Tom,

I have been using Maxsurf last year and it is a good hull desing software. It's optimized for building commercial crafts and the speed prediction program (hullspeed) will not work for a cat.

You will also need a real cad program to produce the plans and to work with the lines. We were using autocad to produce the plans and we used rhino3d for part of the work as well.
Solidwork is not necessarly a good choice. While it is a good solid modeling software, part of the work to produce the hull is the calculation of the hull stability and buoyancy. I did try to use Surface work marine but I have a busy schedule at this time and no real need to design a hull at this time (hopefully this will change soon)

I did have a look at polycad http://www.polycad.co.uk/ and it looks good to me The only problem is the lack of manual with it.

As far as designing a hull, most software are surprising close to the paper and pencil methode so you could get a book on hull drawing or on naval architecture and the hull designing process will be compatible with most of the Hull design software.

As a last comment, the Tornado catamaran is still the fastest cat in the 20' range and it was designed over 30 years ago with pencil and paper.

Charles Leblanc
Nacra 5.2