As for the sail of a sailboat, I'd be interested to learn whether it's a flux problem. As an analogy, comparing it to a solar power panel, the effectiveness of that solar panel depends on on three things, an intensity term (the brightness of the sunlight), an area term (surface area of the solar panel), and a trig function of an angle (representing to what extent does the sunlight hit the panel perpendicularly). It's the same form, three terms multiplied, that we find in problems involving electric flux and magnetic flux. So how about a sailboat? Does the useful force that moves the sailboat have that same form, a multiplication of three terms, with the wind speed as the intensity term, the size of the sail as the area term, and the trig function of the angle having the same role? I'm not so sure. Sadly, I have been advised in the past that a windmill is *not* a simple flux problem -- when I asked around why they don't make the blades of a windmill very wide to catch more wind, I was told that doing that makes it work even worse, because then more of the wind would go around the windmill instead of going through it!