Well, if totally seaworthy be your goal, that makes things a little difficult.
The problem with a "pirate" type vessel, which to me means two or three masts, complete with yardarms and top sails, etc. is that's a lot of cloth to control, and on a small scale many boats would seem to be dangerously over-powered by that amount of sail. If you're looking to build something for just some fun coastal cruising on mild sunny afternoons, that's one thing. But a safe, ocean-going vessel able to handle serious bluewater conditions is something else.
And take it from someone who's sailed for over half his life, captained boats from 12 to 51 ft in length, and lived through a capsize, the wind be a powerful force to recon with. The ocean can be a very inhospitable place at times. Certainly no place for a boat designed as a big toy.
That said, anything can be done if you want it bad enough. Just do your homework first, and do it correctly.
The first thing to do in that case would be to consult a marine architect with your ideas, or at least study the subject in-depth yourself.
The first questions to answer would be the following:
What size boat/ship are you talking about?
What kind of budget do you have in mind?
What type of vessel specifically are you looking for?
Do you plan to build it all yourself?
Also, what is your pervious sailing experience?
-Cap'n Matt