That's where you start.
You then have to figure out how thick to make the frames, what to make the frames out of(wood, FG or other), how much ballast to use, size of the rudder, etc. All of this comes into play when you start building. I'm assuming you're building a sail yacht so you are going to have to figure out how heavy the structure needs to be to handle the stress of the mast(s) and rigging under sail and base everything else from that.