ABSTRACT
Racing sailors are often obsessed with producing the smoothest possible hulls, centerboards or keels, and rudders. Sails should be smooth, too. It seems sensible to assume a smoother surface would be subjected to a smaller viscous force. How smooth is a typical sailboat hull? Is this smooth enough? Is a smooth surface always the best, and if so why? These smoothness questions are trickier than one might think. Some curiosities make one think twice about surfaces. A familiarity with small distances is needed to judge roughness. A micrometer, μm or 10-6 meters, is an appropriate distance to consider. Fluid mechanics suggests that 10 μm is “smooth enough,” but many sailors would not be happy with this roughness. Ten micrometers is about one-seventh the thickness of a human hair or one-fifth the thickness of a standard piece of cellophane tape. This is on the borderline of visibility. Sometimes roughness can be felt even if it can’t be seen. Under ideal conditions a human finger is amazingly sensitive and can detect a surface bump not much more than 1 μm high. There is a difference between a smooth surface and a shiny surface. The wavelength of visible light is about (1/2) μm, and any structure on this small distance scale determines whether the surface is dull or shiny. Although a mirror-like shine is not important, visible bumps and scratches on sailboat surfaces are likely suspects for the instigation of turbulence and increased drag.
