SAILING AND GLIDING
Question. Is it possible to sail on a river on a windless day?
Answer. Yes, sailing with no wind is possible, thanks to the current; Figure 12.1 explains how. The sail in still air acts like a knife in butter: it can only slice through the air, moving along the line of the sail.1 On the other hand, the keel is being pushed by the current, and thus the boat slides as shown, towards the shore at the right angle. So the keel now acts as the sail and the flowing water acts as the blowing wind. And the sail, slicing through the air, acts as if it were a keel! Its just like regular sailing, except upside-down.
The sail-keel symmetry. We just discussed the boat from the point of view of a shore observer. But imagine yourself on that boat; you will then think that the water is still and that the wind is blowing upstream instead. So you would be in a conventional situation of a boat sailing in the wind. To you the sail will act as a normal sail catching the wind, and the keel will be a normal keel slicing though water. This
1 This is an approximation—of course the sail can move in the direction perpendicular to the line S, but we ignore this smaller motion.