To quote 'Sailing Prof' Mark Orams from is article in the NZHerald today, "Team NZ will likely be holding back some "kit" - but all teams are limited in new equipment. They are only permitted a total of four foils (and two matching spares). The main wing, hulls and the jibs (small triangular front sail) are identical on all boats.

Rudders, fairings and the "aero-package" offer potential for change. Aerodynamic and hydrodynamic drag is a big deal, so additional fairings or configurations to reduce drag could be an option, he wrote."

So what is aerodynamic and hydrodynamic drag?  Thats not a question that this writer can answer so a quick Google search came up with:

The aerodynamic or hydrodynamic lift is a force perpendicular to the movement of the fluid. It is created by the suction in a negative pressure zone, formed on top of the profile designed for this purpose. It depends on the displaced mass of fluid.  -  http://www.mecaflux.com/en/portance.htm   - for the full article

and/or

Aerodynamic/Hydrodynamic Drag - A Unit: Dynamics (Forces) & GravitationUnderstand and correctly use the term “drag” when it refers to an object that is slowed down by a fluid.

Visit http://www.mrbigler.com/moodle/pluginfile.php/10258/mod_resource/content/1/269_page_Notes-AP-Physics-1-2016-17.pdf to view the complete unit

By the way the  Google search threw up 317,000 results so within there should be enough tinformation to  answer most questions on how the present lot of America Cup AC50's work. Or just ask any  sailor that you sppot hanging around the Team New Zealand shed on the waterfront; there must be some left on home duties.