As I understand it (And I'm sure Jake will correct me) when you hit 50 knots you require a totally different kind of foil shape in order for the foil to operate as expected. The problem apparently is cavitation that would lead to potentially the loss of flow attached to the surface of the foil. This would in turn lead to a nosedive like we've seen from ETNZ when their foil lifted them out of the water in the LV cup.