Emirates Team New Zealand, America's Cup defender, has been spotted testing a new mainsail (see video). This is apparently inspired by a similar variant that the now retired challenger American Magic used. Because of its similarity to bat wings, it was also given the name "Batwing", based on the comic character Batman.
The batwing sail in the video
It is a mainsail version with a shortened profile length. Simply put, several inches of the leech were cut away, so it's a narrower wing. This should bring advantages in winds of 15 knots or more. Then the sail area would no longer be needed to generate propulsion and would only produce unnecessary aerodynamic drag. The fact that the battens protrude aft like a bat wing has to do with the minimum sail areas prescribed in the regulations and with the fact that the same mechanics are apparently used as with the large sail. Only the top four meters of the main sail may be adjusted by a mechanic (twist), and also only the bottom 1.5 meters. The area in between adapts, interpolates, so to speak.
The previous mainsail of the New Zealanders (left) and the batwing sail of the Americans