I am still learning this game but last night I had the opportunity to try out some privateers while playing as Spain. I had built them because I liked the idea of being able to raid another civilizations surface fleet without having to declare war. I was not aware of their "enslavement" function. After building my first one I sent it out on the high seas to look for some vitims. I ran across a fleet (5-6) Byzantine Galleys. I thought OK lets take one on. I was victorious and thus created another Privateer. A handful battles later and all but one of the Galleys were mine. These things are awesome. At this rate I should be able to control the seas until the industrial era!!