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Harnessing the wind

How sails work

Sails do not actually push a vessel along.  They work more like an aerofoil.  The cloth of the sail interferes with the flow of the wind, creating an area of low pressure behind the sail.  This low pressure is what pulls the vessel along, very much in the way that the low pressure area above a wing lifts an aircraft.  This helps explain how a vessel can sail ‘up wind’, in other words towards the direction the wind is blowing from.  The sails are set at such an angle to the wind that the low pressure area is towards the direction the ship needs to sail.  No ship can sail directly into the wind, but much effort has gone into designing sails so that ships (and especially racing yachts) can ‘point’ as closely as possible into the direction of the wind.

There are two basic forms of sails, or rigs: square rig and fore-and-aft rig.  Square rig is older, as the earliest illustrations show ships with a single square sail.  Fore-and-aft rig may have had its origins in the lateen sails, which largely replaced square sails in Mediterranean craft in the first millennium AD.  In fact, both forms of rig have their advantages and disadvantages.  


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