Andrew Pace
Sailing boats have long, thin underwater parts that prevent them from being pushed sideways. The fastest way to sail is not to have the wind pushing directly into the back of the sail, but instead to travel perpendicular to the wind. This is because sails work like wings, creating lift as the wind pushes them into a curved shape. The angle of the sail is important when sailing perpendicular to the wind or towards where the wind is blowing from. The keel, which must be symmetrical, also helps the boat to move to either side of the wind.
| Characteristics | Values |
|---|---|
| Shape of underwater part | Long and thin |
| Fastest sailing direction | Travelling directly perpendicular to the wind |
| Sail function | Acts as a wing, not a parachute |
| Sail shape | Curved |
| Sail angle | Important when sailing perpendicular to the wind or towards where the wind is blowing from |
| Keel function | Provides sideways lift |
| Keel shape | Symmetrical |
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What You'll Learn
The physics of sailing
Sailing boats have long, thin underwater parts that make it difficult for them to be pushed sideways. The fastest way to sail is when you're travelling directly perpendicular to the wind. This is because sails work like wings, and the wind pushes the sail into a curved shape, like the wing of a plane. The wind pushes the sail into a nice curved shape, just like the wing of a plane.
The force of the wind on a sail works in the same way as a wing on an aeroplane or the blades on a wind turbine. Air moves across the surface of the wing or blade and creates a force called lift. Lift on a sail is created when the air travelling around the outside of the curve of the sail takes a longer path than the air around the inside of the sail.
A keel must be symmetrical for the sailboat to move to either side of the wind. A keel works only if the motion of the boat is not exactly in the direction in which it’s pointed. The boat must be moving somewhat sideways. In that “crabbing” motion, the keel moves through the water with an angle of attack. Just as for the sails in the wind, that causes the water on the “high” (more downstream) side of the keel to move faster and create a lower pressure. Again, the net lift force on the keel is due to the combination of that decreased pressure on the high side and increased pressure on the other (low) side.
When sailing perpendicular to the wind or towards where the wind is blowing from, the angle of the sail becomes important. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat.
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How the wind works with the sails
The wind works with the sails of a boat in a similar way to how it works with the wings of an aeroplane. The wind pushes the sail into a curved shape, and air travelling around the outside of the curve of the sail takes a longer path than the air around the inside of the sail, creating a force called lift. The fastest sailing is when you’re travelling directly perpendicular to the wind, as the sails can be used as wings rather than parachutes.
The angle of the sail is important when sailing perpendicular to the wind or towards where the wind is blowing from. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat.
A keel is also important for a sailboat to move in the direction in which it’s pointed rather than just drifting downwind. The keel must be symmetrical for the sailboat to move to either side of the wind. The boat must be moving somewhat sideways, in a “crabbing” motion, for the keel to work.
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How the wind works with the keel
The wind works with the keel to help a sailboat move in the direction in which it's pointed, rather than drifting downwind. The keel must be symmetrical for the sailboat to move to either side of the wind. The boat must be moving somewhat sideways, in a "crabbing" motion, for the keel to work. In this motion, the keel moves through the water with an angle of attack, causing the water on the "high" (more downstream) side of the keel to move faster and create a lower pressure. The net lift force on the keel is due to the combination of that decreased pressure on the high side and increased pressure on the other (low) side.
The fastest sailing is when you’re travelling directly perpendicular to the wind. The wind pushes the sail into a nice curved shape, just like the wing of a plane. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat.
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How to sail perpendicular to the wind
Sailing perpendicular to the wind is the fastest way to sail. This is because modern sailboats use their sails as wings instead of parachutes. The wind pushes the sail into a curved shape, like the wing of a plane, and the force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat.
To sail perpendicular to the wind, the angle of the sail is important. The sail must be curved so that the air travelling around the outside of the curve takes a longer path than the air around the inside of the sail. This creates lift, which moves the boat forward.
The keel is also important for sailing perpendicular to the wind. The keel must be symmetrical for the sailboat to move to either side of the wind. The boat must be moving somewhat sideways, in a "crabbing" motion, for the keel to work. This sideways motion, combined with the angle of attack of the keel, creates a lower pressure on the "high" (more downstream) side of the keel, resulting in a net lift force on the keel.
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How to sail upwind
Sailing boats have long, thin underwater parts that make it difficult for them to be pushed sideways. The fastest way to sail is when you're travelling directly perpendicular to the wind. This is because the sails are used as wings, rather than like parachutes, and the wind pushes the sail into a curved shape, like the wing of a plane.
However, sailing upwind is more difficult, as you are sailing against the wind, the only apparent force that pushes you along. To sail upwind, you need to sail in a zigzag course, with each leg of the zigzag approximately 45 degrees away from the direct line between your starting point and your destination. This is because a sailboat cannot make any forward progress directly into the wind. When you try to sail too close to the wind, the sails will simply flap and you will lose headway. This is called 'luffing' and you may even put the boat in irons.
When sailing upwind, you need to trim the sails according to the wind direction relative to the boat. To start, trim the jib sheet tightly and trim the mainsail to the point that its luff just stops bubbling. The boom will lie a little off-centre. You can then experiment with small changes to the trim of both sails.
A keel must be symmetrical for the sailboat to move to either side of the wind. The boat must be moving somewhat sideways, in a 'crabbing' motion, and the keel moves through the water with an angle of attack. This creates a lower pressure on the 'high' side of the keel and increased pressure on the other side, which helps the boat move in the direction it's pointed.
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Frequently asked questions
A keel helps the sailing boat move in the direction it's pointed. A keel must be symmetrical for the sailboat to move to either side of the wind.
The fastest sailing is when you’re travelling directly perpendicular to the wind.
The wind pushes the sail into a curved shape, just like the wing of a plane. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat.
Sailing boats have long thin underwater bits that make it very hard for them to be pushed sideways.
A keel works only if the motion of the boat is not exactly in the direction in which it’s pointed. The boat must be moving somewhat sideways.
