The Hrabri class of submarines consisted of two vessels built for the Kingdom of Serbs, Croats and SlovenesYugoslavia from 1929 onby Vickers-Armstrong in the United Kingdom. Launched in 1927, the boats were named (Brave) and (Fearless). Their design was based on that of the British L-class submarine of World War I, and they were built using parts from L-class submarines that were never completed. The Hrabri-class were the first submarines to serve in the Royal Yugoslav Navy (KM), and after extensive sea trials and testing they sailed from the UK to the Adriatic coast of Yugoslavia, arriving in April 1928. They were armed with six bow-mounted torpedo tubes, two deck guns, one QF 2-pounder () L/39 anti-aircraft gun and two machine guns. Their maximum diving depth was restricted to by Yugoslav naval regulations.
Prior to World War II, both submarines participated in cruises to Mediterranean ports. In 1930, Nebojša was damaged in a collision with a merchant ship. In 1933–1934 both boats were refitted, their superstructure was extensively modified and the 2-pounder gun on each submarine was replaced with a single Hotchkiss M1929 anti-aircraft machine gun. By 1938 the class was considered to be obsolete, but efforts to replace the two old boats with modern German coastal submarines were stymied by the advent of World War II, and the class remained in service.
Immediately before the April 1941 German-led Axis invasion of Yugoslavia, the two boats conducted patrols in the Adriatic Sea. Hrabri was captured by the Italians at the time of the Yugoslav surrender in mid-April, and after an inspection she was scrapped. Nebojša evaded capture and made it to Egypt to join the British Royal Navy (RN). Along with other vessels and crews that had escaped during the invasion, Nebojša formed part of the KM-in-exile, which operated out of eastern Mediterranean ports under the operational command of the RN. Nebojša was overhauled and initially served with RN submarine forces in the Mediterranean Sea as an anti-submarine warfare training boat. At the end of 1941 the RN prohibited her from diving and she was employed as a battery charging station for other submarines. In May 1942 her crew were removed and placed in a British military camp following a revolt by Yugoslav generals based in Egypt, and she received an almost entirely RN crew. Nebojša underwent another extensive overhaul by the RN, then she was briefly utilised for training in Beirut. The boat was formally handed back to the KM-in-exile in mid-1943, after which she underwent a further substantial refit. Nebojša eventually made her way to Malta where the headquarters of the KM-in-exile was then located. After the war in Europe ended, Nebojša was transferred to the new Yugoslav Navy and renamed Tara. She was used in a static training role until 1954, when she was stricken.
Background
The naval policy of the Kingdom of Serbs, Croats and Slovenes (the Kingdom of Yugoslavia from 1929) lacked direction until the mid-1920s, although it was generally accepted by the armed forces that the Adriatic coastline was effectively a sea frontier that the naval arm was responsible for securing with the limited resources made available to it. A few naval officers attended submarine warfare courses in the United Kingdom and France in the hope that funds might be forthcoming from the government to create a submarine arm for the navy. In 1925, King Alexander visited the navy for the first time and several senior naval officers suggested to him that a submarine arm was needed. In the middle of 1926, the submarine visited the Adriatic to familiarise Yugoslav naval officers with the class and undergo trials to demonstrate its capabilities. In the same year, a modest ten-year construction program was finally initiated to build up a force of submarines, coastal torpedo boats, torpedo bombers and conventional bomber aircraft for coastal defence. The two Hrabri-class submarines, named (Brave) and (Fearless), were some of the first new acquisitions aimed at developing a naval force capable of meeting this challenge. Nebojša was also named after the eponymous medieval tower near the Yugoslav capital of Belgrade, which had played a prominent part in battles between Serb and Ottoman Empire forces during the wars of Serbian independence from 1804 to 1817, and again during the Austro-Hungarian Empire campaigns against the Kingdom of Serbia in World War I.
The Hrabri class was built for the Kingdom of Serbs, Croats and Slovenes by the Vickers-Armstrong Naval Yard on the River Tyne in the United Kingdom. Their design was based on that of the L class, and they were built using parts originally assembled for the Group III boats HMS L-67 and HMS L-68, which were not completed due to the end of World War I. The British Royal Navy cancelled the order for the two submarines in March 1919, and the partially constructed hulls were launched on 16 June and 2 July respectively to free up the slipways on which they were being built. In November the hulls were sold by the Royal Navy (RN) to the shipyard, and once the contract with the Yugoslavs was signed they were brought back onto the slipways and completed to a modified design. The British boats were chosen for two reasons: the visit by HMS L53, and a British credit for naval purchases which included spending some of the funds in British shipyards.
A total of 33 L-class boats were built by the British between 1917 and 1927. The L-class boats were an improved version of the and achieved a better relationship between displacement, propulsion, speed and armament than their predecessors, including a powerful armament of both torpedoes and guns. The class was designed for operations in the North Sea in World War I, but due to their considerable range they were deployed around the world during the interwar period by the RN, including in the Mediterranean, and three were still in service at the outbreak of World War II.
Description and construction General The Hrabri-class boats were of a single hull design with a straight stem, circular cross section and narrow pointed stern. The ballast and fuel tanks were configured as saddle tanks, one along each side, tapered at either end. The keel was straight until it inclined upwards as part of the pointed stern, and a detachable ballast keel was also fitted. The submarines had two shafts each driving a three-bladed propeller, and boat direction was controlled using a semi-balanced rudder. The forward hydroplanes were positioned about aft of the bow, and the aft set were positioned aft of the propellers. The two anchors could be lowered from vertical tubes in the bow. The boats had a narrow steel-plated deck, and a tall, long and narrow conning tower with a navigation bridge positioned in the centre of the deck. Both fore and aft and at the same level as the conning tower were gun platforms protected by high bulwarks. The superstructure was partially raised both fore and aft of the gun platforms, with the torpedo hatch and a small torpedo crane forward, and the machinery hatches aft. Aft of the machinery hatches was a small step down which ran to short of the stern and contained a small ship's boat. The deck was too narrow and slippery for the crew to walk along it while underway.
Internally, transverse bulkheads divided the hull into seven watertight compartments. The first two compartments housed the torpedo tubes and reserve torpedoes, respectively, along with sleeping accommodation for some of the crew. The forward trim tanks were located underneath these two compartments. The upper levels of the third and fourth compartments contained accommodations for the officers and additional crew, respectively, and the lower levels each contained 112-cell electric batteries. In the middle of the boat, underneath the conning tower, was the control room. Aft of that, the fifth compartment was taken up by the machinery room, containing two diesel engines for surface running. Underneath that were diesel fuel tanks. The sixth compartment contained two electric motors on the upper level and an electric battery compartment with another 112 cells on the lower level. At the stern, the seventh compartment contained the steering machinery on the upper level and the aft trim tanks on the lower level.
Several innovations in the Hrabri-class boats distinguished them from the original L-class design. At the suggestion of Yugoslav naval officers, gyrocompasses salvaged from former Austro-Hungarian Navy ships were fitted. A central pumping station was also installed, allowing for all ballast tanks to be blown at once from a single position. This enhancement saved the boats from sinking at least twice during their service. The final difference was that one of the three periscopes was modified to enable observation of the skies to warn of impending air attacks.
The two Yugoslav boats had an overall length of , a beam of , and a surfaced draught of . Their surfaced displacement was or submerged. Their crews initially consisted of five officers and 42 enlisted men, but by 1941 this had increased to six officers and 47 enlisted men. They had a diving depth of , but Yugoslav naval regulations restricted them to a maximum depth of . They were expected to dive to periscope depth in 70 seconds, but at one point the crew of Hrabri could achieve this in 52 seconds.
Propulsion For surface running, the Hrabri-class boats were powered by two Vickers V12 diesel engines designed in 1912 that were rated at a combined at 380 rpm. Each engine weighed , was made up of six parts, each of which held two cylinders. The parts were held together by screws. The screws were subject to great stress during navigation in rough weather, and they often cracked. This was a severe burden on the submarine's engine crews, as the work to replace them was exhausting, and forced the navy to stock the boats with a significant number of spare screws, adding significant weight. The naval historian Zvonimir Freivogel states that on a 15-day Mediterranean cruise, Nebojša needed of replacement screws.
A total of of fuel was carried, in each of the saddle tanks and the rest inside the hull. As the fuel in the saddle tanks was used it was replaced with sea water to maintain displacement, buoyancy and trim. They could reach a top speed of on the surface, less than the contract speed of . On the surface using the diesel engines, the submarines had a range of at .
When submerged, the two propeller shafts were driven by two electric motors generating a total of at 300 rpm. The submarines also had a small electric motor for silent underwater running. The battery storage consisted of three hundred and thirty-six 3820 LS Exide cells, which had a combined weight of . The boats could reach on their electric motors when submerged, again less than the contract speed of . With the silent running motor, the boats could achieve a nominal speed of underwater. Underwater, the boats had a range of at .
Armament
The Hrabri class were fitted with six bow-mounted torpedo tubes and carried twelve Whitehead-Weymouth Mark IV torpedoes, six in the tubes and six reloads. They were also equipped with two Mark IV L/40 guns, one in each of the mounts forward and aft of the bridge. Each gun weighed . Due to the raised gun platforms and protective bulwarks, the guns could be brought into action before the submarine reached full buoyancy and the crews could work them in heavy seas without the danger of being washed overboard. The guns could fire a shell up to twelve times a minute to a maximum range of . Until the destroyer leader was commissioned in 1932, the Hrabri-class boats had the heaviest armament of any Royal Yugoslav Navy vessel.
Modifications in Yugoslav service