Updated: New classes added. Originally posted 10th June 2017.
This article relies heavily on information from unnamed individuals. Sincere thanks to everybody who has helped. Information and corrections welcome. Contact the author HERE
DG-550
Drass SDVs and submarines
It seems probable that Italian firm Drass is behind the construction of two new submarines for Qatar. The exact model is unclear but they are likely to be in the 'compact' or 'midget' categories of Drass' range.
The secrecy surrounding Italian Swimmer Delivery Vehicle (SDV) manufacture is legendary. Drass are an established name in the defense industry supplying hyperbaric chambers for diver decompression. They also offer a range of SDVs and midget submarines which are the evolution of the previous Cos.Mo.S designs. Their connection to Cos.Mo.S has been known in relevant circles but it is only very recently that they went public with their new-build SDV offering.
Quick reference table to DRASS and historical Cos.Mo.S types. In all cases DRASS designs have been updated.
Before involvement with the Cos.Mo.S designs, Drass (formerly Roberto Galeazzi) had previously designed SDVs:
From Drass website
Based in La Spezia, conveniently near to the Italian naval Special Forces (COMSUBIN), Drass were close neighbors of Cos.Mo.S during the time that the latter was a major midget submarine and SDV exporter. So it is not entirely Surprising that Drass have taken over the Cos.Mo.S designs after they went out of business (see notes HERE). Although the fundamental designs are established classics from Cos.Mo.S, Drass has incorporated up to date lithium-ion battery technology, a new control and navigation module and the latest generation of sensors.
Get The essential guide to World Submarines
This Covert Shores Recognition Guide Covers over 80 classes of submarines including all types currently in service with World Navies.Check it out on Amazon
DG-900 Compact Submarine
The DG900 is the largest design in the compact submarine range. As the name implies, it displaces 900 tons. The operational is that the submarine is large enough to function as a regular patrol submarine, yet still compact enough to operate in the special inshore roles. Relative to Drass' smaller types it offers improved underwater performance, endurance and armament.
The most distinctive feature is its novel sail design with sculptured forward surfaces and the dive planes mounted relatively far aft. This contains several non-penetrating masts including a DRASS optronic mast. The hull uses a double-hull construction but with the pressure hull frames on the inside. This unusual arrangement is the evolution of the Cos.Mo.S midget submarines design. At the stern new X-form control surfaces lead to a large-diameter basic pump jet.
Inside it is comparatively traditional, at least compared to the smaller Drass designs. There are six 533mm (21 inch) torpedo tubes in the bow. These enter a torpedo room with capacity for two reloads. And two additional torpedoes can be mounted under the casing in external tubes. Beneath the tubes is a modest cylindrical array sonar.
In common with other Drass types the design emphasizes minimum crew needs. It is also able to host AUV (Autonomous Underwater Vehicles) and SDVs (Swimmer Delivery Vehicles). Drass builds both.
DG-550 Compact Submarine
This design has a similar overall form to earlier Drass / Cos.Mo.S designs but with some enhancements similar to the DG-900. These include new non-penetrating masts and X-form rudders.
At least one DG-550 is under construction.
DG-450 Shallow-water Attack Submarine
The largest type in the DRASS line up, the 450 ton DG-450 is significantly larger than any reported Cos.Mo.S designs. It can carry two 6-man SDVs and three heavyweight torpedoes making a truly multi-role platform in littoral waters.
DG-450 Specification
Displacememt: 369 tons surfaced, 451 tons submerged
Length: 34.8 m
Width: 3 m hull diameter, 4.3 m with hydroplanes
Height: 7.15 m
Crew: 9 + 12 PAX
Range: 3,000 nm, 200 nm on batteries
Speed: 12 kt max, 5 kt cruising, 9 kt surfaced
Operating depth: 150 m
Armament: 3 x 533mm torpedoes standard, plus 4 x lightweight torpedoes or 8 x mines in lateral tubes as mission-specific load
Other: 2 x 6-man SDVs or 2 x 8-man inflatable boats and up to 12 Special Forces containers
Equipment: Passive Sonar, Obstacle Distance Measurement System, SWR Intercept Sonar, Radar Warning Receiver, Electronic Support Measure (OPTIONAL), TV Periscope System (2nd Periscope Optional), HF Transceiver, UHF/VHF Transceiver, Underwater Communication System, Sound Powered Telephone, Intercom Network, Inertial Navigation System, Echo Sounder with Sound Velocity, Doppler Velocity Log
DG-350 Shallow-water Attack Submarine
DG-350 Specification
Displacememt: 291 tons surfaced, 352 tons submerged
Length: 34.8 m
Width: 3 m hull diameter, 3.5 m with hydroplanes
Height: 7.15 m
Crew: 9 + 12 PAX
Range: 2,500 nm, 200 nm on batteries
Speed: 12 kt max, 5 kt cruising, 9 kt surfaced
Operating depth: 150 m
Armament: 3 x 533mm torpedoes standard, plus 2 x lightweight torpedoes or 8 x mines in lateral tubes as mission-specific load
Other: 2 x 4-man SDVs or 2 x 8-man inflatable boats and up to 12 Special Forces containers
Equipment: Passive Sonar, Obstacle Distance Measurement System, SWR Intercept Sonar, Radar Warning Receiver, Electronic Support Measure (OPTIONAL), TV Periscope System (2nd Periscope Optional), HF Transceiver, UHF/VHF Transceiver, Underwater Communication System, Sound Powered Telephone, Intercom Network, Inertial Navigation System, Echo Sounder with Sound Velocity, Doppler Velocity Log
DG-160 Shallow-water Attack Submarine
The 140 ton DG-160 is reminiscent of the unbuilt Cos.Mo.S X-201 SWATS submarine concept but with a more conventional ‘+’ tail section based on the MG-120 design.
left: photo of Cos.Mo.S MG-120 bow with twin torpedo tubes and sonar dome on top, right: CG of DG-160 with two DRASS DG2 SDVs on deck.
CLICK for HIGH-RESOLUTION image. 
DG-160 Specification
Displacememt: 141 tons surfaced, 169 tons submerged
Length: 32.24 m
Width: 2.5 m hull diameter, 4.12 m with hydroplanes
Height: 6.45 m
Crew: 7 + 8 PAX
Range: 2,000 nm, 160 nm on batteries
Speed: 10 kt max, 5 kt cruising, 8.5 kt surfaced
Operating depth: TBC
Armament: 2 x 533mm torpedoes standard, plus 2 x 533mm torpedoes or 6 x mines in lateral tubes as mission-specific load
Other: 2 x SDVs or up to 6 x inflatable boats
Equipment: Passive sonar, active sonar, SWR Intercept Sonar, Radar Warning Receiver, TV periscope, Underwater Communications, Ring-Laster gyrocompass, echosounder, doppler velocity log, Electromagnetic log
DG-120 Shallow-water Attack Submarine
The DG-120 sacrifices Special Forces payload for a larger complement of wire-guided 400mm torpedoes with 11 nautical miles range. Only two commandos can be carried in addition to the seven-person crew.
DG-120 Specification
Displacememt: 115 tons surfaced, 130 tons submerged
Length: 28.2 m
Width: 2.3 m hull diameter, 3.92 m with hydroplanes
Height: 6.4 m
Crew: 7 + 2 PAX
Range: 1,800 nm, 120 nm on batteries
Speed: 9 kt max, 4 kt cruising, 7.5 kt surfaced
Operating depth: TBC
Armament: 6 x 400mm torpedoes, 6 mines.
Other: 1 x inflatable boat
Equipment: Passive sonar, active sonar, SWR Intercept Sonar, Radar Warning Receiver, TV periscope, Underwater Communications, Ring-Laster gyrocompass, echosounder, doppler velocity log
DG-100 Compact Submarine
This design bears a passing resemblance to the Russian Piranha design. This is because it carries its torpedo tubes in the unusual should position. Overall it is similar to the DG-550 and DG-900 but scaled down to just 100 tons.
DG-85 Shallow-water Attack Submarine
The smallest of the DRASS midget submarine designs, the DG-85 is intended for a) attacks on ships moored in harbor or harbor installation using the 6 mines which are carried externally, b) patrol of coastal areas as deterrent against enemy ships approaching own coastal targets using 400mm wire-guided torpedoes, c) Intelligence gathering using acoustic, electronic and communication (SIGNIT) sensors, and d) deployment of mines in very shallow waters / harbor mouths.
DG-85 Specification
Displacememt: 81 tons surfaced, 94 tons submerged
Length: 21.06 m
Width: 2.2 m hull diameter, 3.82 m with hydroplanes
Height: 5.75 m
Crew: 6 + 1 PAX
Range: 1,500 nm, 100 nm on batteries
Speed: 7.5 kt max, 4 kt cruising, 6 kt surfaced
Operating depth: TBC
Armament: 2 x 400mm torpedoes, 6 mines.
Equipment: Passive sonar, active sonar, SWR Intercept Sonar, Radar Warning Receiver, TV periscope, Underwater Communications, Ring-Laster gyrocompass, echosounder, doppler velocity log
DG2-EN 2-man SDV
The DG2 EN design is a direct evolution of the Cos.Mo.S CE2F X100 design and is intended purely as an offensive system to carry two combat swimmers From a host submarine (or vessel) to within range of their enemy ships in port or at anchor.
Main article: 
Cos.Mo.S CE2F chariot
The hull is still recognizable as the CE2F but is rearranged with the crew air bottles moved inside the hull (not below the hull) and the belly payload removed. This is similar to the Cos.Mo.S CE3F design which was produced for South Korea. And like the CE3F, the canopy structure has lost its ‘head and shoulders’ cross-section. The payload is carried behind the crew in a locker on the back of the boat.
DG2 EN Specification
Length: 7.56 m
Width: 0.8 m without hydroplanes
Height: 1.3 m
Weight (dry): 2.4 tons
Crew: 2
Range: 80 nm
Speed: 4.8 kt max, 3.5 kt cruising
Operating depth: TBC
Host submarine transit depth: 100m
Payload: Special Forces stores or 6 limpet mines carried in locker on back of craft
The ultimate book of Special Forces subs Covert Shores 2nd Edition is the ONLY world history of naval Special Forces, their missions and their specialist vehicles. SEALs, SBS, COMSUBIN, Sh-13, Spetsnaz, Kampfschwimmers, Commando Hubert, 4RR and many more.
Check it out on Amazon
DS4 4-man SDV
The DS4 is a minimal evolution of the Cos.Mo.S CE4F project for the Turkish Navy's SAT (Su Altı Taarruz) in the late-1990s to early-2000s.
The design does not have some of the post-delivery alterations which were added to the CE4F (see below) but does have the additional stabilizers running along the lower hull.
Main article: 
Cos.Mo.S CE4F
DS4 Specification
Length: 8.6 m
Width: 1.44 m hull diameter, 2.0 m max with
Height: TBC
Weight (dry): 3.49 tons
Crew: 2 + 2 PAX
Range: 110 nm
Speed: 7.5 kt max, 5 kt cruising
Operating depth: 50 m
Host submarine transit depth: 100m
Payload: Two sealed containers for Special Forces stores or limpet mines. Provision for 8 mini-torpedoes.
These photos, used by Drass in reference to the DS4, are of the Cos.Mo.S CE4F before delivery to Turkey
Get The essential guide to World Submarines
This Covert Shores Recognition Guide Covers over 80 classes of submarines including all types currently in service with World Navies.
DS6 6-man SDV
The DS6 is Drass’ answer to the 6-man SDV market which is dominated by US, French and British designs. This allows a team of four Special Forces operators to be transported for a cross-beach mission.
Analogous to the unbuilt Cos.Mo.S CE6F design, the DS6 is a lengthened DS4 with an additional row of diver’s sitting in the rear compartment. The air supply is increased but otherwise most systems remain the same, leading to a slight compromise in performance to accommodate the extra passengers.
DS6 Specification
Length: 9.1 m
Width: 1.44 m hull diameter, 2.16 m max with hydroplanes
Height: TBC
Weight (dry): 3.66 tons
Crew: 2 + 4 PAX
Range: 70 nm
Speed: 7 kt max, 5 kt cruising
Operating depth: 50 m
Host submarine transit depth: 100m
Payload: Two sealed containers for Special Forces stores or limpet mines. Provision for 8 mini-torpedoes.
DS8 8-man SDV
The SDV carried is likely to be the Drass DS-8 which is generally analogues to the US Navy's SDV Mk.VIII Mod-0 and newer Mk.XI. It is larger than the rival Italian Deep Shadow from CABI Cattaneo.
This 9 meter long wet-sub can transport 8 combat swimmers 60 nautical miles at depths of up to 30 meters (100 meters transit depth). It is 1.42 meters wide and capable of 7 knots.
DS-8 SDV. Image, Drass
The design was previously shown with a cruciform tail similar to the DS6, but is now illustrated with the X-form control surfaces.
Related articles (Full index of popular Covert Shores articles)
Drass DG-900 Compact Submarine
Cos.Mo.S CE2F chariot
Cos.Mo.S Nessie Fast SDV submersible boat
Cos.Mo.S CE4F 4-man SDV
Vogo 'Chariot' SDVs (SDV-300, SDV-340...)
COMSUBIN's submersible boat
Mignatta first human torpedo
Decima MAS attack on New York 1943 (CA Class)
Deep Shadow SDV
EMT FWS-1 to -5 Barracuda SDVs