At first glance, sailboats don’t look much like an airplane, but physics-wise, they’re closely related. Both the sail and hull of a sailboat act like wings turned on their side. Just as with airplane wings, the driving force for a sail comes from a difference in pressure across the two sides of the sail. The same effects applied to the hull and its keel (the wing-like extension that sits below the hull) provide the force that keeps a sailboat from slipping sideways as it cuts a path through wind and water.
Like airplane wings, sailboats also generate tip vortices: one from the top of the sail, the other from the bottom of the keel. Those vortices are typically invisible, but in foggy weather, like in the photo below, you can see the tracks they leave behind. (Image credits: top – Ludomił; bottom – D. Forster; research credit: B. Anderson; submitted by Lluís J.)
Follow along all this week and next as we celebrate the Olympics with sports-themed fluid dynamics.