It is an absolutely clear and calm day, and the wind and water are moving in the same direction. This is not the most ideal situation for any sailor as there is not a good amount of energy to extract for movement. Energy is captured from the difference in motion, between the wind and water. The sailboat captures this energy by utilizing the sail(s) and the hull(s).
The sailboat and wind usually have different directions, and this difference of motion is the relative or apparent wind. A sail uses this apparent wind and generates lift. This is an example of an airfoil, and using Bernoulli's Principle the airfoil creates lift much like an airplane does. In fact, depending on the orientation of the sailboat, its point of sail, and its design it could conceivably obtain a speed faster than the true windspeed.
Using the sail, the boat will be able to move in the water. But being able to move in the water is useless if direction cannot be manipulated as well. To help direct the boat an underwater component is required, such as a centerboard, underwater foil, keel, or even the hull itself can be employed. The combination of the sail and underwater component allow for movement in any direction, except into the wind.
The tacking angle, the angle of the boat’s movement relative to the true wind, usually ranges between 35 and 80 degrees. Using a 35 degree tacking angle, on either side of the wind, a sailboat could cover over 290 degrees of the boat compass. Sailing into the wind can be accomplished indirectly by sailing close-hauled, about 45 degrees, and then tacking and sailing in a zigzag fashion. This is a prime example of both the sail and underwater component working in tandem to move in a given direction.
Understanding the dynamics between capturing and using the energy from the motions of wind and water can help answer the following questions:
1) How do sailboats move?
2) How can a sailboat sail upwind?
3) How can a boat sail faster than the wind?
Most sailors are not physicists but they are undoubtedly cognizant of the physical principles that govern basic movements in the water when sailing. Without these principles, a boat and its crew would be dead in the water.
