I am saying that 10' should be the beam. You should take into account that there are three major 20' boats with 10' beams":
- Tornados were once the olympic catamaran with lots out there
- Marstrom 20' not a lot out there but very fast
- Nacra F20 the latest 20' on the market

I would take the specs of these boat and use the largest numbers for the outside of your box. This way all these older and newer boats would be automatically eligible and they could be developed to the new specs over time.

Also if the Inter 20's and the Supercats etc... wanted to join the class, they could develop their boats as such. I think the Inner 20 would perform well with a 10' beam and I think my Supercat 20 would perform well with a 10' beam.