Hull Design Software

I’ve been using Maxsurf to design hulls and find the software fantastic to use, view and print out stations etc.
but… i’ve been having problems with the velocity prediction software “span” also made by Formulation Design. the problem is it wont calculate accurately on anything rc size. it calculates all heel angles in different wind strength and angles but the hullspeed for each strength/angle comes out at around 3knts for every one! i think it assumes with the sail area the hulls carry that it will reach max hull speed in every aspect of strength/angle. Real pity because i love span it can print out all your polar graphs, hull drag at angles of heel and much more.
Only way i’ve got it to work out is to scale the hulls up then i can compare them against each other but I’m not sure if this would prove accurate when scaled back down.

If you got 2 rc sized boats one quick one slower and scaled them up (exactly so the waterlines match etc.)would the speed difference still be as great or greater??? (or vise versa)

Has anyone used theese software packages with any sucsees or does anyone know of a velocity prediction package that does small scale?

Cheers in advance.

The Wolfson Unit VPP has a special section for r/c boats.