Sailing

http://www.phys.unsw.edu.au/~jw/sailing.html

Sailing with the wind behind you: it is easy to see that, when sailing downwind, the wind blows into the sails and exerts a forces on them. The sails push back against the wind with an equal but opposite force. The diagram above, as well as the picture below, shows a spinnaker. A spinnaker is a large sail only used when sailing downwind. The increased surface area of the sail allows for increased force to be applied by the wind.

Sailing upwind: A sailboat cannot sail directly into the wind as seen in the above diagram; the sails will just flail back and forth.

Spinnaker

http://www.elisetomlinson.com/sailing-alaska.html

Sailing towards the wind: much like an airplane wing turned upright, the sail of a sailboat creates lift.The difference in pressure has to do with Bernoulli's principle: an increase in speed of a fluid creates a decrease in pressure.

http://www.caranddriverthef1.com/

In the diagram below we can see the two component forces adding up to the resultant force. Since the keel is under the boat it is harder to visually understand that it is also creating a force. The force it is creating is F perpendicular. The wind is hitting the sails and the boat, trying to move it downwind. The keel prevents this from happening by pushing against the water and adding to the force created by the sails.

http://www.physicsclassroom.com/class/vectors/u3l3b.cfm

Now that we better understand how the sailboat is moving we need to learn about velocity made good, or more commonly, "vmg." If a boat was trying to sail westward against a west wind the boat would need to alternate between sailing north west and south west. This is called tacking. The boat should start out pointing as close to west as it can. But, the boat will need to veer off at more of an angle in order create more pressure onto the sails. So, there is a give and take; in order to go faster you must veer from the oncoming wind, but this points you at a less advantageous angle towards your destination. Fortunately, it becomes very apparent when you reach vmg because there is a characteristic jerk of the boat followed by acceleration.