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Now that the experts are assembled, perhaps a conundrum can be resolved. My fancy, and expensive, new sails have thick threads running through them. They're really quite rough. What ever happened to smooth?

Don't we exalt smooth, seek smooth, lust for smooth? Is the world gone mad?

Regards
Chet


Karl is basically right - there are two basic types of boundary layers, laminar and turbulent.

Flows across sails start as laminar and transition to turbulent at some point:

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Laminar flow has less friction, but is unstable. Turbulent boundary layers are more stable and less likely to separate from the sail.

The transition from laminar to turbulent depends on the viscosity of the fluid, the speed of the fluid and the roughness of the surface.

The bottom line is that the stitching and seams on a sail are within that viscous sublayer that's at the bottom of the sail's turbulent boundary layer that starts on the mast in all but the lightest windspeeds.