Sailboats may vary in size, design and use, but all float on water and move by harnessing wind energy. They rely on basic principles of physics to float and maneuver, and sail shape continues to evolve to maximize efficiency.
The mainsail captures and harnesses the power of the wind, but it also creates resistance against the water by generating drag. A keel, a long, slim plank that juts out from the bottom of the boat, provides an underwater balancing force that reduces this drag. Smaller sailboats might also use a centerboard or daggerboard, which serve the same purpose.
As the boat moves through water, it generates a bow wave with a wavelength equal to its speed. Observing this wave, one sees a series of crests and troughs moving past the hull, with a single well-formed trough in between. The longer waves generated on the outer, convex side of the boat have greater length than those on the inner, concave side. This longer-path difference creates lift that counteracts the drag created by the curved hydrofoils of the sails.
Sailors often talk about true wind (the force that blows) and apparent wind, which is what they feel as the boat moves through the water. Bernoulli’s theorem and Newton’s Third Law explain the interaction between wind, sails and the keel that produces lift. Both involve the concept of a longer path for air particles or water molecules. This difference creates pressure increases on the high-pressure side and decreases on the low-pressure side. The keel also creates a twist in the flow, creating vortices behind the bottom rear of the keel.
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