Estimated Block Coefficient for Catamaran Vessel

You need to calculate first the estimated weight and displacement to fit your SOR. Your Fn number will guide you on the l/b (hull spacings of cats) to find deck area.

With the Froude number you have, you need semi displacement hull. For a quick glance, consult the ratios and coefficient of NPL type for a parent hull. These are transom styled hull with partially or non immersed transom type. Use Mercier and Savitsky or Holtrop and Mennen for resistance and powering. You can jump to the lower Cb range in the fast craft to destroyer type since you have a catamaran (the spreadsheet serves only a guide in the hull type). Using the 100 ft length, vary the L/B ratio to find the least resistance.

Using the derived displacement, you can now analyze the length to displacement ratio which will count a lot. It is this ratio that matters most. You will see that length is most beneficial.

 

The length/displacement ratio is the key factor, along with the length/ demihull beam, not sure why the OP was concentrating on the block co-efficient.

 

Not sure also. He will soon find out when he does the number crunching.

 

Maybe you started with these table? A range of Cp for every Fn plus a corresponding Length to Displacement ratio (in Imperial System) or Volumetric coefficient. That is a design lane. So is Watson's line in which Cb is given.