I realize a sailboat can’t go directly at the wind (or maybe it can🤷🏽♂️) but for the life of me I can’t picture it going anywhere but where the wind is bowing.
Also, lets say you were in a round pond, could you sail to any point you wanted to in the pond with the wind blowing steady in one direction?
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Imagine a circle around the sailboat, and for the sake of simplicity let’s say the wind is blowing from the top of this circle, and that point is 0 degrees. In a clock face it would be 12. Take two 45 degree slices of that circle, left and right from 0 degrees, which gives us a total slice of 90 degrees, or a quarter of the circle, or to more easily visualise, in a clock face with the wind blowing from 12 o clock the slice would be from 10 to 2. That slice is the area where you cannot point your boat to since it’s either blowing directly opposite to the boat or close to that.
Sailboat sails move the boat through two means. One is that they act as a foil, which is the scientific term for a wing, and the other is that they work through drag, which is what you and most people are familiar with when they’re thinking of sails. When the boat is pointing as close to the wind as possible, the sail acts as a foil. This means that it redirects the wind coming over it and deflects it, which creates a force on the sailboat that’s trying to push the boat diagonally. Underneath the sailboat is a large fin, the keel, which also acts as a foil but it’s much smaller since water is denser so it doesn’t have to be as big as a sail to have an effect. The keel resists the sideways drift of the boat and creates a force that’s perpendicular to that of the sails. So basically it resists the tendency of the boat to drift diagonally. Since the sails produce a force that’s diagonal in relation to the bow of the boat, and the keel produces a force equal to it and diagonal from the other side, the resultant force pushes the boat straight down the middle of those two, so forward.
This effect is dominant when the boat is pointing towards the wind, up until the wind is perpendicular to the boat, so coming from the side. Once the boats steers past that, and starts pointing in a direction closer to the direction of the wind, up until 180 degrees, or 6 on a clock face, the main component of the propulsion the sails create comes from drag, which is that they basically catch wind coming from behind and use the wind’s kinetic energy to move the boat.
Both of these forces are at play in varying degrees depending on the heading of the boat relative to the wind. To sail upwind a maneuver called tacking has to be performed, which is when a sailboat zigzags from left to right and always keeping the wind either 45 degrees to their right or 45 degrees to their left. This means they’re sailing in the general direction the wind is blowing from but not literally.
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