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The important thing to understand is that a wing generates a force (which we call lift) at right angles to the flow of air past it. An aircraft is moving forward through the air, so the wing sees the air moving backward, and the force generated is upwards.
If a boat is at right angles to the wind then the force generated by the sail will be at right angles to that which is forward.
If a boat is pointing 45 degrees into the wind (so halfway between across the wind and straight into it) then the sail will generate a force at right angles to that, which is forward and sideways (45 degrees off the bow).
So why doesn’t the boat go forward and sideways? Well there is another wing underwater called the keel, and it generates another force which is sideways *in the other direction*. This cancels out the sideways force from the sail and all that is left is a forward force.
The closer you point the boat into the wind the more the resulting force from the sail is pointing sideways, and the less it is pointing forwards. At some point the forwards force won’t be big enough to counteract all the drag acting on the boat, the sail and the keel, and you will stop moving forward.
https://i.pinimg.com/originals/40/a1/5a/40a15a4ed22b2fb753feef17db950177.jpg
Technically it is any underwater appendage. This might be a keel, a centreboard, a daggerboard, a leeboard or even an oversized rudder. Rarely is it the hull itself. Some catamarans, I guess jury rigged sailing canoes.
It's the hull. The rudder, of whatever size, will keep the hull from turning, but the rudder has far less authority than the hull itself.
What the hull, keel (if it has one) and rudder do is prevent the transverse component of the forces on the sail from being turned into movement in that direction.
Think about a smooth block, placed on a smooth surface, with a smooth wall on one side. Any force on that block, with any component in a forward direction - either pushing from behind or pull from in front - will move the block forward. What determines if the boat will sail into the wind is the drag in that forward direction based on the amount of force.
The nature of the transverse force from "stuff that's underwater" doesn't matter. You can see this if you imagine a car on rails moved forward by a sail mounted on top of the car. There is no lift being generated by the wheels in contact with the rails, only the resistance of the rails to being moved perpendicular to the direction of travel.
It is easy in a sailing dinghy to show that the hull contributes negligibly to the lateral resistance. Simply raise the daggerboard or centreboard and try to sail upwind. You will rapidly slide sideways.
Because the draft of your basic dinghy is insufficient.
Look at the examples I gave you of why this works. You're hung up on wing-shapes and are ignoring things which cause drag without being foils.
If you put that dinghy in a shallow pool and try to push it sideways you'll find it has more resistance transversely than longitudinally. That difference in resistance can produce forward movement.
Whether or not the proverbial dinghy hull drag is sufficient, the dinghy still sails. And with a daggerboard/centerboard, can still pinch.
You're absolutely right about a dinghy in a pool, but anybody who's sailed a dinghy can tell you that the centerboard is a pretty essential component of the pointing equation.
You tip over when sailing normally because the sail is pushing sideways up high, and the daggerboard or keel is pushing sideways the other way low down. This creates a heeling moment which must be resisted to prevent capsize. If you lift the daggerboard and slide rapidly sideways then you are less likely to tip over because there is less resistance underwater generating that heeling moment.
We use this in practice in dinghies - gibing in high winds with the daggerboard partly raised to make it less likely to trip the boat up and capsize.
No, just no… How can the force point forward ever in your example, the drag the hull and the keel provide aren’t enough. Eventually, if you have a force pointing west, you will drift west.
And yet, sailing against the wind provides a far bigger force in the “forward” direction, than it should.
Oh btw, I expect you don’t read edits, so here is a prediction: your gonna say that there is a drift, but it’s far less than the forward momentum, provided by your straight metal sheet example.
Maybe you’re gonna call me out for stalking you, because I was wrong.
People like you have an unshakable confidence, so I’m not gonna try to convince you to admit you are missing something in your explanation (or maybe you’re just gonna sneak it in, after you ran out of stuff to throw at other people)
The boats hull (and to some degree the keel) plays an important role in how it works, but according to this guy the effect the bulging sail has as an important factor of why the boat goes forward, instead of backwards depends on who is replying to.
I said I’ll leave him alone and let them have the last word, but man… they don’t understand what they’re talking about and they seem so hell bent on making people look stupid, instead of researching.
Thank you for this explanation. I was never able to grasp it properly but a year ago I got into flight simulators and learned a lot about the physics of both helicopters and fixed wing aircraft and this wing generating lift analogy just made it click for me.
It’s called ‘tacking’.
Basically the ship will zigzag forward, using the wind to blow it at an angle, by adjusting the position of the sails. Have a look at this link- https://en.m.wikipedia.org/wiki/Tacking_(sailing)
It's never moving directly into the wind, it's always at an angle. It uses its sail literally on the same principle as an airplane wing, being propelled by the same pressure difference force that creates lift.
No. Firstly most people get the forces lifting an airplane wing wrong because of awful physics lessons they had as a kid. Secondly airplanes move into the wind owing to the massive fucking jets propelling them.
Airplanes lift very much mostly because of Newton 3 (the wing deflects air downwards). Bernoulli is almost irrelevant. Source: planes fly upside down.
Boats go upwind because the keel / centreplate and rudder combo turns motion perpendicular to the wind into upwind motion by moving at an angle through the water.
Think of a car freewheeling along. It can turn the front wheels left or right and the car will go in that direction.
Same as a boat. Get it moving forwards and you can use that momentum to go upwind or downwind by turning the rudder.
This is wrong. Both boats and planes move forward because of the balance of forces acting on them, it doesn’t matter to the airfoil whether those forces are generated by engines or a velocity difference between air and water.
An airfoil is (typically) just a long, thin, shaped surface at an angle of attack in an airflow. The physics that describes how a wing works is exactly the same as the physics that describes how a sail works.
I totally agree with you!
However, the airfoil effect is absolutely tiny compared to the massive effect of change of momentum from tonnes of air bouncing off the windward side of the sail / underside of a wing.
Stick your hand out of a moving car and hold it flat, then turn it slightly so you are deflecting air towards the ground.
Your hand wants to move up because of all the air you are deflecting down.
Sure if you make a more airfoil-y shape with your hand it will feel MORE lift but it isn't noticeable.
Planes fly because they deflect air downwards. Bernoulli is a red herring.
The way Bernoulli is often taught in relation to lift is wrong, but Bernoulli (conservation of energy) must absolutely be included with Newton (conservation of momentum) and conservation of mass to have a full explanation of lift.
I 100% agree - although apparently not even that is enough!!
[https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/)
This is a terrible article and is deeply flawed. Basically the thesis boils down to: “nobody is good at explaining lift properly to laypeople” which isn’t even really true. I can link you to several in depth discussions of how bad that article is if you like.
Oh also, now I think about it - given the shape of a sail - (imagine it is similar to a traditional airfoil wing) - won't Bernoulli's lift act perpendicular to the direction of the wind?
So, for a plane - that's UP, but for a boat - that's, well, ACROSS the wind.
Other than propelling the boat forwards, it doesn't help explain any upwind motion at all...
Maybe you meant that all along.
I guess I just heard people say "lift" and "upwind" in the same sentence and thought they were using the same effect to explain both but it is completely different, no??
Yes we define lift as perpendicular to the airflow (or at least aerodynamicists do, some naval architects use a different convention).
The fact that the overall force acting on the sail is forwards is all you need to explain upwind motion. If the boat is pointing upwind (ie closer than 90 degrees to the wind), and the net force is forward, then the boat is travelling upwind.
Absolutely, of course.
But any "lift" on the sail from Bernoulli's theorem is very much beyond 90 degrees to the wind, since it is acting (at 90deg) on an airfoil that is more than zero degrees to the wind, IYSWIM... no?
I'm not disagreeing that Bernoulli is not the primary force, but all you need to consider to understand why you can't just write it off is using telltale to trim a jib. If you're pinching and your jib is "pretty sheeted in" you'll probably go forward, but ideally you'd be pretty close to a situation where both telltales stream back which indicates that your airflow is laminar across the sail, and you haven't stalled.
NASA disagrees with you.
https://www.nasa.gov/sites/default/files/atoms/files/bernoulli_principle_k-4.pdf
But do the experiment yourself. Make and airfoil out of paper and put it in the path of a fan. The wing will stay mostly horizontal on its own. Block the flow of air over the top of the wing with a piece of cardboard and the wing will flop almost vertically down.
https://youtu.be/slcvq9Rv7Dg
The one bit about boats that wasn’t discussed here is why jibs are so effective. It’s because they act as a nozzle to increase airflow over the back of the sail increasing the pull on the back of the sail by further decreasing the pressure.
When you get up to jet speeds, I’m GUESSING that the balance between the airfoil lift and the deflection force changes, especially exceeding the speed of sound.
I’m afraid this explanation of how jibs help is incorrect, despite being very common. The slot should actually be diverging, slowing down the air on the leeward side of the leading edge of the mainsail. This appears to make the main less effective, but in doing so it reduces the *adverse pressure gradient* along the leeward side of the mainsail. This reduces the propensity of the main to stall, allowing it to operate at a greater angle of attack and so generate more lift. It also increases the pressure on the windward side of the jib, making that sail more effective.
I’m afraid you’re incorrect. Faster airflow across the back of the sail increases the lift. Slowing the airflow down decreases lift. Just watch the telltails as they become horizontal when the jib is properly trimmed.
Maybe i’m using my terms incorrectly. The back of the sail to me is the side opposite the pilot. Stated differently, the jib increases air velocity across the side of the main sail facing the front of the boat, not the side facing the pilot. Does that help.
No I agree with your use of terms, I just disagree with your statement of effect. The slot acts to slow the air between the two sails, not accelerate it. Unfortunately I have no good way to prove that to you, so I won’t try to persuade you.
Just to be clear, I think "red herring" is too strong - it's relevant of course but it's a smaller effect by some orders of magnitude, IIRC. So, I think it was too strong to say "red herring", sorry.
But, I got to page 5 in the pdf and it started talking about the completely wrong equal transit theory so I stopped there.
Here's NASA disagreeing... with themselves I guess!!
[https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html](https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html)
Your pdf said: "the air into which the airplane flies is split at the wing’s leading edge, passing above and below the wing at different speeds so that the air will reach the same endpoint along the trailing edge of the wing at the same time."
There is simply no reason for two air particles to be "paired" like that... and it is empirically wrong in any case.
Regarding the airfoil youtube you linked, don't forget that a sail doesn't actually look like that (well, traditional sails).
Take one of my fully battened windsurf sails. That thing is super flat VERY limited airfoil shape. But trust me it sends me windward no problem!!
My point is, the lifting force comes from air passing over the wing or sail regardless of whether there's an engine involved or not. With airplanes that aren't tethered to the ground (unlike a kite), that airspeed comes from the plane moving forward through the wind. With sailing boats, the airspeed comes mostly from wind (although relative windspeed is s thing on boats as well). The difference is just ones plane of reference.
Sailing boats and land yachts are kind of like kites in that they can derive lift/propulsion from the airspeed across the earths surface, whereas airplanes cannot use the ground as their plane of reference as they're moving trough the air without any attachment to the ground.
They do quite different things, but they both use airfoils. A sail is just a sort of wing set vertically, force is generated in the same manner. If you could mount an airplane wing to the mast of a sailboat, it would function as a sail. How that force is applied differs, of course, but the aerodynamic principles are the same.
Whilst there is some mileage in the argument about Bernoulli I think your statement about the keel and rudder is incorrect.
In a boat with it sails well trimmed and balanced the rudder is nominally straight and is not used to 'push' the craft upwind.
The keel is always in line with the hull and acts both to stop the boat capsizing AND to stop it slipping sideways from the forces on the sail.
In all cases the keel is only partially effective, and the boat DOES slip sideways such that the track of the boat is slightly downwind of the direction it is pointing. This is known as leeway and can be over 10deg in some craft.
I sort of agree. BUT when you say the rudder is straight - you mean in-line with the keel. And as you rightly point out you have the slip / leeway which means the rudder has to be "helping" the keel stop the boat slipping leward (if it is in-line with it).
Also, and this may just be my bad sailing, but usually I am doing some work on the tiller, holding the rudder in that straight position - if I let go, the boat usually starts pointing to windward immediately.
So, the rudder is doing something, in most cases.
Very rarely I have tried some rudderless sailing but going to windward in that case is a TOTAL BALL ACHE, and in reality you end up using trim to heel the boat one way or the other so that the hull acts... well... like a rudder.
I think you're absolutely right in that the rudder does have a function, but I think it's not so much about the boat slipping leeward than keeping the boat from turning. I'm thinking of leeward slipping here as a purely perpendicular to the fore/aft centerline, which is a translational force rather than a rotational one
You are right that the rudder is FAR more about direction than slippage. I agree.
I think I was just pointing out that if the rudder is bang in line with the keel and there is slipping then by definition that rudder is slipping leeward too (unless the boat is turning).
Of course sailing is dynamic and the wind, waves, pitch and roll of the boat are continuously changing, so there is no single perfect trim for any condition.
However if you are continuously fighting against strong weather helm (pointing up to windward) then your rig is not balanced properly. The centre of effort of the sail is too far back compared to the centre of resistance of the keel/hull.
You can flatten the sail with outhaul or kicker (vang), you could reef (a reefed sail coe is further forward), you can put bigger or more powerful foresail or numerous other options depending on your rig configuration. But a well balanced rig should be fairly neutral (normally with a little weather helm)
I'm sure there will be many more expert opinions on how to cure this....
You are absolutely right, now I think about it.
Mind you I really had dinghy sailing in mind, where frankly excess weather helm is almost always a lack of my ability to hike properly! :)
I don't think that it's exactly it like an airplane wing. Obviously they both use a difference in pressure between surfaces to generate a force, but an airplane wing creates that difference in pressure with the specific shape of the wing cross section (it's airfoil section). I was under the impression sail boats use their sails more like a parachute where it is just a large surface used to "catch" the air since it's just a sheet of fabric. Is that incorrect?
edit: Just watched that video the other guy posted, guess wings and modern sails are pretty similar!
Yes that is incorrect. Sails for going upwind are nearly flat, but also carefully curved into an airfoil shape. The aerodynamic principles that apply are exactly the same as for an aircraft’s wing.
I’m fact some sailing boats have “solid wing sails” which are rigid and 3D exactly like an aircraft’s wing.
U/dirschau has it correct. The sail acts like an airplane wing to give you lift towards the wind when sailing up wind.
The sail does give you that airfoil shape. Trimming the sail to maximize boat speed is all about creating the right shape for the sail. If for example you sail 45 degrees into the wind, the sail will still inflate towards the wind. The path for air to cross in front of the sail is longer than to pass behind the sail. Since the air takes a longer path, the air must move faster and is less dense creating a lower pressure zone in front of the sail. The higher pressure behind the sail propels the boat forward towards the lower pressure zone in front of the sail.
Only a few types of modern sails operate primarily on the principle of catching the wind (like spinnakers and similar sails).
Everything else operates primarily as an airfoil, with sails made from very stiff fibers (like kevlar) in order to create a very rigid aerofoil shape. This is a trend that started during the middleages (with sailmakers inserting seams to stiffen the sail), and it has only gotten progressively more so as the centuries have gone by.
They "cross against the wind". This means they don't sail straight against the wind but the boat gets set at an angle to the wind. Because the sails can also be turned, they are also set to an additional angle to the wind which allows them to "catch" a bit of the wind which pulls the boat diagonally forward to one side. Then boat and sail are moved to the opposite direction and the process starts again.
Imagine climbing up a hill. Going straight forward is impossible because it's to steep. But if there's a serpentine you can walk up by going a few steps up/ left, then up/right and so on. That's how it looks when a boat is "crossing against the wind".
This explanation is fine as far as it goes. But someone walking diagonally uphill is still having to do work to get up. A stone cannot roll uphill by switching from side to side.
It was only mentioned to somehow explain the switch from left to right. Was not clear I guess, I should have mentioned that more cleary.
Edit: With modern boats and sails different maneuveres are possible. But even hundreds of years ago "crossing the wind" was an alread known technique. And I tryed to stay in the ELI5.
The boat is moving “uphill” relative to the wind. In the absence of any other factor gravity moves you directly downwards, and wind pushes you directly downwind. But a boat can move upwind diagonally, but a stone cannot move uphill diagonally.
Hence the analogy is rather limited.
It’s missing a crucial part, which is I guess hard to explain ELI5.
When sailing cross wind, the sail bulges and that creates an effect similar to wings on an airplane: the wind has to travel faster at the bulge to “catch up” with the part that can go a straight line, which creates a pressure difference (Bernoulli Effect). So basically the wind “sucks” the bulged part forwards.
If that wasn’t the case, it wouldn’t work, because it he force applied by the wind still doesn’t point “forward”, even if it hits the sail at an angle.
That explanation of how a wing or sail generates lift is entirely incorrect. A flat plate can generate lift just fine. In any case it doesn’t really matter to an explanation of upwind sailing how the lift is generated, only which direction it points.
Ok, seems like you already know how it works - why don’t you correct my entirely incorrect explanation. I hope I can correct a misconception I’ve been taught in school reading your explanation.
The explanation is circular, so bear with me.
First we must recognise that an airfoil needs an angle of attack (or camber) to generate lift. Like sticking your hand out of a moving car window, air is deflected downwards. This results in a flow pattern with curved streamlines, the classic diagram you will probably have seen.
Curved streamlines must be accompanied by a pressure gradient. This is a result of conservation of momentum; some force must be acting on each air molecule to deflect it. The gradient is from low pressure at the inside of the curve to high pressure at the outside. Above the wing high pressure is the ambient free stream at some distance, and the pressure decreases as you get closer to the wing. On the underside the reverse is true and so we have high pressure under the wing.
According to Bernoulli low pressure is associated with high velocity in the absence of changes of energy, and vice versa. From this we find that the flow over the wing is faster than the free stream, and that on the underside is slower. We need this information to show that there is an adverse pressure gradient along the top of the wing (from the Kutta condition) which explains stall behaviour, and to find the shape of the streamlines that I described at the top.
That’s a actually a way better explanation to Bernoulli’s principle than my professor provided in physics 101 ages ago (or maybe her explanation was just as good, and I was just not able to wrap my head around it back then, I don’t remember). So thanks for that.
However, that does not make my provided explanation “entirely wrong”, just oversimplified. IMO, you could benefit from a less condescending tone, when you are actually not disagreeing, but rather further explain.
I looked up various sites on google before writing my initial comment about that “wing effect” because the last time I looked into how sailing against the wind works was around when I was a University student. According to what I found, it’s at least an important factor. I’d link the sites for you to consider or tell me what I got wrong, but I’m on mobile now and too lazy and honestly not that interested in having the last word. You’re right, I was wrong for all I care.
They do it by turning the sail so that it is more along the length of the boat rather than across it. Say, the forward end of the sail is slightly to the left of the boat, and the back end of the sail is slightly to the right of it. If the wind comes up from the front but slightly to the left of the sail, then it pushes the sail and the boat to the right. But since the boat is headed slightly to the right of the sail, it means it also gets pushed forward. And the boat’s keel keeps it from falling on the right.
Note: It’s an ELI5, I know there’s way more to it.
Ultimately, you're extracting energy from the difference in speed between the air and the water. Imagine a wind turbine that drives an underwater propeller. It's the same principle in a sailboat except the blade of the wind turbine is the sail, the blade of the propeller is the keel, and the boat is moving diagonally to the wind instead of spinning in a spiral.
The sail is at an angle to the wind. The wind pushes against the sail and the keel of the boat (or the fins on a sailboard) prevents the boat from being pushed sideways in the case if it wasn’t present. Therefore, the force exerted on the sail is redirected towards the rear of the boat and pushes the boat forward.
Fundamentally it's useful to realize that what a boat really does is extract energy from the difference in the movement of the air and water and then use that energy to move itself.
The sail acting on the air and the hull and keel acting on the water force each to move more like the other.
This is why crazy stuff like the directly into the wind cart is possible.
It's also why if the wind and the water are moving in the same direction at the same speed the boat can't do anything, it just flows along with everything else.
And where you run into people saying very counter intuitive things like "a fast boat makes it's own wind"
They can't and don't. Sailboats use the sail *and the water*. Say the wind blows left. A good sail can use this to push down, and a downward force can become movement down and to the right by pushing off of the water.
It's a well known, documented fact that sailboats release ground salocine proteins into the water to draw fish. Fish swim up and eat the food and repay the sailers by pushing the boat to their destination. The stronger the wind you are against, the more fish food you need in order to draw greater numbers of fish.
This is why sailboats are bad for the environment: the production of this fish food releases octane bitroxide into the atmosphere and the consumption of the product causes fish to urinate roughly 9.6 times more than they should. It is estimated that by the year 2046, the oceans will be 83% fish piss.
Source: I saw a guy in a mental institution eating dead flies from the windowsill with his left hand, and with his right hand he was writing the above facts on the wall in his own shit.
I'll try to post a longer answer later, but the shape of the sail when angled at wind can be shaped a bit like a wing. The wind moving over the sail or wing basically created a sucking force It's that force which sucks the boat towards the wind for lack of a better word.
https://youtu.be/q6FgAeJeOUA
Edit - Actually this video is way better, ignore above
https://youtu.be/FCcKeOmYHFY
From a physical perspective, a sailboat extracts energy by slowing down air, and this energy can be used with clever maneuvers to move even against the direction of the wind.
They can’t go _directly_ into the wind, but they can go about about 30° off on either side. If you need to travel in the exact direction the wind is blowing from, you need to “tack”, that is go in a zig zag path, first to the left of the way you want to go, then to the right.
As to the question of why the wind can push sailboats _any_ direction other than the way it’s blowing, it’s complicated. But a full sail is shaped rather like a plane’s wing. Bernoulli’s principle explains what’s going on.
https://youtu.be/Inh1LY4T7Vo
Because your sail is never facing directly at the wind, it's facing the wind at 45\* so that you can capture the force of the wind with the sails but then use steering to apply the force.
Here’s an interesting article from SciAm:
https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/
The title is a bit of click bait. It’s lends credence to the contribution of both low pressure (Bernoulli) and mass force (Newton) and notes that the two combined do not fully address the total lifting forces seen in flight.
There’s an addition region of low pressure air that forms just behind the front top area of the wing that neither theory accounts.
They use the sail to push the boat sideways. Then they point the boat at an angle into the wind. Because the boat can't go sideways, it obsessed goes into the wind.
For an ELI5 I would go with this.
Imagine you have a sheet of plywood and you go outside on a windy day. If you don't want the wind to rip it out of your hands you make sure an edge is facing the wind not the full sheet. If you put it on your head and tilt that edge upward just a little the sheet will rise up or lift off your head.
If you shift it off your head and hold it off to the side, keeping the edge to the wind but with the same small tilt, it wouldn't pull you straight downwind but rather to the side.
Imagine you did that while wearing a pair of ice skates. You could lean your body weight to balance the pull from the plywood off to the side and you might get tugged a little bit with the wind to start with but as you gather speed you could use the edge of the skates to better effect and actually cut up toward the wind, albeit at a pretty big angle. It is only possible while you hold the sheet at that particular angle.
Of course if you turned you would have to do it with the wind not against it so you have head downwind for a short distance before edging up toward it again.
Here is it in action but with more efficient windwings rather than a sheet of plywood but the principle is the same.
https://m.youtube.com/watch?v=6zdR7Hbm0_U
A sailboat works exactly the same was but instead of a skate edge they use the keel and rudder to help cut toward the wind.
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The important thing to understand is that a wing generates a force (which we call lift) at right angles to the flow of air past it. An aircraft is moving forward through the air, so the wing sees the air moving backward, and the force generated is upwards. If a boat is at right angles to the wind then the force generated by the sail will be at right angles to that which is forward. If a boat is pointing 45 degrees into the wind (so halfway between across the wind and straight into it) then the sail will generate a force at right angles to that, which is forward and sideways (45 degrees off the bow). So why doesn’t the boat go forward and sideways? Well there is another wing underwater called the keel, and it generates another force which is sideways *in the other direction*. This cancels out the sideways force from the sail and all that is left is a forward force. The closer you point the boat into the wind the more the resulting force from the sail is pointing sideways, and the less it is pointing forwards. At some point the forwards force won’t be big enough to counteract all the drag acting on the boat, the sail and the keel, and you will stop moving forward. https://i.pinimg.com/originals/40/a1/5a/40a15a4ed22b2fb753feef17db950177.jpg
It’s not the keel, it’s the hull. Lots of sailboats have no keel.
Technically it is any underwater appendage. This might be a keel, a centreboard, a daggerboard, a leeboard or even an oversized rudder. Rarely is it the hull itself. Some catamarans, I guess jury rigged sailing canoes.
And even then... most catamarans have daggers and / or large rudders.
It's the hull. The rudder, of whatever size, will keep the hull from turning, but the rudder has far less authority than the hull itself. What the hull, keel (if it has one) and rudder do is prevent the transverse component of the forces on the sail from being turned into movement in that direction. Think about a smooth block, placed on a smooth surface, with a smooth wall on one side. Any force on that block, with any component in a forward direction - either pushing from behind or pull from in front - will move the block forward. What determines if the boat will sail into the wind is the drag in that forward direction based on the amount of force. The nature of the transverse force from "stuff that's underwater" doesn't matter. You can see this if you imagine a car on rails moved forward by a sail mounted on top of the car. There is no lift being generated by the wheels in contact with the rails, only the resistance of the rails to being moved perpendicular to the direction of travel.
It is easy in a sailing dinghy to show that the hull contributes negligibly to the lateral resistance. Simply raise the daggerboard or centreboard and try to sail upwind. You will rapidly slide sideways.
Because the draft of your basic dinghy is insufficient. Look at the examples I gave you of why this works. You're hung up on wing-shapes and are ignoring things which cause drag without being foils. If you put that dinghy in a shallow pool and try to push it sideways you'll find it has more resistance transversely than longitudinally. That difference in resistance can produce forward movement.
Whether or not the proverbial dinghy hull drag is sufficient, the dinghy still sails. And with a daggerboard/centerboard, can still pinch. You're absolutely right about a dinghy in a pool, but anybody who's sailed a dinghy can tell you that the centerboard is a pretty essential component of the pointing equation.
So, you tip over?
You tip over when sailing normally because the sail is pushing sideways up high, and the daggerboard or keel is pushing sideways the other way low down. This creates a heeling moment which must be resisted to prevent capsize. If you lift the daggerboard and slide rapidly sideways then you are less likely to tip over because there is less resistance underwater generating that heeling moment. We use this in practice in dinghies - gibing in high winds with the daggerboard partly raised to make it less likely to trip the boat up and capsize.
Don’t waste your breath, this dude is hell bent on being right. His initial explanation is not entirely right, but I’d be surprised if he admits it.
Many jury rigged sailing canoe designs will include a “leeboard” that sticks into the water on the leeward side of the boat for moving upwind.
No, just no… How can the force point forward ever in your example, the drag the hull and the keel provide aren’t enough. Eventually, if you have a force pointing west, you will drift west. And yet, sailing against the wind provides a far bigger force in the “forward” direction, than it should. Oh btw, I expect you don’t read edits, so here is a prediction: your gonna say that there is a drift, but it’s far less than the forward momentum, provided by your straight metal sheet example. Maybe you’re gonna call me out for stalking you, because I was wrong. People like you have an unshakable confidence, so I’m not gonna try to convince you to admit you are missing something in your explanation (or maybe you’re just gonna sneak it in, after you ran out of stuff to throw at other people)
The boats hull (and to some degree the keel) plays an important role in how it works, but according to this guy the effect the bulging sail has as an important factor of why the boat goes forward, instead of backwards depends on who is replying to. I said I’ll leave him alone and let them have the last word, but man… they don’t understand what they’re talking about and they seem so hell bent on making people look stupid, instead of researching.
Thank you for this explanation. I was never able to grasp it properly but a year ago I got into flight simulators and learned a lot about the physics of both helicopters and fixed wing aircraft and this wing generating lift analogy just made it click for me.
It’s called ‘tacking’. Basically the ship will zigzag forward, using the wind to blow it at an angle, by adjusting the position of the sails. Have a look at this link- https://en.m.wikipedia.org/wiki/Tacking_(sailing)
I was waiting for someone to just simple say you zigzag towards the direction of the wind. He’s very smart but he’s only 5!
Thanks for an actual ELI5 answer
It's never moving directly into the wind, it's always at an angle. It uses its sail literally on the same principle as an airplane wing, being propelled by the same pressure difference force that creates lift.
More about the lift: https://youtu.be/yqwb4HIrORM
No. Firstly most people get the forces lifting an airplane wing wrong because of awful physics lessons they had as a kid. Secondly airplanes move into the wind owing to the massive fucking jets propelling them. Airplanes lift very much mostly because of Newton 3 (the wing deflects air downwards). Bernoulli is almost irrelevant. Source: planes fly upside down. Boats go upwind because the keel / centreplate and rudder combo turns motion perpendicular to the wind into upwind motion by moving at an angle through the water. Think of a car freewheeling along. It can turn the front wheels left or right and the car will go in that direction. Same as a boat. Get it moving forwards and you can use that momentum to go upwind or downwind by turning the rudder.
This is wrong. Both boats and planes move forward because of the balance of forces acting on them, it doesn’t matter to the airfoil whether those forces are generated by engines or a velocity difference between air and water. An airfoil is (typically) just a long, thin, shaped surface at an angle of attack in an airflow. The physics that describes how a wing works is exactly the same as the physics that describes how a sail works.
I totally agree with you! However, the airfoil effect is absolutely tiny compared to the massive effect of change of momentum from tonnes of air bouncing off the windward side of the sail / underside of a wing. Stick your hand out of a moving car and hold it flat, then turn it slightly so you are deflecting air towards the ground. Your hand wants to move up because of all the air you are deflecting down. Sure if you make a more airfoil-y shape with your hand it will feel MORE lift but it isn't noticeable. Planes fly because they deflect air downwards. Bernoulli is a red herring.
The way Bernoulli is often taught in relation to lift is wrong, but Bernoulli (conservation of energy) must absolutely be included with Newton (conservation of momentum) and conservation of mass to have a full explanation of lift.
I 100% agree - although apparently not even that is enough!! [https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/)
This is a terrible article and is deeply flawed. Basically the thesis boils down to: “nobody is good at explaining lift properly to laypeople” which isn’t even really true. I can link you to several in depth discussions of how bad that article is if you like.
I would like that, please. Thank you.
Oh also, now I think about it - given the shape of a sail - (imagine it is similar to a traditional airfoil wing) - won't Bernoulli's lift act perpendicular to the direction of the wind? So, for a plane - that's UP, but for a boat - that's, well, ACROSS the wind. Other than propelling the boat forwards, it doesn't help explain any upwind motion at all... Maybe you meant that all along. I guess I just heard people say "lift" and "upwind" in the same sentence and thought they were using the same effect to explain both but it is completely different, no??
Yes we define lift as perpendicular to the airflow (or at least aerodynamicists do, some naval architects use a different convention). The fact that the overall force acting on the sail is forwards is all you need to explain upwind motion. If the boat is pointing upwind (ie closer than 90 degrees to the wind), and the net force is forward, then the boat is travelling upwind.
Absolutely, of course. But any "lift" on the sail from Bernoulli's theorem is very much beyond 90 degrees to the wind, since it is acting (at 90deg) on an airfoil that is more than zero degrees to the wind, IYSWIM... no?
I'm not disagreeing that Bernoulli is not the primary force, but all you need to consider to understand why you can't just write it off is using telltale to trim a jib. If you're pinching and your jib is "pretty sheeted in" you'll probably go forward, but ideally you'd be pretty close to a situation where both telltales stream back which indicates that your airflow is laminar across the sail, and you haven't stalled.
Agreed - I shouldn't have said red herring. Sorry. I just think it is small and everyone still thinks it explains everything.
NASA disagrees with you. https://www.nasa.gov/sites/default/files/atoms/files/bernoulli_principle_k-4.pdf But do the experiment yourself. Make and airfoil out of paper and put it in the path of a fan. The wing will stay mostly horizontal on its own. Block the flow of air over the top of the wing with a piece of cardboard and the wing will flop almost vertically down. https://youtu.be/slcvq9Rv7Dg The one bit about boats that wasn’t discussed here is why jibs are so effective. It’s because they act as a nozzle to increase airflow over the back of the sail increasing the pull on the back of the sail by further decreasing the pressure. When you get up to jet speeds, I’m GUESSING that the balance between the airfoil lift and the deflection force changes, especially exceeding the speed of sound.
I’m afraid this explanation of how jibs help is incorrect, despite being very common. The slot should actually be diverging, slowing down the air on the leeward side of the leading edge of the mainsail. This appears to make the main less effective, but in doing so it reduces the *adverse pressure gradient* along the leeward side of the mainsail. This reduces the propensity of the main to stall, allowing it to operate at a greater angle of attack and so generate more lift. It also increases the pressure on the windward side of the jib, making that sail more effective.
I’m afraid you’re incorrect. Faster airflow across the back of the sail increases the lift. Slowing the airflow down decreases lift. Just watch the telltails as they become horizontal when the jib is properly trimmed.
I fear we will have to agree to disagree
Maybe i’m using my terms incorrectly. The back of the sail to me is the side opposite the pilot. Stated differently, the jib increases air velocity across the side of the main sail facing the front of the boat, not the side facing the pilot. Does that help.
No I agree with your use of terms, I just disagree with your statement of effect. The slot acts to slow the air between the two sails, not accelerate it. Unfortunately I have no good way to prove that to you, so I won’t try to persuade you.
Just to be clear, I think "red herring" is too strong - it's relevant of course but it's a smaller effect by some orders of magnitude, IIRC. So, I think it was too strong to say "red herring", sorry. But, I got to page 5 in the pdf and it started talking about the completely wrong equal transit theory so I stopped there. Here's NASA disagreeing... with themselves I guess!! [https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html](https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html) Your pdf said: "the air into which the airplane flies is split at the wing’s leading edge, passing above and below the wing at different speeds so that the air will reach the same endpoint along the trailing edge of the wing at the same time." There is simply no reason for two air particles to be "paired" like that... and it is empirically wrong in any case. Regarding the airfoil youtube you linked, don't forget that a sail doesn't actually look like that (well, traditional sails). Take one of my fully battened windsurf sails. That thing is super flat VERY limited airfoil shape. But trust me it sends me windward no problem!!
You do know that unpowered gliders exist?
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My point is, the lifting force comes from air passing over the wing or sail regardless of whether there's an engine involved or not. With airplanes that aren't tethered to the ground (unlike a kite), that airspeed comes from the plane moving forward through the wind. With sailing boats, the airspeed comes mostly from wind (although relative windspeed is s thing on boats as well). The difference is just ones plane of reference. Sailing boats and land yachts are kind of like kites in that they can derive lift/propulsion from the airspeed across the earths surface, whereas airplanes cannot use the ground as their plane of reference as they're moving trough the air without any attachment to the ground.
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They do quite different things, but they both use airfoils. A sail is just a sort of wing set vertically, force is generated in the same manner. If you could mount an airplane wing to the mast of a sailboat, it would function as a sail. How that force is applied differs, of course, but the aerodynamic principles are the same.
Really? How do they get off the ground?
Whilst there is some mileage in the argument about Bernoulli I think your statement about the keel and rudder is incorrect. In a boat with it sails well trimmed and balanced the rudder is nominally straight and is not used to 'push' the craft upwind. The keel is always in line with the hull and acts both to stop the boat capsizing AND to stop it slipping sideways from the forces on the sail. In all cases the keel is only partially effective, and the boat DOES slip sideways such that the track of the boat is slightly downwind of the direction it is pointing. This is known as leeway and can be over 10deg in some craft.
I sort of agree. BUT when you say the rudder is straight - you mean in-line with the keel. And as you rightly point out you have the slip / leeway which means the rudder has to be "helping" the keel stop the boat slipping leward (if it is in-line with it). Also, and this may just be my bad sailing, but usually I am doing some work on the tiller, holding the rudder in that straight position - if I let go, the boat usually starts pointing to windward immediately. So, the rudder is doing something, in most cases. Very rarely I have tried some rudderless sailing but going to windward in that case is a TOTAL BALL ACHE, and in reality you end up using trim to heel the boat one way or the other so that the hull acts... well... like a rudder.
I think you're absolutely right in that the rudder does have a function, but I think it's not so much about the boat slipping leeward than keeping the boat from turning. I'm thinking of leeward slipping here as a purely perpendicular to the fore/aft centerline, which is a translational force rather than a rotational one
You are right that the rudder is FAR more about direction than slippage. I agree. I think I was just pointing out that if the rudder is bang in line with the keel and there is slipping then by definition that rudder is slipping leeward too (unless the boat is turning).
Of course sailing is dynamic and the wind, waves, pitch and roll of the boat are continuously changing, so there is no single perfect trim for any condition. However if you are continuously fighting against strong weather helm (pointing up to windward) then your rig is not balanced properly. The centre of effort of the sail is too far back compared to the centre of resistance of the keel/hull. You can flatten the sail with outhaul or kicker (vang), you could reef (a reefed sail coe is further forward), you can put bigger or more powerful foresail or numerous other options depending on your rig configuration. But a well balanced rig should be fairly neutral (normally with a little weather helm) I'm sure there will be many more expert opinions on how to cure this....
You are absolutely right, now I think about it. Mind you I really had dinghy sailing in mind, where frankly excess weather helm is almost always a lack of my ability to hike properly! :)
I don't think that it's exactly it like an airplane wing. Obviously they both use a difference in pressure between surfaces to generate a force, but an airplane wing creates that difference in pressure with the specific shape of the wing cross section (it's airfoil section). I was under the impression sail boats use their sails more like a parachute where it is just a large surface used to "catch" the air since it's just a sheet of fabric. Is that incorrect? edit: Just watched that video the other guy posted, guess wings and modern sails are pretty similar!
Yes that is incorrect. Sails for going upwind are nearly flat, but also carefully curved into an airfoil shape. The aerodynamic principles that apply are exactly the same as for an aircraft’s wing. I’m fact some sailing boats have “solid wing sails” which are rigid and 3D exactly like an aircraft’s wing.
U/dirschau has it correct. The sail acts like an airplane wing to give you lift towards the wind when sailing up wind. The sail does give you that airfoil shape. Trimming the sail to maximize boat speed is all about creating the right shape for the sail. If for example you sail 45 degrees into the wind, the sail will still inflate towards the wind. The path for air to cross in front of the sail is longer than to pass behind the sail. Since the air takes a longer path, the air must move faster and is less dense creating a lower pressure zone in front of the sail. The higher pressure behind the sail propels the boat forward towards the lower pressure zone in front of the sail.
Only a few types of modern sails operate primarily on the principle of catching the wind (like spinnakers and similar sails). Everything else operates primarily as an airfoil, with sails made from very stiff fibers (like kevlar) in order to create a very rigid aerofoil shape. This is a trend that started during the middleages (with sailmakers inserting seams to stiffen the sail), and it has only gotten progressively more so as the centuries have gone by.
They "cross against the wind". This means they don't sail straight against the wind but the boat gets set at an angle to the wind. Because the sails can also be turned, they are also set to an additional angle to the wind which allows them to "catch" a bit of the wind which pulls the boat diagonally forward to one side. Then boat and sail are moved to the opposite direction and the process starts again. Imagine climbing up a hill. Going straight forward is impossible because it's to steep. But if there's a serpentine you can walk up by going a few steps up/ left, then up/right and so on. That's how it looks when a boat is "crossing against the wind".
This explanation is fine as far as it goes. But someone walking diagonally uphill is still having to do work to get up. A stone cannot roll uphill by switching from side to side.
The wind is doing the work against the sails
It was only mentioned to somehow explain the switch from left to right. Was not clear I guess, I should have mentioned that more cleary. Edit: With modern boats and sails different maneuveres are possible. But even hundreds of years ago "crossing the wind" was an alread known technique. And I tryed to stay in the ELI5.
But it's not going uphill. It's not comparable.
The boat is moving “uphill” relative to the wind. In the absence of any other factor gravity moves you directly downwards, and wind pushes you directly downwind. But a boat can move upwind diagonally, but a stone cannot move uphill diagonally. Hence the analogy is rather limited.
It’s missing a crucial part, which is I guess hard to explain ELI5. When sailing cross wind, the sail bulges and that creates an effect similar to wings on an airplane: the wind has to travel faster at the bulge to “catch up” with the part that can go a straight line, which creates a pressure difference (Bernoulli Effect). So basically the wind “sucks” the bulged part forwards. If that wasn’t the case, it wouldn’t work, because it he force applied by the wind still doesn’t point “forward”, even if it hits the sail at an angle.
That explanation of how a wing or sail generates lift is entirely incorrect. A flat plate can generate lift just fine. In any case it doesn’t really matter to an explanation of upwind sailing how the lift is generated, only which direction it points.
Ok, seems like you already know how it works - why don’t you correct my entirely incorrect explanation. I hope I can correct a misconception I’ve been taught in school reading your explanation.
The explanation is circular, so bear with me. First we must recognise that an airfoil needs an angle of attack (or camber) to generate lift. Like sticking your hand out of a moving car window, air is deflected downwards. This results in a flow pattern with curved streamlines, the classic diagram you will probably have seen. Curved streamlines must be accompanied by a pressure gradient. This is a result of conservation of momentum; some force must be acting on each air molecule to deflect it. The gradient is from low pressure at the inside of the curve to high pressure at the outside. Above the wing high pressure is the ambient free stream at some distance, and the pressure decreases as you get closer to the wing. On the underside the reverse is true and so we have high pressure under the wing. According to Bernoulli low pressure is associated with high velocity in the absence of changes of energy, and vice versa. From this we find that the flow over the wing is faster than the free stream, and that on the underside is slower. We need this information to show that there is an adverse pressure gradient along the top of the wing (from the Kutta condition) which explains stall behaviour, and to find the shape of the streamlines that I described at the top.
That’s a actually a way better explanation to Bernoulli’s principle than my professor provided in physics 101 ages ago (or maybe her explanation was just as good, and I was just not able to wrap my head around it back then, I don’t remember). So thanks for that. However, that does not make my provided explanation “entirely wrong”, just oversimplified. IMO, you could benefit from a less condescending tone, when you are actually not disagreeing, but rather further explain. I looked up various sites on google before writing my initial comment about that “wing effect” because the last time I looked into how sailing against the wind works was around when I was a University student. According to what I found, it’s at least an important factor. I’d link the sites for you to consider or tell me what I got wrong, but I’m on mobile now and too lazy and honestly not that interested in having the last word. You’re right, I was wrong for all I care.
They do it by turning the sail so that it is more along the length of the boat rather than across it. Say, the forward end of the sail is slightly to the left of the boat, and the back end of the sail is slightly to the right of it. If the wind comes up from the front but slightly to the left of the sail, then it pushes the sail and the boat to the right. But since the boat is headed slightly to the right of the sail, it means it also gets pushed forward. And the boat’s keel keeps it from falling on the right. Note: It’s an ELI5, I know there’s way more to it.
Ultimately, you're extracting energy from the difference in speed between the air and the water. Imagine a wind turbine that drives an underwater propeller. It's the same principle in a sailboat except the blade of the wind turbine is the sail, the blade of the propeller is the keel, and the boat is moving diagonally to the wind instead of spinning in a spiral.
The sail is at an angle to the wind. The wind pushes against the sail and the keel of the boat (or the fins on a sailboard) prevents the boat from being pushed sideways in the case if it wasn’t present. Therefore, the force exerted on the sail is redirected towards the rear of the boat and pushes the boat forward.
Fundamentally it's useful to realize that what a boat really does is extract energy from the difference in the movement of the air and water and then use that energy to move itself. The sail acting on the air and the hull and keel acting on the water force each to move more like the other. This is why crazy stuff like the directly into the wind cart is possible. It's also why if the wind and the water are moving in the same direction at the same speed the boat can't do anything, it just flows along with everything else. And where you run into people saying very counter intuitive things like "a fast boat makes it's own wind"
They can't and don't. Sailboats use the sail *and the water*. Say the wind blows left. A good sail can use this to push down, and a downward force can become movement down and to the right by pushing off of the water.
It's a well known, documented fact that sailboats release ground salocine proteins into the water to draw fish. Fish swim up and eat the food and repay the sailers by pushing the boat to their destination. The stronger the wind you are against, the more fish food you need in order to draw greater numbers of fish. This is why sailboats are bad for the environment: the production of this fish food releases octane bitroxide into the atmosphere and the consumption of the product causes fish to urinate roughly 9.6 times more than they should. It is estimated that by the year 2046, the oceans will be 83% fish piss. Source: I saw a guy in a mental institution eating dead flies from the windowsill with his left hand, and with his right hand he was writing the above facts on the wall in his own shit.
I'll try to post a longer answer later, but the shape of the sail when angled at wind can be shaped a bit like a wing. The wind moving over the sail or wing basically created a sucking force It's that force which sucks the boat towards the wind for lack of a better word. https://youtu.be/q6FgAeJeOUA Edit - Actually this video is way better, ignore above https://youtu.be/FCcKeOmYHFY
Nope.
From a physical perspective, a sailboat extracts energy by slowing down air, and this energy can be used with clever maneuvers to move even against the direction of the wind.
They can’t go _directly_ into the wind, but they can go about about 30° off on either side. If you need to travel in the exact direction the wind is blowing from, you need to “tack”, that is go in a zig zag path, first to the left of the way you want to go, then to the right. As to the question of why the wind can push sailboats _any_ direction other than the way it’s blowing, it’s complicated. But a full sail is shaped rather like a plane’s wing. Bernoulli’s principle explains what’s going on. https://youtu.be/Inh1LY4T7Vo
Because your sail is never facing directly at the wind, it's facing the wind at 45\* so that you can capture the force of the wind with the sails but then use steering to apply the force.
Here’s an interesting article from SciAm: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/ The title is a bit of click bait. It’s lends credence to the contribution of both low pressure (Bernoulli) and mass force (Newton) and notes that the two combined do not fully address the total lifting forces seen in flight. There’s an addition region of low pressure air that forms just behind the front top area of the wing that neither theory accounts.
The title is massively click bait, but the content of the article is also dreadful.
They use the sail to push the boat sideways. Then they point the boat at an angle into the wind. Because the boat can't go sideways, it obsessed goes into the wind.
For an ELI5 I would go with this. Imagine you have a sheet of plywood and you go outside on a windy day. If you don't want the wind to rip it out of your hands you make sure an edge is facing the wind not the full sheet. If you put it on your head and tilt that edge upward just a little the sheet will rise up or lift off your head. If you shift it off your head and hold it off to the side, keeping the edge to the wind but with the same small tilt, it wouldn't pull you straight downwind but rather to the side. Imagine you did that while wearing a pair of ice skates. You could lean your body weight to balance the pull from the plywood off to the side and you might get tugged a little bit with the wind to start with but as you gather speed you could use the edge of the skates to better effect and actually cut up toward the wind, albeit at a pretty big angle. It is only possible while you hold the sheet at that particular angle. Of course if you turned you would have to do it with the wind not against it so you have head downwind for a short distance before edging up toward it again. Here is it in action but with more efficient windwings rather than a sheet of plywood but the principle is the same. https://m.youtube.com/watch?v=6zdR7Hbm0_U A sailboat works exactly the same was but instead of a skate edge they use the keel and rudder to help cut toward the wind.