Their vechicle technology page [1] has some details about the design, with pictures of the underwater parts too. It seems to be a combination of the rigid sail, the crossing "spar", and some sub-surface features.
Also surprised at the scale of the things, the smallest model is 7 m (23 ft) long, the largest is a whopping 22 m (72 feet).
Pretty cool things!
[1]: https://www.saildrone.com/technology/vehicles