You say "usable". Do you mean something that will float? Something that you can put on a pond and watch it sail around? Something you can drop a battery, receiver, and servos in and use as an RC boat? Do you want a hull that's a solid block, or do you want to build a planked timber framed hull?
If it's the first, it's fairly easy: most wood floats, so if you go buy a cheap plastic model and cut wood to look like it, it will probably float. If the mast is too tall it may tend to tip over, so you might need to add some weight to the bottom: maybe drill some holes in the bottom and fill them with lead, or something like that.
If the second, I'd recommend going to a hobby shop, if you can find one, and looking at free-sailing models. That's also a good search term to use -- "free-sailing model boat plans" turns up a lot of results. I've spent some time looking at http://www.john-tom.com/SailBoatPlan...BoatPlans.html
, which seems to have some good plans.
If you want RC capability... that gets more complicated, because you have to start looking at servo throw length, carrying capacity, hull weight, balance points, and all sorts of things like that. I was planning to build an RC sailboat until I started looking at the details there. Now I'm planning to wait until I have more money and BUY an RC sailboat.