They discovered that when the boat was traveling at 200 mph at the end of the straightaway and then turned left, the change in airflow over the wing was causing a sudden change in the boats handling. With the fin sticking up above the right side of the horizontal wing the airflow was harnessed in such a way that it corrected this. If you look back at the evolution of the unlimiteds, particularly through the 1970's, you will see all sorts of wing shapes both vertical and horizontal.