Fig. 2. Theoretically predicted hysteresis behaviour of an inextensible sail profile of given slackness in inviscid flow, showing changes in sail shape and the lift coefficient as angle of attack is decreased from a high value (1) through ideal incidence (2) and beyond, when the sail becomes S-shaped (3) and finally pops through (broken line), adopting a wholly convex shape with negative camber (4). A subsequent increase in the angle of attack from this point causes the profile to become S-shaped again (5) before positive camber is restored. Profile chord lines are indicated in red.