But it is possible for this movement to become reversed because the sail of a moving sailboat is shaped like an airfoil like an airplane's wing.
When the air moves over the wing of a plane from the front and going backward, the wind that flows over the wing's top has to travel farther than the wind that flows beneath the bottom surface of the wing. This creates a difference in pressure to lift the airplane.
On sailboats, the wind that blows at an angle against the boat inflates the sail. It forms a foil shape similar to the airplane. It creates a pressure difference pushing the sail perpendicular to the direction of the wind.
The force from the foil shape of the sail is balanced and combined with other forces including the keep of the boat. The keel is the thin, long piece jutting down from the boat bottom.
From the water, the forces of drag simultaneous with wind pressure against the sail pushes the craft onwards. It moves at angles opposite the wind direction. in sailing terminology, this is called windward.
The keel is of particular importance because without its balance action. boats would drift simply downwind. Sailing windward won't work either if boats are directly pointed opposite the direction of the wind. Instead, the wind has to move against the boat at angles of about forty degrees for many sailboats.
When you angle your sailboat too sharply into the wind will cause the forces on the craft to become imbalanced. When this happens, the boat will then move sidewards into the water.
It is possible to sail against the wind when your sailboat's sail is slightly angled in a direction that is more forward than the force of the sail. The boat can then move forward in this aspect because the centerline or the keel of the boat does to the water what the sail is doing to the wind.
The sail's force keeps its balance by the keel's force. This keeps the boat from moving into the sail force's direction. A proper angle of attack moves the boat forward even if the total force of the sail is to the side when the boat sails into the wind.
In other words, when the sail is angled away from the hull's centerline, the more the force is pointing forward rather than pointing to the side. When you combine the forward force's slight adjustment with the water's opposition to the air, the boat can then shoot windward because you have found a way to sail a course of least resistance against the wind.
A sailboat sailing against the wind will turn through the point on each tack. This is the point in which the boat is neither on the starboard tack or the port tack and is directly headed against the wind.
On the other hand, boats are not able to sail directly against the wind. Thus, f a boat heads into the wind it is said to be "in irons" when it loses steerage. For this reason, a boat sailing against the wind is sailing with the sails trimmed tightly, also known as sailing "close-hauled."
When it comes to how to sail against the wind, keep in mind that when a sailboat sails too close to the wind, or with an angle too small to the wind, the term is called "pinching." This is also a phrase used in colloquial expression that means "recklessness."
To reach its target, sailors that intend to travel windward to a point in line with the exact wind direction will need to zig-zag in order to reach its destination. This technique is tacking. Sailors can reach a point in any direction using the technique of tacking and traveling at angles closest to the wind direction.
Sailing against the wind in practice is usually achieved at a course of and angle of around forty-five degrees to the oncoming wind. To reach specific points, alternating the wind's direction between the starboard and the port is sometimes necessary. The term for this is "tacking."
Tacking is when a yacht or a sailboat sail against the wind. Counterintuitively, this means that compared to having a weak wind behind you, it is always better to have the strong wind in the direction opposite your craft. Having no wind is the worst-case scenario. Think of vectors.
The wind generates forces against the boat's hull through the momentum change that the sails cause. The force goes both towards the direction of where you are going and perpendicular to the motion. The keel takes up the perpendicular force and leans the yacht. Motion is then created by the remaining forward vector.
If your destination is located upwind, how are you going to sail there? Because of the lift created by wind blowing across and not against them, the sails propel the boat forward. This happens unless the wind blows from directly over the back of the boat (astern).
As you begin steering in the direction of the wind, you trim the sails tighter in and keep them full, so that lift is continuously generated. However, sailing too close to the sail and wind will "luff."
This means the edge of the forward sail begins to flutter inwards and outwards and the boat slows down. If you begin turning more into the wind, the whole sail will soon be flapping like a king-sized bedsheet you hung out to dry.
However, don't stop turning into the wind and you will soon see the sail filling on the other side of the boat. This is called tacking and the scientific reasons are explained as you read further down.
Sailboats made today can sail up to around a forty-five-degree angle against the wind. For example, if the north wind is blowing into your sail, the boat can sail on a port tack about the northeast.
The boat can sail all the way through to northwest, west, south, and east on the starboard tack, or wind coming from the boat's right side. Port tack means that the wind comes over the left side of the port. Tack means which side of the boat the wind blows from.
Even if you can't sail your boat literally directly into the wind, sailors call this tacking or beating to windward. You will find that on the newer tack, you sail in the direction that's at about right angles to the old tack. This occurs with the wind still at about forty-five degrees but this time on the other side. The zig-zagging and the repeated tack will move the boat upwind.
Four forces act on a sailboat trying to sail against the wind. The two that directly affect the boat are the viscosity force of the water and the force of the wind, which propels the boat.
The water's viscosity slows down the boat and helps her keep on-course. The remaining two forces are buoyancy and gravity. Buoyancy pulls up the sailboat and gravity pulls her down. All of these forces keep the boat afloat as it sails against the wind.
The combined effect of the water and the wind is a net force pushing the boat diagonally against the wind. The resistance of the water combined with the force of the wind determines the direction in which a sailboat sails. On the sail, the force exerted by the wind has two components:
- The lift component pushing the sail into the wind perpendicularly.
- The drag component pushing the sail into the direction of the wind.
Because of the lift, the direction of the wind-force varies from the direction in which the wind blows. The angle between the wind and the sail shape of the sail will determine what direction the wind force goes.
The forward motion of the boat and her slippage sidewise slows down due to water resistance. For boats to sail against the wind diagonally, the sidewise slippage needs to be minimal compared to the motion forward. Sidewise slippage is significantly reduced with the keel.
If a keel somewhat eliminates the sidewise slippage, sailboats can only move in the keel's direction. This is also the direction of the sailboat's centerline. Whenever the wind-force total diagonally points forward in relation to the keel, the boat will then move forward in the keel's direction.
If the keel is pointing diagonally into the wind, and the wind-force diagonally points forwards, the boat will then diagonally sail into the wind.
On the other hand, the boat won't be able to diagonally sail into the wind if the sidewise slippage is too big. Just like everything else, sailing against the wind takes practice. Master this and you can sail yourself anywhere in the world and through anything.