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 82nd 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: