Hi Pete

its dated but think it was Skene,s elements of yacht design that got me started ,-
http://www.sportsreading.com/Skenes_Elements_of_Yacht_Design_0396079687.html

It goes thru the basics ,though applied more to sailing yacht than beach cats ,--but the basic calc and formulas apply ,--just differently with two hulls and no lead mine keel below.
model building or the actual thing , then application of these and final design becomes all to real as any error becomes obvious ,-this is the hard way but best way to really learn . Hope we see a resurgence of cat designs and interest encouraged by limited development type racing classes to test then out in.
You read about a number of home builders or catsailors planning a cat project.