It doesn't seem to be a rudder directing this boat," it's even simpler than that - the bar that can be seen being flipped from one side of the boat to the other at <a href=""https://www.youtube.com/watch?v=b6utGZQ9Sks&amp;t=1m00s"">1:00</a> changes the anchor point of the cable", allowing the current to turn the entire boat toward the opposite bank.