The Yugoslav navy developed its own submarine-building capability during the 1960s. In 1990, the main combat units of the submarine service were three Heroj class submarines armed with 533 mm torpedoes. Two smaller Sava class submarines entered service in the late 1970s. Two Sutjeska-class submarines had been relegated mainly to training missions by 1990. At that time the navy had apparently shifted to construction of versatile midget submarines. Four Una-class midgets and four Mala-class swimmer delivery vehicles were in service in the late 1980s. They were built for use by underwater demolition teams and special forces. The Una-class boats carried five crewmen, eight combat swimmers, four Mala vehicles, and limpet mines. The Mala vehicles carried two swimmers and 250 kilograms of mines.