I had a crisis of confidence earlier this year (or recognised that you can't be an expert in everything) and asked Richard Roake about foil design. Based upon this conversation unless you want to play around with xfoil or similar go with a Eppler E836 and approximately 12% thickness to chord ratio. Assuming you are designing for an adult driving from trapeze you are going to need a bigger foil than the A cat as you are going slower, have similar righting moment but a shorter heeling arm (mast). So I'd go widish on the section maybe 175mm (7").