels completed for Iraq were unable to sail home due to the war since Iran dominates the Straits of Hormuz, these vessels remained in their Mediterranean ports during the war and after... some are still there. The largest of these vessels is the armed replenishment tanker Agnadeen, she remains ported in Alexandria. Two Mussa Ben Nussair class frigates were completed and remained in Italy during the Iran-Iraq war. They were scheduled to sail to Iraq in the winter of 1990, Iraq invaded Kuwait and Italy denied them permission to leave. They sat and rotted in La Spezia with only the minimum of maintenance and remain their today.

The Assad Al Tadjier.
Photo: US Navy

Six Assad class corvettes were built but not delivered four were sold to Malaysia while two remain in La Spezia. Four Lupo class multipurpose frigates were built but not delivered, they were seized by Italy and incorporated in to the Marina Militare as the Artigliere class after having their ASW weapons removed. The final vessels were three SX-756/W and /S midget submarines.

Artigliere class frigate

Photo: Jorge Guerra Moreno

Desert Storm

Lurssen TNC 45 Missile Boat

Photo: Lurssen

When Iraq invaded Kuwait it captured a sizable (for the region) naval force including one FPB-57 and five TNC-45 missile boats, 100 Exocet anti-ship missiles, and a number of smaller patrol craft. Stemming from the unfamiliar systems of the Kuwait vessels none of them contributed in a meaningful way to the defense of Iraq against the US lead coalition, five of them were sunk or damaged during the war and one was recaptured. During the invasion of Kuwait the Iraqi's lost four vessels taking Bubiyan Island, three by a Kuwaiti missile boat that escaped the conquest of their country.

Iraqi leader Saddam Hussein had threatened to use oil as a weapon against the US led Coalition. Iraq as a oil producing country possessed several merchant tankers each one a potential floating bomb. One of the first Iraqi vessels attack was the tanker Amuriyah along with a British made Mk 6 hovercraft. Several Mk 20 Rockeye cluster bombs ignited the Kerosene aboard her and the resulting explosion destroyed her. The hovercraft was sunk with missiles and cluster bombs.

Polnocny class LSM
Photo: US Navy

During the Battle of Khafji an Iraqi amphibious force including 15 small craft attempted to cut off the city in Saudi Arabia that had been taken by Iraqi armored and mechanized forces. This operation was stopped by Coalition aircraft that sank three of them, the remainder took shelter in a Kuwait port that was off limits to Coalition bombing due to the risk of collateral damage, eventually permission was granted to attack them and 10 were sunk by aircraft.

An Iraqi Yevgenya Minesweeper under fire from Coalition forces
Photo: US DoD

In the Battle for Qurah and Umm al Maradim (two Kuwaiti islands) a patrol boat and two mine warfare craft were sunk by USN A-6s while another minelayer struck one of its own mines attempting to escape.

The Tripoli in dry dock after striking a LUGM-145 mine.
Photo: US DoD

In the northern gulf area the Iraqis laid approximately 1,300 sea mines; two US warships fell victim to these both on February 13th within minutes of each other: the Missile Cruiser Princeton and the Helicopter Assault Ship Tripoli. The Princeton fell victim to a Italian made Manta bottom mine in a area already declared clear of mines, the cruiser was able to limp back to port. Only three crew were injured. The Tripoli hit a LUGM-145 mine injuring four sailors. The ship was able to continue its duties for several days before returning to port for repairs.

LUGM-145 and Manta mines.
Photo: US Navy

The small number of Coalition ships damaged by mines can be partially attributed to the mishandling of the mines on the Iraqi's part. Mines were laid in strait lines and not random patterns. Ground mines were laid in their packing crates or were not fitted with their sensor packages, moored mines had their cables fouled or their acid horns still covered. In all the Iraqis laid six mine fields, each field had three barriers containing three lines of mines each. There were also four separate mine lines laid. Coalition mine clearince operations continued until mid July although US and Japanese clearance efforts continued in one problem area until September 1st.

Shelling from the WWII era USS Missouri on February 23 drew unwanted attention of the Iraqis two days later.
Photo: US Navy

The only other notable attack by Iraqi naval forces was the missile attack against the battleship Missouri on February 25. This attack was conducted by shore based HY-2 'SEERSUCKER' missiles (more commonly known as 'SILKWORMS'). One of the missiles crashed short of its target; ether decoyed or due to malfunction, the second missile was intercepted by a Sea Dart missile from the British destroyer Gloucester and it crashed 640 meters away from the American battleship.

An example of a HY-2 missile captured in 2003.
Photo: US Government.

Facing destruction by US lead forces the Iraqi's Navy (just as parts of its Air Force had) attempted to flee to neutral Iran, this resulted in what would be called the "Bubiyan Turkey Shoot". USN A-6s and F/A-18s used laser guided bombs to sink several Iraqi ships, Canadian CF-18s sank three captured Kuwaiti missile boats with 20mm cannon fire and damaged an Osa that escaped to Iran. British Lynx helicopters using Sea Skua missiles damaged or sunk several Iraqi ships. An interesting footnote is the lost of a captured Kuwaiti Maymoon class patrol craft sunk by gunfire along with 14 other boats by the battleship Wisconsin during the shelling of the Khawr al-Mufattah marina in Kuwait; possibility the last warship sunk by a battleship in combat.

One Polnocny LSM, one Osa PTG, and one Bogomol PC were among the major craft to escape to safety. The fate of the two patrol craft is uncertain but the Polnocny was recently seen as the subject of a life fire exercise by the Iranians in 2010. In all 140 craft of all kinds were destroyed or interned in Iran in this operation.

Saddam's Yacht
In 1983 Iraq took delivery of the $50 million Finnish made yacht al-Mansur. The ship was 120 meters long, capable of speeds of 20 knots and had a helicopter pad. The ship was furnished to excess, it featured a dining hall for up to 200 guests seated on velvet lined chairs, 24k gold toilet seats were provided, the passenger space was taken up with five large staterooms and for Saddam himself a one man escape submarine accessed from his stateroom (the submarine was never delivered). In 2003 the ship (which had survived the two previous wars unscathed) was attacked with a Maverick missile and three 500 lb bombs launched from US Navy aircraft. Additonal bomb hits tore the yacht loose from its anchor. Following looting by locals the vessel capsized and in 2005 was scrapped.

The bombed hulk of the al-Mansur.
Photo: AP

Saddam also had two other yachts, the small al-Quadisya for voyages along the Tigris and Euphrates rivers which was sunk during Desert Storm, and the Qadissat Saddam which never arived in Iraq and was first given to the King of Saudi Arabia (a gift that was later retracted) then sold to its current French owner.

Iraqi Freedom
Following the near total destruction of its navy and UN sanctions in new military purchases Iraq only source of new warships and arms was its self. At this time their foreign built ships, the Ibn Marjid frigate, the remaining Osa I, Bogomol and SO-1 class subchaser remained mostly immobile due to a lack of spare parts or unrepaired battle damage.



Around 80 Sawari class patrol boats were built following the '91 Gulf War. The Sawari was a simple gasoline powered patrol boat that could be armed with a 12.7 mm machine gun and up to four mines. The aft deck could be covered with a tarp or sheets to conceal the presence of mines aboard.

Mines and fake oil drums aboard an Iraqi Tug.
Photo: UK MoD

Another development of this time was outfitting their remaining non-combat vessels as mine layers. In 2003 Coalition forces captured two two tugs outfitted for mine laying in the Khor Abd Allah waterway. One had oil drums that would cover the mines on its deck. The other tug towed a barge with a set of hidden mine laying rails installed inside. Between them they could carry 100 sea mines incl. LUGM-145 moored mine and the hard to find Manta bottom mine.



Rebuilding
Following the fall of the Saddam regime Iraq found it self in need of a new navy and had virtually a clean slate to build it on. The first vessels (which had been designed before the war) were the al-Uboor class patrol boats, these craft turned out to be a failure with one never passing sea trials. All five were eventually replaced by foreign built boats.

Predator class patrol boat at sea
Photo: MoD

One of the more successful boats are the five Predator class 81' patrol boats (also called Nasir class). Built in China for the Saddam regime these boats were impounded in the UAE until after the war. These craft are lightly armed capable of caring only one or two 7.62mm machine guns depending on configuration.

Iraqi Saettia class patrol ship Majed ariving in Umm Qsar.
Photo: MoD

Four Saettia Mk 4 class offshore patrol vessels are to be built for the new Iraqi Navy. These patrol vessels are armed with a Otobreda 25mm cannon, have a helipad capable of handling a Huey sized helicopter and is equipped with a stern ramp for launching a RHIB.

Defender class boat.
Photo: MoD

Iraq's southern border with Iraq is the Shatt al Arab waterway which includes the major port of Basra, to the south west is the Khor Abd Allah waterway near the border with Kuwait. Small and fast boats are commonly used for patrol on these rivers. The US built 25' Defender class response boat is one of the more common ones with 26 vessels acquired. Smaller craft such as RHIBs and Airboats are also used. Swiftships has recently received the contract to build nine 35 meter patrol boats.



Sources:
Globalsecurity
Wikipedia
Harpoon HQ database
www.history.navy.mil
MIW 21st Century eBook
Iraq and the War of Sanctions (Via Google Books)
Gulf War Mining Case Study by "Seaman84"
Desert Storm at sea: What the Navy really did (via Google Books)
Timelines.com
warandgame.com
Iran-Iraq War in the Air, 1980-1988

Yugoslavia's Sabotage Submarines

Croatian Navy (HRM) P-01 Velebit, formerly P-914 Soca

Small submarine operations in the former Yugoslav Navy (JRM) were a part of the SFRY’s (Tito’s Yugoslavia, the Socialist Federal Republic of Yugoslavia) defensive partisan warfare doctrine, which was intended to deter both the west and the Warsaw Pact from invading, and failing deterrence, to combat one or both opponents using partisan and guerrilla tactics. Following the collapse of the SFRY, the FRY consisting of Serbia and Montenegro carried this doctrine forward in the RMVJ (Navy of the Yugoslav Army, the SFRY’s naval force).  Under the doctrine of “Total National Defense,” the conventional submarine force constituted part of the country’s initial line of defenses, while the midget submarines, known as “sabotage submarines” in Yugoslav terminology performed a diversionary role, laying offensive and defensive minefields, striking enemy forces in port, and at anchored invasion forces once a beachhead had been established. A discussion of Yugoslav defensive naval doctrine will form the basis of a separate post. 

P-916 Vrbas

Although the JRM had experience operating a former Italian CB-class midget submarine in the 1950s, the requirement to build a small “sabotage” submarine was only established in 1977.  During the early 70's, while the B-71 and B-72 (Heroj and the Sava) programs still were still underway, development of small submarines suitable for operation in the area of about two-thirds of the surface of the Adriatic Sea north of a line from Molat Island to Ancona in depths of less than 150 meters was discussed. Operation of large submarines  along the coastal shelf on the Italian side of the Adriatic, some 10-20 nm wide, with depths of less than 20 meters was out of the question. Operations there would necessitate development of midget submarines for the JRM.

P-912 Una in the Tivat Repair Arsenal, ca. 2008

The first preliminary analysis of a small torpedo-attack submarine type M-100 and a small sabotage submarine-type M-40 was conducted in 1974. This studied a requirement for a submarine that could transport, disembark and re-embark underwater demolitions teams and their equipment, lay offensive and defensive minefields, have the capability of carrying two torpedoes, and collect tactical intelligence. While larger submarines could be and were used for such operations in the JRM, operational experience pointed out the greater suitability for small or midget submarines in this role, which led to the establishment of a 100-ton upper limit for design displacement, and gave the class its designation of “M-100.” It should also be pointed out that the term “sabotage submarine” did not imply “submarine sabotage,” which in the Yugoslav context meant “saboteurs” inserted individually or as a team into the immediate area of operations upon exit from the submarine, and which more normally implied “wet” submarines, i.e., swimmer delivery vehicles such as the R-1 and R-2. 

Design specification requirements included:

• 
  
  Shallow water (10-15 meters) maneuvering capability
• 
  
  Maximum diving depth 150 meters at a 4kt speed, with 80% battery expenditure
• 
  
  Working depth of 90 meters
• 
  
  Good habitability sufficient for 48 hours autonomy
• 
  
  Maximum underwater speed of 6-7kts
• 
  
  Good controllability and maneuverability at speed of only 1kt

 Tests conducted on P-911 indicated the design met all requirements, with the overall class technical specifications resulting as follows: 
Length: 19.52 m

Width: 3.64 m

Diameter: 2.70 m

Height to deck superstructure: 3. 38 m

Height to periscope:  5.30 m

Displacement, surface:  79.58 t

Displacement underwater:  90.27 t

Reserve Buoyancy: 10-14%

Operational Diving Depth: 105 m

Maximum depth: 120 m

Computational depth: 181 m

Minimum depth of the sea for diving: 10 m

Underwater speed, max:  7.48 knots

Underwater speed, economical:  4 knots

Surface speed:  5.9 knots

Range of navigation with 80% of Battery Expenditure:

at  6.4 knots and external attachments (R-1, etc) : 106 nm 

at  4 knots and clean hull: 254 nm

at  5.9 knots on the surface: 90 nm

Total range with 100% expenditure battery at 3 knots: 270 nm

Autonomy:

with full crew (6 people): 160 hours

for 10 people (4 crew and 6 div.): 96 hours

Sabotage resources:

2x R-1 vehicles mounted forward under casing hatch

2x R-1 vehicles mounted aft under casing hatch

Div. mines, M 66 or M 71: 6 or 12 pieces.

Armament:

Mines:  AIM - M 70 or AIM - M 70/1: 4 pcs.

Pistols: 4 pcs.

flare guns: 1 pc.

The M-100s were assigned to the 88th Submarine Brigade, but supported the 82^nd Maritime Center, stationed at Kumbor in Boka Kotorska, Montenegro. In operational practice, they proved to be an extremely quiet and likely very effective design. Yugoslav sources claim that no M-100 was ever detected during an operation, nor even on a “friendly” training exercise with JRM forces. During initial testing, P-911 repeatedly crossed the hydrophone line at the entrance to Lora inlet, where the main JRM base was located at Pula. At no time was the boat detected by the hydrophone array. It is also reported that while conducting a patrol 30 nautical miles out from the Montenegrin coast just before the Allied bombing campaign of 1999, P-913 encountered a NATO submarine. Remaining behind at a distance of 100 meters, the “uninvited guest” reportedly failed to detect the midget. If this incident is true, and we have no reason to doubt its author, this represented a significant vulnerability on NATO’s part, or a significant strength on the part of the RMVJ, depending on one’s point of view. 

P-914 Soca launch

As built, the M-100s were powered solely by batteries with no on-board recharging capability. P-914, which had been left behind in Split upon Croatia's secession from Yugoslavia, was rebuilt and lengthened approximately one meter to incorporate a diesel generator. P-914 was renamed Velebit (P-01) upon recommissioning into the Croatian Navy, serving until 2006. Some confusion among outside observers has arisen concerning the presence or absence of sails/conning towers on these boats. Local writers comment that these "sails" are actually removable plastic units, which serve as crew protection, and were most often used only in port. The exception to this is Velebit, which after its modifications, appears to have retained the sail permanently. As a class, the M-100s have fared well, with all but one surviving the scrapper's torch, being donated to museums around the former Yugoslavia. 

RMVJ units on parade in Boka Kotorska, mid to late-1990s

 Fates of the P-911s are shown below:

BSO =  Brodogradiliste Shipyard, Split

Sources: 

http://www.paluba.info/smf/brodovi-jrm/diverzantske-podmornice-klase-p-911/ "Diverzantske podmornice klase P-911 "Una""

http://www.podmornicar.com/istorijat.html

http://www.forum.hr/showthread.php?t=522605,"Yu-Podmornice"
http://www.avijacijabezgranica.com/forum/showthread.php?4804-Retrospetkiva-Yu-podmornicarstva-II-put, "Retrospektiva Yu podmornicarstva - II put"

Italian Small Submarines

(Work in Progress, to be continued)

Since before the Second World War, Italy has been a prolific designer and producer of small submarines. The CA and CB-class midget submarines and their operational histories are already well-known, but those produced post-WWII are much less known, poorly documented and the subjects of confusion and misinformation. Fincantieri, Maritalia and Cos.Mo.S. are all known to have produced viable small submarine designs, however only Cos.Mo.S. is known to have definitely produced actual small submarines, albeit all for export.

This file is intended to serve as a repository for actual and projected post-World War 2 Italian small submarines designs. As always, comments and additional information are welcomed.

Cos.Mo.S. SpA, Livorno

Overview

Over the course of its 48-year history from 1955 to 2003, Cos.Mo.S. is believed to have produced a total of 24 midget submarines. Of these, 20 were delivered to five export customers, three were sold to Iraq but never delivered, and one remained at the Cos.Mo.S. factory as a demonstrator and developmental testbed. For further information on Cos.Mos.S. history, see separate entry under Italian Chariots.

Because of Cos.Mo.S' very secretive business practices, it may never be possible to completely ascertain all their boats' deliveries, but the most likely breakdown of their actual midget submarine deliveries is as follows (Provisional):

Type Date Quantity Customer

SX-404 1969 2 Taiwan

SX-404/B 1972 6 Pakistan

SX-506 1973 2 Colombia

SX-506 198? 2 Republic of Korea (i.e. South)

SX-756/K 198? 5 Republic of Korea

SX-756/W 1985 3 Pakistan

SX-756/W 1989 2 Iraq (never delivered)

SX-756/S 1989 1 Iraq (never delivered)

MG-120/ER unknown 1 Demo boat retained at factory

Cos.Mo.S' midget submarine designs were an ongoing evolution and expansion of essentially the same design over the course of 45 years from the mid-1960s to the company's demise in 2003. The company frequently marketed the same design under different designations to multiple potential clients. The reason for this practice is not known; possibly it was intended to reduce the possibility of clients becoming aware of each others' capabilities and Cos.Mo.S' relationship with them, or possibly it was simply to create the impression that Cos.Mo.S produced and sold more units than was actually the case.

(click to enlarge)

SX-404

SX-404 was Cos.Mo.S’ first known venture into midget submarine design and production. This was a 40-ton boat intended to transport naval SOF operators into hostile waters over distances greater than those that could be achieved by the company’s Chariot swimmer delivery vehicles (SDVs). All Cos.Mo.S’ midget submarines were designed with the capability of carrying two of the company’s CE2F-series SDVs.

Cos.Mo.S built two subvariants of this design, the SX-404, two examples of which were delivered to Taiwan, and the SX-404/B, six of which were delivered to Pakistan. The two subtypes are easily distinguished from each other by the presence or absence of a large horseshoe step on the sail. Pakistan’s SX-404/Bs have this step, Taiwan’s do not.

Displacement: 40 tons

Length: 59.1 feet

Width: 6.5 feet

Draft: 8.4 to 10.4 feet

Surface Speed: 10kts (designed), actual 4kts

Submerged: 7.2 kts

Max depth: 131.2 feet

Range: 1235 nautical miles

Crew 4 Officers, 2 NCOs, 6 passengers

Payload: 2 SDVs (CE2F Chariots)

SX-404 (above) and SX-404/B (below), showing evolutionary changes between the two sub-types

SX-404

Taiwan

Taiwan was Cos.Mo.S’ first midget submarine customer, taking delivery of two in 1969. As originally delivered, Taiwan’s boats had rounded bows, which were later modified to the more common ship-type bow seen on most other of Cos.Mo.S’ boats. The two boats, S-1 and S-2 served until 1973. Both boats are preserved and are on public display at the Taiwan Naval Academy.

Taiwanese SX-404, original configuration

Taiwanese SX-404, final configuration

source: Cos.Mo.S. SpA

SX-404/B

Pakistan

In the late 1960s, Pakistan ordered six SX-404s to a slightly modified design. The Pakistani Navy deployed its six boats against the Indian Navy during the 1971 war. One of them, reportedly fitted with external torpedo tubes fired on an Indian naval frigate, INS Kukri, but the torpedo remained stuck in its external launcher. Of the six, one was lost with all hands as a result of an accident on December 27, 1976. Following removal from service, four were scrapped and one was placed ashore as an exhibit in the Pakistan Maritime Museum in Karachi.

Pakistani SX-404/B on display at Pakistan Maritime Museum, Karachi

source: internet

SX-506

Colombia's two SX-506s in port, Cartagena, Colombia, November 2009

source: authors

S20 ARC Intrepido

S21 ARC Indomable

1972 - 07 August: Arrival in Colombia

1973 - 17 April Intrepido commissioned

1973 - 03 July Indomable commissioned

1980 - both modernized, lengthend 6ft to accommodate air conditioning unit

1995 - batteries replaced; auxiliary systems updated to incorporate automatic load control

1998 - 1st participation in annual UNITAS exercises

2003 - steel plating on outer hull casings replaced with anticorrosive steel at COTECMAR

Missions: shallow-water operations, commando operations, tactical amphibious reconnaissance, underwater demolition, attacks on port facilities, offshore installations, anchored or moored targets, beach surveys and intelligence missions.

Propulsion:

1x Cummins diesel-electric, 300hp

1x 75hp engine generator

Battery: 24V 1100 Amp

1x screw

Displacement

Surface: 75 tons

Submerged: 90 tons

Autonomy: 20 days

Dimensions (2010):

Length: 23 meters

Beam: 2.2 meters

Height: 4 meters

Armament:

8x explosive charges from 50kg to 2050kg, including the charges onboard chariots.

Mines: 6x Mk-21; 8x Mk-50.

Other Equipment:

2x CE2F/X-60 Chariots capable of carrying 8 submarine commandos and explosive charges.

Note: SX-506s are almost indistinguishable externally from SX-756s. The two Colombian boats can be distinguised from one another when their snorkel masts are raised. The ball mechanism on top of Intrepido's mast is flattened, whereas Indomable's is spherical. Both are distinguishable from South Korea's SX-506s by the step mounted around the sail, which is absent on the Korean units. Colombia's SX-506s are sometimes referred to as SX-506/B as a result of their lengthening, however this designation is not used by the Colombians, who usually refer to them as "Submarinos Tacticos."

One of South Korea's two SX-506s prior to delivery

source: Cos.Mo.S SpA