According to
boardseekermag.com/ Windsurfer, well 2 windsurfers actually. Yesterday 2 windsurfers broke the 50knot barrier with a spped of 52.64 knots on a Tandem, in winds of only 34knots. The board in question was a custom design that was narrow enough to allow such speeds, it also took a lot of fin and sail testing to get the most effective combination. The windsurfers Steve Todd and his wife Jane also said they had to try loads of different fins but eventually they found that the 32cm Foil from Australian brands Ka Pro A series was up to the job.
...Oh, and it's still am in the UK on April 1