Surely you mean the Hobie Fox...Formula 18 is very popular.

Mary, If I were to guess (and this is just a guess), it was to simply try and keep things simple and prevent as many loop-holes as possible. I can't imagine what kind of loophole comes into play from having a curved beam though...maybe less freeboard and windage in the hull? I do know that among other important measurements, some mast and spinnaker measurements are based from the main beam - but I'm not really sure.