Massively outnumbered by the Black Sea Fleet, the Romanian ships were kept behind the minefields defending Constanța for several months after the start of Operation Barbarossa on 22 June 1941, training for convoy escort operations. Beginning on 5 October, the Romanians began laying minefields to defend the route between the Bosphorus and Constanța; the minelayers were protected by the destroyers. After the evacuation of Odessa on 16 October, they began to clear the Soviet mines defending the port and to lay their own minefields protecting the route between Constanța and Odessa. On 1 December "Regele Ferdinand", her sister ship and the flotilla leader "Mărăști" were escorting a convoy to Odessa when a submarine unsuccessfully attacked the convoy. It was quickly spotted and depth charged by "Regele Ferdinand" and "Regina Maria" with the latter claiming a kill. Soviet records do not acknowledge any losses on that date. The sisters escorted another convoy to Odessa on 16–17 December, the last one before ice closed the port. As the convoy passed Jibrieni, "Regele Ferdinand" spotted a submarine's periscope and depth charged the submarine after evading a pair of torpedoes. The ship reported spotting debris and an oil slick; she may have sunk .