I remember a particular graveyard watch on a transatlantic crossing. The windvane was steering the Van der Stadt 37 through the huge rollers, so I was just sitting alone in the cockpit, staring at it. Having studied a little control theory, I was wondering what the equation looked like, and whether the designers had actually used a mathematical formalism, or just tried different mechanical configurations and settings. Then, just for the hell of it, I reached down and torqued the rudder. To my amazement, the vane moved! With all of its gears and levers and counterweights, the thing was backdrivable!! So the pressure of the water, as well as the pressure of the wind, were in the control equation. I simply couldn't believe it, and instantly gave up trying to figure out how a human could come up with a design for such a system. Windvanes are amazing!