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.
First, the sails.
[Wikipedia has a picture that can help with the explanation](https://commons.wikimedia.org/wiki/File:Points_of_sail.svg).
There are several ways to set the sails. If the wind is directly behind the way the boat wants to go, or up to about 45 degrees on either side, the wind can blow directly into the sail. Going straight on is generally called running. As you start to reach farther angles it is a reach, a broad or beam reach. Turning the sail a bit can compensate for the direction change and that can get you to nearly a 90 degree on either side, or “blowing sideways” as you put it.
Once it goes even farther, you can use a sail to beat upwind if that’s the direction you need to go. You can set the sails so they work more like an airplane wing providing lift, except they’re doing it at an angle. That’s close hauled. Depending on the boat and the sail, you can do that to about 35 degrees or 45 degrees against the wind. You can zig-zag, or tack, to go back and forth to compensate for the direction it pushes the boat.
So using nothing but the wind you can move in almost any direction, except for directly upwind. With tacking you can travel upwind, it’s just somewhat slower.
Second, the rudder.
A boat has a big movable blade either on the hull or behind the hull that steers the water. There’s a wheel that lets you turn the rudder. As the boat cuts through the water, the rudder pushes the water to either side forcing the boat to turn. This also changes the direction the boat will travel.
Putting them together:
Let’s say the wind is blowing directly from the north, so heading due south, but you want to head due west. You would set your sails to a beam reach, at about a 45 degree angle to the wind. This makes the wind push the boat in a southwest direction. Next, you would turn your rudder to turn you starboard, or to the left, to make up for the other 45 degrees to finish pushing you to the west. You’ll end up traveling somewhat slower at this angle, but still pick up plenty of speed over time.
Let’s say you wanted to beat upwind, say the wind was still heading due south but you wanted to travel north. You would set the sails close hauled so it pulls to one side like a wing lifts an airplane wing up, and turn the rudder so instead of just a 45 degree angle on the sail and the rudder, the sail would be almost straight in line along the boat on the port side, and the rudder turned to starboard. Now your ship would be heading northwest. It won’t have the same force as heading downwind, but will pick up speed over time. Then you would tack, turning from facing northwest to facing northeast, reset your sail so the sail would be along the boat on the starboard side, and turning hard to port. Now your boat is heading northeast, making up for the westward movement. By tacking, alternating between northeast and northwest, your boat will average out to a north direction.
You’re also no limited by the wind speed, since the sail is operating more like lift on an airplane wing. You can gain more and more speed even when traveling directly upwind, losing very little when you tack.
Surprisingly to many people running directly downwind is often the slowest way to sail. As you may have learned in math class, the hypotenuse of a triangle is always longer than either leg. If you’re traveling diagonally with the wind, the wind is pushing you at a certain speed (perhaps 15 knots) in one direction, but your direction is a diagonal line, and may reach 20 knots or more. If you’re looking for speed and running downwind, a racing sailboat will still tack downwind with a broad reach sail. By tacking in a zig-zag downwind they can travel quite a bit faster than windspeed, some racing sailboats can hit 3x the windspeed, and even beating upwind racing sailboats can exceed windspeed.
Wind blowing sideways does tend to push the boat sideways. If the boat were just a flat shape floating on top of the water, it would be pushed sideways a lot, with nothing to stop it. But boats aren’t shaped like that. They sink into the water somewhat, with the bottom of the boat below the water line. Also, boats are longer than they are wide. Together, this means that it is easier to push a boat through water along its long axis (forward or backward) than it is to push sideways. Because, when you try to push sideways, you have to displace a lot of water that gets in the way of this wide underwater surface.
Some boats are designed to resist being pushed sideways even more, for instance by attaching a fin or board to the bottom of the boat, protruding downward into the water. This provides even more surface area to resist sideways motion, while adding minimal resistance to forward motion. This fin also stops the boat from rolling easily (which sailboats are susceptible to as wind pushes against the sails which sit mostly above the boat’s center of gravity).
so if I were explaining like you were 5…
yes. the wind pushes the boat sideways.
But.
The boat has a long thin hole right in the middle, you can see the water through the hole.
In that hole you put something called a “centerboard” which is kind of like an airplane wing. It sticks down deep in the water, so you have to take it out when you get in shallow water to dock and put it back in when you start sailing.
Some centerboards are mounted under the boat on a hinge and can fold up under the boat with a lever or a motor, but simple boats have an actual hole you can see on the deck and you have to put the board in and out by hand.
https://en.wikipedia.org/wiki/Centreboard
It’s like when you stick your hand out a car window – if the board is turned to one side the moving water pushes it back so it always goes through the water skinny side first, just like your hand or like a weathervane in the wind.
So when the wind tries to push the boat sideways, the centerboard in the water keeps it from sliding that way and helps keep the boat straight.
It should and it does, but the boat’s keel doesn’t let the boat drift sideways so it’s forced to go forward.
Think of an F1 car. It is powered by the rear wheels which are always parallel to the car so the car should always go straight forward but the front wheels are turned not allowing the car to go straight so it has to go around the corners.
It does push the boat off course slightly over longer distances but that can be counteracted by how you turn the boat. The boat hull is designed in such a way that there’s a lot less resistance between the hull and water for the boat to go forward than there is for it to be going sideways. That means that even if the wind is blowing at an angle from the side and not directly behind, the special shape of the sail can convert most of that energy into forward momentum. In fact most sailboats sail the fastest with a wind at an angle, since if the wind is directly behind you you might think that that would be the fastest but that also means your sail creates the most drag since it has to be completely perpendicular to the direction you’re moving.
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.
First, the sails.
[Wikipedia has a picture that can help with the explanation](https://commons.wikimedia.org/wiki/File:Points_of_sail.svg).
There are several ways to set the sails. If the wind is directly behind the way the boat wants to go, or up to about 45 degrees on either side, the wind can blow directly into the sail. Going straight on is generally called running. As you start to reach farther angles it is a reach, a broad or beam reach. Turning the sail a bit can compensate for the direction change and that can get you to nearly a 90 degree on either side, or “blowing sideways” as you put it.
Once it goes even farther, you can use a sail to beat upwind if that’s the direction you need to go. You can set the sails so they work more like an airplane wing providing lift, except they’re doing it at an angle. That’s close hauled. Depending on the boat and the sail, you can do that to about 35 degrees or 45 degrees against the wind. You can zig-zag, or tack, to go back and forth to compensate for the direction it pushes the boat.
So using nothing but the wind you can move in almost any direction, except for directly upwind. With tacking you can travel upwind, it’s just somewhat slower.
Second, the rudder.
A boat has a big movable blade either on the hull or behind the hull that steers the water. There’s a wheel that lets you turn the rudder. As the boat cuts through the water, the rudder pushes the water to either side forcing the boat to turn. This also changes the direction the boat will travel.
Putting them together:
Let’s say the wind is blowing directly from the north, so heading due south, but you want to head due west. You would set your sails to a beam reach, at about a 45 degree angle to the wind. This makes the wind push the boat in a southwest direction. Next, you would turn your rudder to turn you starboard, or to the left, to make up for the other 45 degrees to finish pushing you to the west. You’ll end up traveling somewhat slower at this angle, but still pick up plenty of speed over time.
Let’s say you wanted to beat upwind, say the wind was still heading due south but you wanted to travel north. You would set the sails close hauled so it pulls to one side like a wing lifts an airplane wing up, and turn the rudder so instead of just a 45 degree angle on the sail and the rudder, the sail would be almost straight in line along the boat on the port side, and the rudder turned to starboard. Now your ship would be heading northwest. It won’t have the same force as heading downwind, but will pick up speed over time. Then you would tack, turning from facing northwest to facing northeast, reset your sail so the sail would be along the boat on the starboard side, and turning hard to port. Now your boat is heading northeast, making up for the westward movement. By tacking, alternating between northeast and northwest, your boat will average out to a north direction.
You’re also no limited by the wind speed, since the sail is operating more like lift on an airplane wing. You can gain more and more speed even when traveling directly upwind, losing very little when you tack.
Surprisingly to many people running directly downwind is often the slowest way to sail. As you may have learned in math class, the hypotenuse of a triangle is always longer than either leg. If you’re traveling diagonally with the wind, the wind is pushing you at a certain speed (perhaps 15 knots) in one direction, but your direction is a diagonal line, and may reach 20 knots or more. If you’re looking for speed and running downwind, a racing sailboat will still tack downwind with a broad reach sail. By tacking in a zig-zag downwind they can travel quite a bit faster than windspeed, some racing sailboats can hit 3x the windspeed, and even beating upwind racing sailboats can exceed windspeed.
Wind blowing sideways does tend to push the boat sideways. If the boat were just a flat shape floating on top of the water, it would be pushed sideways a lot, with nothing to stop it. But boats aren’t shaped like that. They sink into the water somewhat, with the bottom of the boat below the water line. Also, boats are longer than they are wide. Together, this means that it is easier to push a boat through water along its long axis (forward or backward) than it is to push sideways. Because, when you try to push sideways, you have to displace a lot of water that gets in the way of this wide underwater surface.
Some boats are designed to resist being pushed sideways even more, for instance by attaching a fin or board to the bottom of the boat, protruding downward into the water. This provides even more surface area to resist sideways motion, while adding minimal resistance to forward motion. This fin also stops the boat from rolling easily (which sailboats are susceptible to as wind pushes against the sails which sit mostly above the boat’s center of gravity).
so if I were explaining like you were 5…
yes. the wind pushes the boat sideways.
But.
The boat has a long thin hole right in the middle, you can see the water through the hole.
In that hole you put something called a “centerboard” which is kind of like an airplane wing. It sticks down deep in the water, so you have to take it out when you get in shallow water to dock and put it back in when you start sailing.
Some centerboards are mounted under the boat on a hinge and can fold up under the boat with a lever or a motor, but simple boats have an actual hole you can see on the deck and you have to put the board in and out by hand.
https://en.wikipedia.org/wiki/Centreboard
It’s like when you stick your hand out a car window – if the board is turned to one side the moving water pushes it back so it always goes through the water skinny side first, just like your hand or like a weathervane in the wind.
So when the wind tries to push the boat sideways, the centerboard in the water keeps it from sliding that way and helps keep the boat straight.
It should and it does, but the boat’s keel doesn’t let the boat drift sideways so it’s forced to go forward.
Think of an F1 car. It is powered by the rear wheels which are always parallel to the car so the car should always go straight forward but the front wheels are turned not allowing the car to go straight so it has to go around the corners.
It does push the boat off course slightly over longer distances but that can be counteracted by how you turn the boat. The boat hull is designed in such a way that there’s a lot less resistance between the hull and water for the boat to go forward than there is for it to be going sideways. That means that even if the wind is blowing at an angle from the side and not directly behind, the special shape of the sail can convert most of that energy into forward momentum. In fact most sailboats sail the fastest with a wind at an angle, since if the wind is directly behind you you might think that that would be the fastest but that also means your sail creates the most drag since it has to be completely perpendicular to the direction you’re moving.
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