Friday, September 9, 2011

Italian Small Submarines, Updated

(updating ongoing work in progress)
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
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.

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. Its midget submarine designs were originally designed to transport naval special warfare forces and swimmer delivery vehicles into a target area, and as such lacked an organic attack capability.  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.


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)

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.

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.


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.
1x Cummins diesel-electric, 300hp
1x 75hp engine generator
Battery: 24V 1100 Amp
1x screw
Surface:              75 tons
Submerged:       90 tons
Endurance:        20 days
Dimensions (2010):
Length:               23 meters
Beam:                 2.2 meters
Height:               4 meters
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."

Key to drawing:

1. Draft marker (painted);  2. diver lockout; 3. crew and passenger quarters; 4. forward battery pack; 5. permanent ballast (8 concrete blocks); 6. fuel and water ballast tanks; 7. control station; 8. large sabotage mines (6x 300 kg mines); 9. aft battery pack; 10. diesel service fuel tank; 11. diesel engine; 12. electric propulsion motor; 13. hydrophone; 14. rescue bouy; 15. periscope with television camera; 16. access hatch; 17. attack periscope; 18. retracting snorkel mast; 19. small (i.e., limpet) mines; 20. ballast tanks; 21. SDV/torpedo tube attachment points.

Instruments: Hydrocompass, autopilot, two sonars, automatic direction finder, bathythermograph, underwater telephone, radio and satellite communications, optical and television periscopes, navigation console, doppler log, and underwater collision avoidance apparatus.

The SX-756 is a straightforward adaptation of the SX-506, with increased range, endurance and weapons carrying capacity, resulting in a 2.2 meter increase in length.  South Korea and Pakistan are the only two countries known to have operated SX-756s. Cos.Mo.S. fabricated five of these boats on-site in South Korea from prefabricated sections shipped out from the company’s factory in Livorno.  Three more were purchased by Pakistan. In service, Korea’s boats were designated SX-756/K while Pakistan’s were referred to as SX-756/W.  The three Pakistani boats were later upgraded to MG-110/LR (see later entry). 

Intended missions for the SX-756 were largely the same as for Cos.Mo.S’ previous boats, with the additional option of a torpedo attack capability carried as an add-on external tube.       

Crew: 6  plus 8 swimmers. 
1x 200-hp Diesel engine,
1x 55-hp electric motor;
Cruising range on diesel engines, 1600 miles at 6 knots; on the electric motor 60 miles at 4 knots. 
Full speed 8.5kts surfaced,  6kts submerged. 
Displacement: 73 tons surface, 80 tons submerged.
Endurance:  20 days
Dimensions: Length: 25.2 meters (82.68 feet), Beam 2.1 meters

Armament options:
(A)  6x  Mk. 21 bottom-type mines (300 kg explosives), 8x Mk. 11 charges (50 kg of explosives)  and  40x small magnetic mines;
(B)  6x ammunition charge containers with an overall weight of 1800kg;
(C)  2x CE2F/60 (or 100) chariot SDVs  and 8x Mk.11 charges or 8x magnetic mines;
(D)  2x 324-mm torpedo tubes and two spare torpedoes (tube mounted externally in same place as chariots);
(E) 6x Mk.21/W bottom mines (300 kg explosives) and 8x Mk.11/W mines.
The torpedo apparatus and containers, bottom mines are placed on external mounts, the small demolition charges are carried between the casing and upper hull cylinder.


(work in progress)


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  1. What kind of SDV is this?

  2. It's a Yugoslav-built R-2 or R-2M Mala SDV. In addition to the JRM, several were operated by the Swedish Navy some years ago.


    here is a link to a video showing a LTTE submarine. Would be good if some research could be done one it.

  4. Extremely great article, never once I found so much info about this subs, heck, no wonder no one knew about them, even in Colombia, my own country!

    Oh, just to let you know, the our units will be decommissioned and replaced by U-206s, either this year or next one, because of their age, and also due to lack of spares.

  5. Hello there, a little update:

    Two Type 206 were bought a few weeks ago (plus another two for spares), they still need modernization and tropicalization, but never the less, as soon as they arrive to Colombia the Cosmos will be decommissioned, and the new units will receive their names.

    As of right now no one knows what will happen to the Cosmos, but since they were our first submarines, is likely both of them will end up as museum ships.

  6. Great article. The MG 120/ER had great specs, too bad its out of production. It arguably is one of the best Midget Submarine ever. You might be interested in this link:

    Just to summarize the specs of the MG 120/ER based on the link:
    Manufacturer: Cosmos
    Displacement: 130 tons
    Range: 2,960km at 7kts surfaced, 740km at 4.5kts submerged with Air Independent Propulsion system
    Crew: 6
    Weapons: 4 530mm Torpedoes

  7. if u have any more articles likethis plaese share the links

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