The Italian submarine Acciaio was the name ship of her class of submarines built for the Royal Italian Navy (Regia Marina) during World War II.
The Acciaio-class submarines were designed as improved versions of the preceding . They displaced surfaced and submerged. The submarines were long, had a beam of and a draft of .
For surface running, the boats were powered by two diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a electric motor. They could reach on the surface and underwater. On the surface, the Acciaio class had a range of at , submerged, they had a range of at .
The boats were armed with six internal torpedo tubes, four in the bow and two in the stern. They were also armed with one deck gun for combat on the surface. The light anti-aircraft armament varied and could consist of one or two or one or two pairs of machine guns.
Acciaio was built in OTO's shipyard at Muggiano. She was laid down on 21 November 1940 and launched on 22 January 1941. She was commissioned on 30 October of the same year. The name Acciaio means "Steel" in Italian. The boatâÂÂs first patrol was on 29 March 1942 and she carried out nine offensive patrols during her service career, against Allied naval forces in the Mediterranean. She had one success, when she sank the British armed trawler on 7 February 1943 off the Algerian coast. Her last patrol was on 10 July from La Maddalena to act against Allied forces involved in Operation Husky, the invasion of Sicily. On 13 July 1943, Acciaio was sunk by the British submarine with the loss of her entire crew of 46.