various "daimyō" clans undertook major naval building efforts in the 16th century, during the sengoku period, when feudal rulers vying for supremacy built vast coastal navies of several hundred ships. the largest of these ships were called "atakebune". around that time, japan seems to have developed one of the first ironclad warships in history, when oda nobunaga, a japanese "daimyō", had six iron-covered "ō-atakebune" ("great atakebune") made in 1576 . these ships were called , literally "iron armored ships", and were armed with multiple cannons and large caliber rifles to defeat the large, but all wooden, vessels of the enemy. with these ships, nobunaga defeated the mōri clan navy at the mouth of the kizu river, near osaka in 1578, and began a successful naval blockade. the "ō-atakebune" are regarded as floating fortresses rather than true warships, however, and were only used in coastal actions.