Is it the apparent wind created by forward momentum? Is it possible to go downwind faster than the wind speed?
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There are two ways to understand this. The first is to think of it as a simple machine like a lever. Normally we use a lever to turn large, low force movements into small, high force movements. Here we are doing the opposite: turning a small, high force movement into a large, low force movement. The sail, however, resembles less a lever than an inclined plane, with fluid as one of the acting members.
The second way to understand it is through energy. A well-made boat takes relatively little effort to move forward. The wind, however, has practically as much energy as we are willing or able to capture, limited only by the size of the sail. I believe the equation is the same as that for a rocket: mass flow rate times velocity squared for the power, although even the best sail won’t be able to capture it all. The mass flow rate is given by density times area: in other words, the larger the sail, the more energy we can capture.
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