The slender, sleek design of a sailboat cuts through the water with minimal drag. Designers have honed the sail shape and hull structure to create boats that are faster and more efficient. But while boat designs are continually changing, one thing has remained unchanged: The passion that sailors have for the salty wind and the challenge of sailing.
The basic concept behind sailing is that the force of the wind combines to push a ship forward. That force is the result of two different kinds of wind: true wind, which makes waves in the water, and apparent wind, which a sailor feels as she steers. True wind pushes a boat away from it when a ship is traveling downwind, while it pulls the boat forward when it’s going toward the wind (upwind). To get that upward pull, a sailor must steer a course called tacking, which involves taking zigzagging paths that allow the ship to approach the wind at an angle rather than head-on.
A sailboat’s resistance to motion is also affected by turbulence, which can increase the amount of force required to move the boat through the water. A smooth hull surface reduces turbulence, but even with the best design, some turbulence will occur.
Using Bernoulli’s theorem and Newton’s Third Law, we can explain this effect. The air particles that rush over the outer, convex side of a sail have a higher pressure than those moving across the inner, concave side. So, if the boat is angled upwind (toward the wind), those particles will migrate from the high-pressure side to the lower-pressure side of the sail — and generate lift, which is what propels the sailboat.
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