Getting rid of the pirate fleets requires some serious money on VH (campaign) it costs about 6000 mnai to take out 6 pirate (2 silver) men out of a fleet of 180. I don't wait to build bigger fleet since the upkeep is horrible, just send 2-3 ships at a time as soon as I build them. They sink my ships but their numbers dwindle...I get most bang for the buck from covered liburnes...