A boat sails by harnessing the power of the wind to propel it through the water. The wind pushes against the sails, which are mounted on masts, causing the boat to move forward. The sailor can control the direction and speed of the boat by adjusting the angle of the sails, known as trimming, and the position of the rudder, which is a device located at the stern of the boat that helps to steer.
The shape of the sails and the position of the boat in relation to the wind also play a key role in how the boat sails. Sailboats are designed to sail into the wind at an angle, known as close-hauled, or with the wind coming from behind, known as a run. The sailor can also control the speed of the boat by adjusting the amount of sail area exposed to the wind.
In addition to the wind, the sailors must also consider other factors such as the water current and the weight and balance of the boat, as well as any other weather conditions that might affect the boat’s performance.
Key Concept: Lift
The lift that propels a sailboat forward is created by the wind as it flows over the sails. When wind flows over the curved surface of a sail, it creates an area of low pressure on the front side of the sail and an area of high pressure on the back side. This difference in pressure results in a force on the sail, pushing the boat forward. This is known as “lift” and the force that is generated by the wind flowing over the sails is called “aerodynamic force”.
The amount of lift generated by the sail can be increased by adjusting the angle of the sail in relation to the wind, known as the “angle of attack.” When the sail is angled more directly into the wind, the angle of attack increases and so does the lift. However, if the angle is too steep, the sail can stall and lose lift. Sailors must find the right balance between lift and drag, which is the opposing force created by the wind pushing against the sail, in order to sail efficiently.
In addition, the shape of the sail also plays a role in how much lift is generated. For example, a triangular sail, known as a “tri-radial” sail, is more efficient at generating lift than a rectangular sail because it is better at capturing the wind and converting it into forward motion.
Overall, lift is the fundamental principle that allows sailboats to move forward and harness the power of the wind. Sailors must constantly adjust the sails, rudder and other controls to optimize the lift and minimize drag in order to move the boat efficiently and safely.
A boat can sail upwind through the following dynamics:
- Air travelling around the outside of the curve of the sail takes a longer path than the air around the inside of the sail. This longer path means the air travels faster around the curve of the path, causing a lower pressure than the air surrounding it.
- Inside of the sail, the air has a shorter path to travel so travels at a much lower speed and has a lot more pressure.
- This difference in pressure between the inside of the sail and the outside of the sail causes lift, a force that helps push the boat through the water.
Key Concept: The Keel & Wind
A keel is a structural element that is typically located at the bottom of the hull of a sailboat and runs along the centerline of the boat. Lift isn’t the only force required for a boat to travel upwind, keels/centreboards prevent the boat from being pushed sideways through the water by the wind. The force applied by the water on the keel acts in the opposite direction to the force of the wind, these 2 forces squeeze the boat, pushing it forwards through the water, similar to when you squeeze a bar of soap in your hands.
So the keel serves several functions, including:
- Providing stability: The keel helps to keep the boat upright and stable by counteracting the forces of wind and waves that can cause the boat to heel, or lean, to one side. The weight of the keel also helps to counteract the force of the wind on the sails, which can cause the boat to tip over.
- Improving steering: The keel provides lateral resistance, which helps the boat to maintain a straight course and improves steering. The shape and size of the keel can also affect the boat’s maneuverability.
- Enhancing performance: The keel acts as a lifting surface and helps to generate lift, especially when sailing upwind. This is because the wind flowing over the keel creates a lifting force that helps to keep the boat moving forward.
- Improving safety: The keel also helps to protect the boat from capsizing or drifting sideways, which improves safety.
The keel is usually made of heavy materials such as lead or iron, which are placed in a long, narrow casting, and is attached to the boat’s hull.
On a sailboat, the keel works in conjunction with the sails and the rudder to allow the boat to move efficiently through the water. Sailors must consider the position and trim of the keel in relation to the wind and the direction of travel in order to optimize performance and safety.