The boats were transferred to the BETASOM Atlantic submarine base at Bordeaux in 1940, but returned to the Mediterranean in 1941, where both were sunk within a few days of the September 1943 Italian armistice.

[2] For surface running, the boats were powered by two 750-brake-horsepower (559 kW) diesel engines, each driving one propeller shaft.

First Lieutenant Alessandro De Angelis was lost overboard and drowned in heavy seas as Argo was returning to port.

After refit, Argo patrolled off Cape Ferrat from 6 to 21 April and off Cap Caxime (Algeria) from 22 to 29 May 1942, where it sustained damage from aerial attacks requiring another period of repairs.

After patrolling off the Balearic Islands from the 15 to 26 October and off the island of Galite (Tunisia) from the 29 to 30 October, Argo entered the Bay of Bougie on 11 November 1942 to torpedo the anti-aircraft ship HMS Tynwald and the troopship Awatea, which had been damaged by aerial bombing during Operation Torch.

After refit at Cagliari and training at Pula, Velella patrolled without success south of Cape Palos (Spain) in April 1942, south of the Balearic Islands in June, along the Tunisian coast in July, and west of the Island of Galite in August, south of the Balearic Islands in September, in the Gulf of Philippeville and the Bay of Bona in November, north of Cape de Fer in April 1943, and off eastern Sicily in July.