The river fleet () was composed of wooden galleys, rowboats (later upgraded to gunboats) and smaller ships, which were capable of sailing up the rivers Danube, Tisza and Sava. The victory at the Belgrade (Nándorfehérvár) in 1456, where the fleet played a significant role in breaking through the Turkish river blockade to bring relief to the besieged city, showed its importance and signaled the beginning of a recognition of its significance. It also encouraged King Matthias to build a larger and better-equipped navy. Since they were manned by South Slavs, mainly Serbs and Croats, the two major ports of operations were Belgrade and Szabács (Šabac). In 1475, concomitantly with the introduction of field guns, he ordered the installation of artillery onto the river barges as well as bombards able to shoot cannonballs ranging from In 1479, he had a mixed fleet of 360 vessels, a crew of 2600 sailors, and a capacity of 10,000 soldiers on board. Matthias also secured an exit to the Adriatic Sea, the city-port of Zengg from which Balázs Matthias could embark for his maritime campaigns. Matthias could also monitor the trade going through the Danube Delta to the Black Sea from the City of Kilia, but during his reign, it was seized by the Moldavian Army supported by the Ottoman Fleet.