The Otranto Barrage was a naval blockade of the Otranto Straits between Brindisi and Corfu, intended to prevent the Austro-Hungarian navy gaining access to the Mediterranean Sea. Although it did keep Austro-Hungarian surface ships in the Adriatic, it had little effect on submarines, which routinely passed through the barrage to conduct anti-shipping operations in the Mediterranean. The idea of the barrage was first brought up by First Lord of the Admiralty Winston Churchill at the May 1915 conference in Paris, on the eve of Italy’s entry into the war. Churchill offered to supply fifty fishing trawlers and fifty miles of submarine indicator nets, in return for the Italians providing crews and armament. The Italians declined, realising that manning and arming the craft would be a significant challenge. During the Dardanelles campaign, British trawlers proved very useful, and as the submarine war in the Mediterranean intensified, the Italians realised that they did not have enough small vessels for anti-submarine duties.
On the 30th August 1915, the British Admiralty issued orders for sixty drifters to be prepared to leave for the Adriatic as soon as possible. These were crewed primarily by fishermen, with divisional officers from the Royal Naval Reserve. Not used to military discipline, the naval officers initially despaired of their charges, though they improved rapidly. Lieutenant M.E. Cochrane, the second in command, commented that “the human material was of the best … it needed only a period of polishing before it would shine with exceptional lustre.”
The first drifters arrived at Taranto on the 22nd September, evidently without warning. Rear Admiral Cecil Thursby complained, “You can imagine my surprise when suddenly sixty drifters were dumped on me with no organisation, provisions, stores or anything else.” The drifters were organised into three divisions of twenty. At any one time, two divisions would be deployed with their nets, while the third would be in Brindisi. Two drifters from each division would be at a subsidiary base at Taranto for docking, boiler cleaning, and repairs. The Italians provided a pair of merchant ships (Gallipoli and Adratico) and a small auxiliary steamer (Mazzini), which was armed with three six-pounders and used for inspections, mail delivery, etc. When they arrived, the drifters had no armament, although Thursby worked to acquire some. On the 12th October, Restore was sunk by the German U39, graphically illustrating the need to provide some form of self-defence. By the 8th November all the drifters were armed, typically with 47mm or 57mm guns and a few three-pounders.
Each trawler carried a series of light steel indicator nets anchored to the sea bed at various depths. These were intended to capture enemy submarines by entanglement, though this rarely worked in practice. Thursby complained that he did not have sufficient drifters, and during winter boats often had to take shelter and nets were lost. On the 15th November 1915, the Admiralty dispatched a further forty drifters, which began to arrive on the 7th December. Warships and aircraft supported the drifters, though at various times during the war other priorities meant that very few warships could be spared for this duty.
After an Italian cruiser and French destroyer were lost in June 1916, the Italians stopped using cruisers during daylight, leaving destroyers to protect the barrage. In July, the Austro-Hungarians raided the line, sinking two drifters, damaging two more and taking nine prisoners. The next day, 10th July, the Italian destroyer Impetuoso was sunk and the drifter line was moved south, to a less vulnerable position.
On the 30th October 1916, a special conference was held regarding the Otranto Barrage. Here it was decided that the drifters would be moved to a line running from Otranto to Asproruga, and that the Italians would lay a minefield ten miles wide, thus reducing the area the drifters had to patrol. Other commitments permitting, the Italians would have two destroyers on patrol. The Italians would also add twenty-two trawlers and eighteen coastal torpedo boats to the barrage, as well as stationing more aircraft at Brindisi and Valona. The Italians did provide twenty additional drifters, as did the British, but promises of extra gunboats were not kept, leaving the drifters vulnerable.
On the night of 22nd December 1916, four Austro-Hungarian Huszár class destroyers raided the line. Six French destroyers in the vicinity sailed to the sounds of gunfire, but there was much confusion in the darkness. Four of the French destroyers lost contact with their leader, and the two destroyers still under command took hits which put one temporarily out of action and reduced the speed of the second. Italian destroyers and the British light cruiser Gloucester had been dispatched to catch the raiding force, but only caused more difficulties when French and Italian destroyers rammed each other in the darkness.
At an Allied naval conference in London in January 1917, it was agreed that the drifters were insufficiently protected. In order to improve organisation, the barrage system was to be placed under the command of a single British officer, Commodore Algernon Heneage. He was to be directly under the Italian Commander in Chief and able to call upon the services of all Allied ships not in use elsewhere. If required, he could also ask for assistance from other Allied ships on the vicinity.
The Italians and French started to argue for a fixed net of around forty miles in length instead of nets towed from trawlers. The British thought that this was impractical, but agreed to the construction of a one mile long trial net. At a conference in April, it was agreed that the barrage did not have sufficient drifters (124 were available in total. Seventy were out at any one time). To counter this, the conference recommended hastening construction of the fixed net barrage.
The Battle of the Otranto Straits
On the night of 14th May 1917, the Austro-Hungarian navy launched their largest raid on the barrage. The raid was carried out by the cruisers Novara, Helgoland, and Saida supported by the destroyers Csepel, Balaton and U-boats U4 and U27, along with German u-boat UC-25, under the command of Admiral Miklós Horthy. A supporting force composed of the armoured cruiser Sankt Georg, two destroyers, and several torpedo boats was on standby. The old pre-dreadnought battleship Budapest and a screen of torpedo boats were also available.
As the force sailed south, they encountered and attacked a small Italian convoy, sinking a destroyer and a munitions ship, and setting another ship on fire, causing it to be abandoned. The Austro-Hungarian force began the attack on the barrage at 03:30, usually warning the drifter crews to abandon ship before opening fire. Forty-seven drifters were on the barrage when the force attacked, of which fourteen were sunk and four were damaged. The remaining drifters withdrew until the Austro-Hungarians returned to port.
Some drifters chose to fight, most notably the Gowan Lee. When ordered by Helgoland to surrender and abandon ship, the captain, Joseph Watt, ordered full speed ahead and called on the crew to give three cheers and fight to the finish. The crew managed to get a single shot off before their six-pounder was disabled, but they kept working to get the gun firing again, despite being under heavy fire. The Gowan Lea was heavily damaged but remained afloat. She picked up survivors from other drifters that had been sunk and came alongside Floandi to remove her dead and wounded. Joseph Watt was awarded the Victoria Cross for his “most conspicuous gallantry”. One of the crew received the Conspicuous Gallantry Medal and two received the Distinguished Service Medal for their parts in the action.
The Allied forces moved to block the Austro-Hungarian retreat. At 07:00 the Italian flotilla leader Mirabello, with the French destroyers Commandant Rivière, Bisson, and Cimeterre intercepted the main Austro-Hungarian force, but being out-gunned, chose to shadow rather than engage. At 07:45, two British cruisers (Dartmouth and Bristol) and five Italian destroyers engaged the destroyers Csepel and Balaton, which had been conducting a diversionary attack off the Albanian coast. After a short fight, in which one Italian destroyer’s boilers were disabled, the Allies retreated as they came into range of the coastal batteries at Durazzo.
At 09:00 Bristol sighted the Austrian cruisers, and the Allied force turned to engage. Some of the Allied destroyers started to suffer from mechanical problems; those that did not were tasked with protecting those that did, so that the two cruisers continued the battle without the destroyers. Meanwhile, reinforcements from both sides were dispatched. By 11:00, the Austro-Hungarian cruiser Novara had been crippled, but Sankt Georg was approaching with a force of destroyers and torpedo boats. Acton, the Allied commander, temporarily withdrew to consolidate his forces, allowing the Austro-Hungarians to take Novara under tow.
Acton broke off the pursuit, though an Italian destroyer misread the signal and attempted to launch a torpedo attack. It was driven off by heavy gunfire. Csepel and Balaton rejoined the others, and the Austro-Hungarian surface force returned to Cattaro. The German u-boat UC25 later caused serious damage to Dartmouth with a torpedo attack, and the French destroyer Boutefeu was sunk by a mine while pursuing UC25.
The Austro-Hungarians planned another, larger attack in June 1918. This was to include all four Tegetthoff class dreadnought battleships, but the attack was called off when the Szent István was spotted and sunk by an Italian MAS boat while sailing south.
At the time, most observers considered the barrage to be a useful and necessary anti-submarine measure. Unexplained losses were generally ascribed to the barrage by the Allies, helping its reputation, though only a single u-boat loss (the U6) has been confirmed as being a result of the barrage. The effort expended by the Austro-Hungarian navy in raiding the line suggests that they also considered it to be of some significant impact. However, it is now known that Austro-Hungarian and German u-boats routinely transited the drifter line with little difficulty, so it must now be seen as a failure. Not only did it do little to impact the u-boat threat, it took up significant resources which could almost certainly have been put to better use elsewhere.