The blockade was enacted in the 45-mile (72km) wide Otranto Straits between Brindisi in Italy and Corfu on the Greek side of the Adriatic Sea. The operation was intended to prevent the Austro-Hungarian Navy from escaping into the Mediterranean and threatening Allied operations there. At the height of the operation there were more than 200 vessels from Allied navies, mainly British and French, involved in the blockade. The anti-submarine force included more than 60 North Sea trawlers, or ‘drifters’ each armed with a 6-pounder gun and depth charges, which dragged nets across the straits to detect submarine movement. The Allied fleet’s capital ships were effective in preventing surface ships from escaping the Adriatic, but it had little or no effect on the enemy submarines based at Cattaro (now Kotor) in Montenegro. RAN participation in this operation began in mid-October 1917 when six Australian destroyers were allocated to the British-led 5th Destroyer Flotilla based in Brindisi. These were HMA Ships Swan, Torrens, Warrego, Huon, Parramatta and Yarra. They operated in two divisions of three ships on rotation of four days on patrol and four days in harbour. During the operation, the Huon, Parramatta and Yarra were fitted with towed observation balloons in which a crew of two would perform four-hour shifts to detect any enemy submarine movements while another destroyer stood by to take up the pursuit. The Australian flotilla remained on this station until the Armistice was signed in November 1918.
