Dating from a period of great expansion for the Italian navy's submarine arm, the 12 'Sirena' class submarines were known also as the '600' class boats. This figure was indicative of their standard surface displacement and, though the final design exceeded it by a considerable margin, they proved very handy boats for the constricted conditions of the Mediterranean. Their detail design was greatly influenced by that of the preceding 'Argonauta' class, but, as they were laid down before the latter's entry into service, they did not benefit from working experience. Simple and robust, they were heavily used and suffered accordingly, only one surviving beyond the armistice of September 1943.
Ten almost identical derivatives, the 'Perla'class, folio wed on. Two of these, Iride and Onice, served somewhat controversially under Spanish Nationalist colours during the Spanish Civil War. During World War II, the Iride, together with the Ambra, were converted to carry SLC human torpedoes. The latter boat had already distinguished herself when, two days after the Battle of Cape Matapan, she had sunk the British cruiser HMS Bonaventure. Once converted, she went on to attack the harbour at Algiers in December 1942, heavily damaging four ships totalling 20,000 gross registered tons.
Yet another virtual repeat class had followed in the 17 'Adua' class boats, launched 1936-8. Two of these also were converted to carry SLCs and one of these the Sore, was particularly successful. She attacked Gibraltar on no less than four occasions, the raid of September 1941 accounting for two ships, including the auxiliary tanker Denbydale. Her greatest coup, however, was in December 1941 when her three SLCs put the battleships HMS Queen Elizabeth and HMS Valiant, together with a tanker, on the bottom of Alexandria harbour. She was finally sunk by the anti-submarine trawler May outside Haifa in August 1942.
The final expression of the '600' type was in the enlarged 13-boat Acciaio' class of 1941-2.
Specification 'Sirena' class Type: sea-going submarine Displacement: between 679 and 701 tons surfaced and between 842 and 860 tons submerged Dimensions: length 60.18 m ( 197 ft 6 in); beam 6,45 m (21 ft 2 in); draught 4.70 m (15 ft 5 in) Propulsion: surfaced diesels delivering 1,200 bhp (895 kW) and submerged electric motors delivering 800 hp (597 kW) to two shafts Speed: surfaced 14 kts and submerged 8 kts Range: surfaced 9000 km (5,590 miles) at 8 kts and submerged 135 km (84 miles) at 4 kts Armament: one 100-mm (3.9-m) gun, two (later four) 13.2-mm(0.52-in) machine-guns, and six 533-mm (21-in) torpedo tubes (four forward and two aft) with 12 torpedoes Complement: 45