Having made this introduction, it became much easier to understand how a sailboat sails against the wind. That experiment where you blow on the side of a sheet of paper and it rises, almost like magic, is a clear demonstration of Bernoulli's Principle! Due to the greater air velocity in the upper part of the paper, there is a lower pressure (to understand, imagine a hose connected to a faucet, and this one is open: with the faucet open, but the nozzle of the hose locked, the force against the walls is huge, but when we unlock the nozzle and the water acquires enormous speed it may be that, even with a hole in the hose, it does not leak through it, but continues to flow), and with that the leaf rises!

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Note that blowing in one direction generated a lift force, in a perpendicular direction, which is amazing!

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Returning to the sailboat, it cannot face the wind head-on and walk against it in this situation, but it can be placed on its side, in such a position that the wind generates lift in the sails from the difference in inclination of its material (the sail is with a more rounded side, like an airplane wing); note that one of the components of lift is in the opposite direction to the force of the winds (and will obviously have a much lower intensity than this one) and the other in a perpendicular direction to this one (also lower intensity).

The tendency is for the sailboat to walk inclined in the direction of the wind (for example, either northwest or northeast, if we are talking about a north-south wind), that is, going sideways.

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And here comes the mistake.

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In a common boat, not a sailboat, the rudder is largely responsible for steering it (the helm changes the position of the rudder). On a sailboat, it also serves to counterbalance the force to one side generated by the sails, but this does not occur as an "inverted lift", or the same lift in the perpendicular but to the other side; in other words, they say that the Bernoulli Principle is at work there, when it is not. What happens inside the water is just action and reaction (that is, something more similar to the wings of an F1, and not to those of a cruising plane): if the boat tends, for example, to the northeast, just turn the rudder to the left, so that the course is corrected, so that the sailboat has the final tendency to walk into the wind. . As we are not talking about an active force, that is, in the end, it is the wind that is setting the tone (note: I am ignoring possible sea currents and proximity to surf zones), the tendency is that, even with the rudder positioned in such a way as to counterbalance the force of the sail in the opposite direction, it will still be displaced in an inclined manner, either to the northwest or to the northeast (in the case of a north-south wind), however the solution is simple: after a while, just invert the position the sails, as well as the rudder, so that the vessel zigzags around the main straight path into the wind.