[RINA] Bottom Pressure for planing boat

I am using RINA(Registro Italiano Navale) scantling rule to calculate bottom pressure and etc for FRP planing boat.
The rule says,

P2 = 15*(1+av)*(W/(L*Cs))*g*FL*F1*Fa

But I couldn't understand what Cs means.
RINA Yacht 2007 book says, Cs is "Support contour of the yacht, in m, defined as the transverse distance, measured along the hull, from the chines to 0.5L. For twin full yachts, Cs is twice the distance measured along the single hull."
Please help me, this Cs changes the pressure a lot so I have to define the value correctly.
Thanks.

 

I took Cs as distance between chines but i am not sure with this. You can check this with iso rules. You should get similiar values. But you are wrigth, if you wrong with Cs value the result will be twice higher or lesser. I had some Rina CD that i get from Rina inspector, any way i can check this value with Rina directly. Regarding rules for bottom pressure you should take a look at one paper: "A comparation of prediction methods for impact pressures of high-speed, light craft". If i can ask, what kind of vessel is your project?