Bernoulli's theorem helps to explain how sailboats can move upwind.

When you see a kite catch the wind and swoop up into the air, you're witnessing lift. You can feel the forward acceleration in the pull on your end of the string. Likewise, the mainsail and jib harness wind energy with their aerodynamic shapes that puff out on one side when the wind hits them. You also might notice that the kite flies angled to the wind, just as the mainsail and jib capture wind when tacking.

Sailing aficionados use two prominent -- yet often disputed -- theories to explain how exactly the wind interaction generates lift: Bernoulli's theorem and Newton's Third Law.



Bernoulli's theorem, also called the Longer Path Explanation, explains lift in terms of high and low air pressures on either side of the sail. Imagine the front of the boat angled upwind, or into the wind. As the breeze hits the sails, the air particles rush over both sides. Theoretically, the air particles moving across the outer, convex side of the sail have a longer distance to travel in the same amount of time as the particles moving across the inner, concave side.

If the particles on the outer side are traveling farther in the same amount of time, they must have a higher velocity, or speed, than the particles on the other side. These higher-velocity particles have more room to spread out, forming a low-pressure area. On the inside of the sail, the slower air particles are packed together more densely, creating a higher-pressure area. This difference in the pressure on the sails acts as a forward suction, producing lift.

Lift also applies to airplane flight. For a more detailed explanation of lift and the Bernoulli and Newtonian theories, read How Airplanes Work.

" " Newton's Third Law at work on a sailboat

Newton's Third Law describes lift in terms of the reaction of the wind's air particles to the mainsail and jib. The law states that every action has an equal and opposite reaction. As the wind hits the sails from an opposing direction (remember, you're sailing upwind to tack), it generates drag, or backward pull. Drag is parallel to the original wind current [source: Glenn Research Center] and occurs naturally when something is moving against a fluid or gas. Swimmers wear specialized suits and caps to reduce drag as much as possible in the water.

Examining lift through the Newtonian lens, the air particles' movement creates an equal, opposite reaction -- or forward pull. It can also be applied to the interaction of the sails and the keel, described in the previous section. The sails and the keel create equal and opposite reactions to focus the boat's energy forward rather than sideways.

Now we'll examine the keel more in depth to see how it contributes to lift and keeps the boat from tipping over when tacking.­