– why can a boat not angle its rudders to move diagonally against the current in a river, the same way a sailboat can angle its sails to move diagonally against the wind?
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A sailboat pushes off of the wind and the water. The resultant force can send the boat in directions other than the direction of the wind or water’s motion.
A boat with no sail only has the water. There are no two competing forces to exploit – only the force of the water pushing it downstream.
You can circumvent this issue by tethering your boat. Now, by balancing the force of the tether and the water, you can indeed move left or right relative to the water’s flow.
The shape of the sail produces lift in a manner similar to an airplane wing. The lift (force) is what propels the sailboat forward. I don’t believe rudder shape would allow this to happen.
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