How a sailboat can sail upwind ( Yeah ! That's amazing ! How ? )

Now that we've seen how and why a sailboat sails forward, it's time to learn how it can sail upwind, which is pretty stunning when you think about it !

Pretty logically, you would think that a sailboat can only sail "with" the wind ( = to the same direction as the wind )

The explanation of this miracle widely resides in what we learnt in section 3 How a sailboat can sail forward

Indeed, if a boat is pointing, say 45° off the wind ( as below ), the force of its sail(s) is pointing downwind

Resultant force of a sail oriented downwind

As the lateral component ( red ) of this force is cancelled by the resistance of the keel ( green ) and only the forward component remains ( blue )...

Sail side component force compensated by keel resistance

...the boat is "pulled" by the forward force ( blue ) and, hence, can sail upwind, even though the resultant force ( black here above ) is oriented downwind

Remaining forward component force
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