If you want some where to start in designing a set of hulls (and that is the first and most important thing that you have to do - the rest, ie rigging mast boards etc follow a more "standard" criteria -) then go around and take all the appropriate measurements of existing 12, 14, 15, and even 16 foot cats that you can find, scale them out on paper with conventional elevation, plan and end elevation drawings, then look at what you want to do with a design around those dimensions. If you have that base data to start with you shouldn't go to wrong with your design and you will gain enormous insight as to how/why different hulls"work/don't work". You have to have some sort of accumulated knowledge to even think of starting to design anything, you either get that through traditional study or experience. It is very, very, rare that a person can just go out and "create" anything based simply on their intuitive brilliance and or genious. You HAVE to put in some of the hard yards before you will get any thing like satisfactory results, but don't get discouraged, even if it doesn't work at first, often the solutions are only found by making the mistakes first, and remember the person who never made a mistake, never made anything!!
Since you first posting some months ago, concerning "free plans on the internet" I have searched extensively and I'm afraid that there is no joy there, I haven't found any suitable site that offer "free" plans, they are all there for sale, and from their descriptions, 99% of them I wouldn't even use for toilet paper.
Darryl