Windsocks are placed at many marinas, coast gaurd stations, and private homes. Most of the time their presence is in vain because most seamen have no clue about how to utilize them. In short it is very simple, and utilizes one of a good captain's favorite tools- estimation. A full sock indicates winds of atleast 15 knots, while it takes atleast 3 knots to lift the sock at all. Obviously wind direction can be observed by the direction the sock is blowing. If the sock is about half full, you can safely assume about 7 knots of wind. A knot is 1.15 mph.