The Marcello class was a class of nine submarines built in 1937 and 1938 by CRDA in Trieste for the Royal Italian Navy (). Two similar submarines built in 1939 at La Spezia by Oto Melara are sometimes considered part of the class. All eleven served in the Mediterranean Sea at the start of the Second World War. After s 1940 sinking, the remaining boats were transferred to the BETASOM Atlantic submarine base at Bordeaux in August 1940. After four boats had been sunk in the Atlantic, and were then selected for conversion to "transport submarines" in order to exchange rare or irreplaceable trade goods with Japan. Cargo capacity of 160 tons reduced reserve buoyancy from 20–25% to 3.5–6%; and armament was reduced to defensive machine guns. Only was in operational condition at the end of the war. Class members Marcello (pennant number ML) was launched on 20 November 1937 and completed on 5 March 1938. When Italy declared war, Marcello was temporarily disabled by air conditioning system leaks. Leakage of chloromethane refrigerants during submerged operations had caused central nervous system poisoning of the crew. After unsuccessful patrols in the Mediterranean, Marcello sailed on 31 October 1940 and passed the Strait of Gibraltar on 5 November for an Atlantic patrol to Bordeaux on 2 December. Marcello sank one ship on its first BETASOM patrol and was lost to unknown causes on its next patrol in late February 1941. Dandolo Dandolo (pennant number DO) was launched 20 November 1937 and completed on 25 March 1938. After unsuccessful patrols in the Mediterranean, Dandolo sailed on 13 August 1940 and passed the Strait of Gibraltar on 16 August for an Atlantic patrol to Bordeaux on 10 September. Dandolo sank one ship and damaged another en route to Bordeaux. After an unsuccessful patrol, Dandolo sank one ship on its second BETASOM patrol. After another unsuccessful patrol, Dandolo sailed from Bordeaux on 26 June, passed the Strait of Gibraltar on 2 July, and returned to Naples on 7 July. Dandolo spent the remainder of the war in the Mediterranean damaging a neutral French tanker on 4 November 1941, sinking the neutral Spanish freighter Castillo Oropesa on 8 November 1941, and damaging the cruiser on 16 July 1943. Dandolo sailed to the United States after the Italian armistice, and was scrapped in 1948. Mocenigo (pennant number MO) was launched 20 November 1937. Mocenigo was sunk off