Exactly how does Suleiman's special work?

Playing Ottoman for the first time and I seem to have far less than 50% conversion of naval barbarians via barbary corsairs. The description merely says there is a 50% conversion rate however and doesn't specify what triggers this so I suspect I'm doing something wrong/


When you get near a barbarian naval unit with your own unit the special ability works. If you are lucky then the barbarian naval unit is converted. If not nothing changes you can attack.