firstly for using the calculator,you need to work out what is the largest diameter propeller that would fit on the propshaft,leaving a few inches clearance from the hull
second to need to work out the maximum weight of the vessel,loaded.
3rd the waterline lenth.
for a catamaran you would also be looking at the formula for a "semi displacement" vessel,and the beam needs to multiplied by 2,ie if one hull is 6 ft wide,your calculation is for a semi displacement
vessel of 40ftis,(aprox) waterline lenth 38ft,by beam 12ft by draft
by total weight = hull speed
once you have a theoretical hull speed,then you can calculate how many horsepower you need to achive that speed.
you then plug
in to the calculator,you hull speed,weight,hp of engine,gearbox reduction ratio,and max rpm of engine and max prop diameter,the calculator will then give you the pitch your propellor needs,changing the diameter of the prop will change the equation an give a new pitch.
generally the lower the rpm at the shaft the bigger diameter prop,and bigger pitch needed to work efficiently.......but the bigger the drag if the prop is not featering or folding when sailing.
on my prout 37ft catamaran the calculation worked out at a 13'diamx9'pitch propellor
with disp 4500kg,wl35ft,beam9ft,draft 2"9',hp 33 ,rpm 3000, reduction 1:1.6, max diam 13'
here is maybe a better calculator that you can download,with more variables that you can play with