201 Wind & Water

How the Wind Moves a Sailboat

A sailboat is able to move through the water in two ways.  The simplest movement is caused by wind pushing on one side of the sail as when you are sailing down wind, which is called a run.

The second way in which a sail boat moves is by the creation of “lift” on the outside curved faces of the sails.  It utilizes a principle discovered in 1738 by Daniel Bernoulli.  Basically it states that an increase in air flow results in a decrease in pressure.

A sail is shaped like a lens that is curved on one side and flat on the other.  Examine the diagrams. 

If wind passes on each side of the sail the wind on the curved side will have to flow faster to meet up with the wind moving along the flat or shorter side of the sail.  The result is that the air pressure is lower on the curved side of the sail and the sail is sucked into this low pressure area.   We call this pressure effect “lift.”  This very same principal allows an airplane to fly.

Now, here is where it gets tricky.  Normally the direction of the “lift” would be roughly perpendicular to the straight side of the sail however; there is a second, hidden force at play, the keel.  The keel resists the sideways motion caused by the lift and the result is that the sail boat “slips” forward through the water.
The Forces At Work
  The Lift Principal
 The Jib's Effect
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