# How do sail boats actually work?

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The sails always seems to be somewhat parallel to the direction of the boat, but if the wind is blowing sideways, should that push the boat off course?

In: 9

The next time you’re at a pool, grab one of those floaty board things, shove it deep underwater, and try to wiggle it around.

You’ll notice that it’s really easy to move the board along its thin directions, but really hard to move it along the flat, wide direction.

Sailboats (and most other boats actually) are like that board. They have long thin underwater bits that make it very hard for them to be pushed sideways.

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Another fun thing about sailboats is that the most intuitive way for them to work, with the wind pushing directly into back of the sail to move it forwards, is actually the slowest way to sail.

The fastest sailing is when you’re travelling directly perpendicular to the wind.

The reason for this is that (modern) sailboats use their sails as *wings* instead of like parachutes.

The wind pushes the sail into a nice curved shape, just like the wing of a plane. Then the pressure differential between the outwardly curved and inwardly curved sides is what pushes the sailboat forwards.

This is what allows sailboats to sail upwind. The wind is *pushing* them back, but the pressure difference across the sail makes the boat move forwards anyway.

Imagine a sailboat like an airplane tipped sideways with one wing in the air and one in the water – the one in the air is the sail and the one in the water is the keel.

The boat moves because the wind is pushing against the water resistance of the keel; the result is low air pressure in front of the sail and high air pressure behind it, combined with water pressure on the lee side of the keel, but no resistance forward or aft.

So all the pressures even out by the boat moving forward.

Sails act like a wing most of the time. The air moving over them generates lift*. Boats have a keel or a dagger/centreboard that stops them drifting sideways to an extent, but they do still drift slightly.

*If a boat is heading away from the wind, the primary mechanism of propulsion may just be the wind pushing the sail.

Okay, so you’re probably looking at sloop rigged sailboats, they look like this: ⛵️

Now, when they’re heading downwind, they can use a sail called a spinnaker or push the mainsail and jib out to roughly parallel to the boat to go faster. They actually do go perpendicular when the wind is pushing them from behind.

So when going upwind (not straight upwind, typically about 45 degrees off of straight into the wind and the sails are off to an angle that creates better lift, but let’s ignore that for now) the sails SHOULD only push the boat downwind, but there’s a clever way to deal with that: a keel. The keel is a straight board in the water that’s parallel with the boat.

So let’s say wind is coming from the top of your phone, straight down. The sail is parallel with the boat/keel, moving 50 degrees to the left of going straight up, with 90 degrees being straight left. (Draw it out if it helps) There’s a component of force down yes, and that should push the boat back, but there’s a leftward component that is bigger than the down component. So the sailboat will go left more than down/back.

Now remember the keel, it’s in water which has much more friction than air. So the keel helps make the boat move in only two directions, forward or back. So whatever force is pushing on the sail pushed down to the keel, and is equalized out. The only part of the force that is allowed to push the boat forward is the remaining leftward force, which pushes the boat left, but that gets equalized out too by the keel, so the boat slips forward in the keel direction too.

In the end the boat is pushed left, but because of the keel, goes forward and left, along that 50 degree bearing.

It’s like a toy car on a string.

In the water, there’s a few fins that work like the tires of the toy car. Above the water, there’s wind that works like somebody pulling the string.

It’s not an exact comparison, but this is ELI5.

When the string is pulled, the car wants to roll in the direction of the tires, not always directly towards the person pulling the string.

The tires can turn much like the rudder of the boat to steer.

Beyond this, things can become slightly more complex when sailing perpendicular to the wind or towards where the wind is blowing from. (But not directly into the wind, that doesn’t work.)

This is where the angle of the sail becomes quite important. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat (in a sense).

Like pinching a bead in your fingertips, the bead can be launched away while neither of your fingers are applying force in the launch direction. It’s two separate forces working together to create a new force in a different direction.

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0

The sails always seems to be somewhat parallel to the direction of the boat, but if the wind is blowing sideways, should that push the boat off course?

In: 9

The next time you’re at a pool, grab one of those floaty board things, shove it deep underwater, and try to wiggle it around.

You’ll notice that it’s really easy to move the board along its thin directions, but really hard to move it along the flat, wide direction.

Sailboats (and most other boats actually) are like that board. They have long thin underwater bits that make it very hard for them to be pushed sideways.

___

Another fun thing about sailboats is that the most intuitive way for them to work, with the wind pushing directly into back of the sail to move it forwards, is actually the slowest way to sail.

The fastest sailing is when you’re travelling directly perpendicular to the wind.

The reason for this is that (modern) sailboats use their sails as *wings* instead of like parachutes.

The wind pushes the sail into a nice curved shape, just like the wing of a plane. Then the pressure differential between the outwardly curved and inwardly curved sides is what pushes the sailboat forwards.

This is what allows sailboats to sail upwind. The wind is *pushing* them back, but the pressure difference across the sail makes the boat move forwards anyway.

Imagine a sailboat like an airplane tipped sideways with one wing in the air and one in the water – the one in the air is the sail and the one in the water is the keel.

The boat moves because the wind is pushing against the water resistance of the keel; the result is low air pressure in front of the sail and high air pressure behind it, combined with water pressure on the lee side of the keel, but no resistance forward or aft.

So all the pressures even out by the boat moving forward.

Sails act like a wing most of the time. The air moving over them generates lift*. Boats have a keel or a dagger/centreboard that stops them drifting sideways to an extent, but they do still drift slightly.

*If a boat is heading away from the wind, the primary mechanism of propulsion may just be the wind pushing the sail.

Okay, so you’re probably looking at sloop rigged sailboats, they look like this: ⛵️

Now, when they’re heading downwind, they can use a sail called a spinnaker or push the mainsail and jib out to roughly parallel to the boat to go faster. They actually do go perpendicular when the wind is pushing them from behind.

So when going upwind (not straight upwind, typically about 45 degrees off of straight into the wind and the sails are off to an angle that creates better lift, but let’s ignore that for now) the sails SHOULD only push the boat downwind, but there’s a clever way to deal with that: a keel. The keel is a straight board in the water that’s parallel with the boat.

So let’s say wind is coming from the top of your phone, straight down. The sail is parallel with the boat/keel, moving 50 degrees to the left of going straight up, with 90 degrees being straight left. (Draw it out if it helps) There’s a component of force down yes, and that should push the boat back, but there’s a leftward component that is bigger than the down component. So the sailboat will go left more than down/back.

Now remember the keel, it’s in water which has much more friction than air. So the keel helps make the boat move in only two directions, forward or back. So whatever force is pushing on the sail pushed down to the keel, and is equalized out. The only part of the force that is allowed to push the boat forward is the remaining leftward force, which pushes the boat left, but that gets equalized out too by the keel, so the boat slips forward in the keel direction too.

In the end the boat is pushed left, but because of the keel, goes forward and left, along that 50 degree bearing.

It’s like a toy car on a string.

In the water, there’s a few fins that work like the tires of the toy car. Above the water, there’s wind that works like somebody pulling the string.

It’s not an exact comparison, but this is ELI5.

When the string is pulled, the car wants to roll in the direction of the tires, not always directly towards the person pulling the string.

The tires can turn much like the rudder of the boat to steer.

Beyond this, things can become slightly more complex when sailing perpendicular to the wind or towards where the wind is blowing from. (But not directly into the wind, that doesn’t work.)

This is where the angle of the sail becomes quite important. The force of the wind acting on the sails and the force of water acting on the fins can squeeze the boat (in a sense).

Like pinching a bead in your fingertips, the bead can be launched away while neither of your fingers are applying force in the launch direction. It’s two separate forces working together to create a new force in a different direction.