In addition to Bill's post, consider the two hull shapes shown below (in a rather simplistic form), hull A would have a low prismatic coefficient and hull B would have a high prismatic coefficient if length and displacement were equal (I hope I have that right).





However, because the prismatic coefficient is a function of the length of the hull, it alone is not enough to define or even compare hull shapes (i.e. a short stumpy hull can have the same PC number as a long fine hull). It's kind of like saying "I live on the east side of town". It tells us the general area but it's not enough to get us to your house for dinner. In fact, it is a number that's really used near the end of the design phase of a hull.

Looking very generally at sailboat hulls, first we define a target speed we want to achieve the highest efficiency at. There are then two routes we can take to define the basic displacement type hull shape (not including planing hulls) and we need to decide which one we must use first. This decision depends on the ratio of hull length to width (finess ratio). Somewhere around 11:1 and lower (a 20' boat with a 1.9' or wider hull; monohull territory) you need to consider the speed that the hull generated bow wave travels in order to try and optimise the hull shape. Unless you've figured out how to make it plane, this wider hull is limited to the speed of that bow wave. However, higher finess ratios than 11:1 (i.e. catamarans) no longer contend with the bow wave and play in a different set of rules since it can drive through the bow wave (and doesn't create much of one). Once you have decided what kind of hull you have, you can calculate a few more parameters to define it's behavior nearing your target speeds at the boat's available power limit. Once you've got all this, you can start drawing your hull to conform to the optimum prismatic coefficient (length and finess ratio being the same). The thing that gets complicated is that the finess ratio, the speed to waterline length, the prismatic coefficient, and any other hull design parameter are all intermingled. If you change one you very likely affect the others - yikes! You can either just start building in your garage like the creators of the Tornado did, experiment with drawing different shapes and recalculating these parameters until you arrive where you want to be, or try and remember all that calculus you studied in college so you can arrive at the hull shape you want in one sitting. This is where my bus stops - it's at my recollected knowledge limit - everyone off!

A great little primer regarding hull shape, speed, and power design for finess ratios less than 11:1...i.e. monohulls...can be found here:

http://www.kastenmarine.com/coefficients_of_form_equations.pdf