I disagree that bernoulli's principle demands equal transit times, but I think it is an irrelevant point.
I believe the key fact is instead that the outer flow is forced into a curve, even though the straight path is technically free (no sail to stop it), because a vacuum (low pressure zone, anyone?) would be generated between the outer flow and the sail.
Hence, the sail needs no thickness to generate its lift, but only to separate the inner and outer flow.
Moreover, due to the viscosity of air, the area of influence of the sails goes well past the immediate layer of outer flow, but rather the layers of outer air closest to the sail have a similar "sail" effect, to a lesser degree, to adjacent layers of air.
I believe the key fact is instead that the outer flow is forced into a curve, even though the straight path is technically free (no sail to stop it), because a vacuum (low pressure zone, anyone?) would be generated between the outer flow and the sail.
Hence, the sail needs no thickness to generate its lift, but only to separate the inner and outer flow.
Moreover, due to the viscosity of air, the area of influence of the sails goes well past the immediate layer of outer flow, but rather the layers of outer air closest to the sail have a similar "sail" effect, to a lesser degree, to adjacent layers of air.