Americaʹs Cup downwind sails—vertical wings or horizontal parachutes?

Analysis of the boat speed polar for a typical International Americaʹs Cup Class yacht shows that the normal downwind apparent wind angles are between 90° and 135° in winds over 5 m/s (10 knot). This means that the lift force generated by the sails is more aligned with the thrust direction than the drag force. Under such conditions the thrust is maximised by trimming the spinnaker to give its maximum lift while still providing significant drag and the mainsail to provide approximately equal amounts of lift and drag. It is therefore concluded that Americaʹs Cup downwind sails need to act both as vertical wings and horizontal parachutes.