It is very logical and intuitive for anyone to understand how a boat moves when the wind enters it from the stern , that is, from behind: it is enough to unfold the sails perpendicular to the direction of the air current , forming a right angle with it. Thus, the sail picks up the wind, inflates and fills with power and pushes the ship forward . But the displacement of sailboats is not produced only by this phenomenon. If so, they would hardly be maneuverable, because they could only navigate in one direction, and yet they are capable of moving against the wind, or almost against it. How can you pilot a ship against such a powerful force and move it forward? In reality, it is impossible to do it completely from the front, but upwind –which is how this course is called in the marine jargon–, forming the smallest possible angle with respect to the current of air that opposes us . To achieve this, the key is to navigate in a zigzag , turning to change the side from which the wind reaches the ship. Thus, the contrary current will affect our sails obliquely –that is, at an angle that is not right–, which will move the boat laterally.
This displacement to one side can be explained thanks to the so-called Bernoulli principle, which also serves to understand the lift of airplanes . Without going into too much detail, this theory explains how the force generated by the wind is divided into two components when it reaches the sail that is presented obliquely: one that pushes the boat in its set direction (forward) and another that It drags it in the direction of the wind, in a course known as drift, and it takes us away from our destination. Correcting drift requires a force to compensate for it . We obtain it thanks to two elements of the sailboat: the keel –the piece that goes from stern to prow along the bottom of the boat, in the water, and on which its frame sits– and the daggerboard, which protrudes from the bottom from the hull and counteracts the force of the crosswind , since water is a relatively dense fluid. This collision of opposing forces results in a forward push that takes us in the desired direction, always zigzagging.
In the case of airplanes, the wings are designed to force the air to flow faster on the upper surface than on the lower one , so the pressure on the latter is greater than on the upper one. This pressure difference provides the lift force that keeps the aircraft in flight.
