3090: Sail Physics

Sail Physics

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Title text: Turning in other directions can be accomplished by using a magnetized centerboard and ocean currents, since a current flowing through a magnetic field induces a Laplace force.

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Explanation

This is one of 52 incomplete explanations: This page was created by a wind-blown electron. Don't remove this notice too soon. If you can fix this issue, edit the page!

This comic starts off looking like a straightforward explanation of how sailboats can travel upwind — a topic that continues to spark debate and refinement in physics circles. However, it quickly veers into a completely fictional and incorrect theory involving triboelectric charging and the Lorentz force, rather than referencing real mechanisms like aerodynamic lift.

This humor works at another level — most interaction of physical things at macro scale (humans and boat sized objects) are electromagnetic in nature. So one unaware of sailing mechanics may start to explain the situation with electromagnetism, and could come to this line of thinking, but it is wrong. If we are to consider this, we find that either no force is appearing in the direction shown, or very little.

Triboelectric effect is acquisition of "static" electric charge by rubbing between two objects, which in turn depends on effective interaction surface area. The charge imparted to light items, such as toy balloons or scraps of tissue, can be sufficient to overcome gravitational force.

For a sailboat, the charge acquired would be very small. There is more charge generated due to the running of the boat through water. In the comic, it is assumed that the push of the wind moves the sail downwind. The charged sail moving through the Earth's magnetic field produces a force (Lorentz force) on the sail, perpendicular to the direction of motion and to the magnetic field.

Earth's magnetic field is effectively that of dipole bar magnet, which is roughly aligned with our planetary rotational axis . Near the equator the Earth's magnetic field runs roughly parallel to the surface. Nearer the poles, the magnetic field is inclined relative to the surface. Earth's magnetic field is typically 25 to 65 micro Tesla's on the surface.

The net Lorentz force on the boat would be F = qvB. Near the equator the velocity of boat and magnetic field lines are in the same plane (as viewed in the above sketches, in a top-down manner), then the force would be perpendicular to the plane. That is, along the direction of gravity for the boat, which will not cause any motion in the plane. Close to the poles, the magnetic field is nearly perpendicular to Earth's surface, the Lorentz force would be perpendicular to the boats motion, along the Earth's surface.

The magnitude of the Lorentz force is very small. If we consider an absurdly large net charge of 1 coulomb across the boat. If we consider field strength of order 0.00001 Tesla (order of magnitude of Earth's field), and take wind speeds of 100 km/s (360,000 km/hour - ludicrously high) then the net force on ship would be 1 N. For a model boat of mass 10 kg would cause a net acceleration of 0.1 m/s^2. Which is not tiny, but due to friction across the boat, would be cancelled out (by 1 or 2 orders of magnitude). This is considering the best possible case. For more reasonable charge values, this force would be about a trillion times weaker, and hence this will not cause any practical motion.^{[citation needed]}

One coulomb is an extremely large amount of charge, at which point, the charge would try to accumulate on sharp objects, and due to this high charge density, either charge will leave the boat through coronal discharge, or the sharp objects would start to break from the boat (this is due to high self electrostatic energy, which is roughly proportional to square of charge density divided by radius of curvature of object, if the bonding between object is not strong enough, then this electrostatic energy can overcome this bonding energy), assuming that this charge does not cause the ship to break or the charge to leak out to water body, as the charge would like to spread to minimise the electrostatic energy.

If the described effect were actually significant, it wouldn't always make boats turn upwind. A positively charged boat would turn counter-clockwise on the northern hemisphere, where the magnetic field points down, and clockwise on the southern hemisphere, where the magnetic field points up. Thus it would turn upwind or downwind depending on which side of the boat faces the wind. A negatively charged boat would turn in the opposite direction. To beat to windward it would be necessary to switch between two sails made of different materials – one sail to accumulate a positive charge, and another to accumulate a negative charge.

Transcript

This is one of 27 incomplete transcripts: Don't remove this notice too soon. If you can fix this issue, edit the page!

[Panel 1]

How sailboats use physics to sail upwind:

[A schematic boat with a sail is shown (top-down view). Winds shown with directional arrows, pointing in the direction of the sail (going towards 4:30 on a clock face).]

[Panel 2]

1. Wind passing over the sail strips away electrons via the triboelectric effect.

[Schematic similar to panel 1, but with charged ions shown across both sides of the sail, representing the triboelectric effect.]

[Panel 3]

2. The positively charged boat is blown downwind; its movement in Earth's magnetic field produces a Lorentz force.

[The same schematic, except a force vector is shown in the direction of the wind, and a perpendicular force vector (along 1:30 on a clock face) is shown with a dashed arrow.]

[Panel 4]

3. The Lorentz force acts perpendicular to the direction of motion, redirecting the boat upwind.

[A net force vector is shown perpendicular to the downstream vector.]

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Discussion

After the last step, the sailors would then need to ground the boat to avoid being pushed in a circle, wouldn't they? Sophon (talk) 20:47, 16 May 2025 (UTC)

But if you ground the boat you won't be able to go anywhere. At least until the tide comes in a bit further.162.158.216.191 15:00, 19 May 2025 (UTC)

Note that for eastward wind, the boat will be propelled upwards, while the opposite is true for westward winds. This provides a basis for the functioning of airships and planes (Helicopters are more complicated, and additionally rely on their own magnetic fields) 162.158.217.45 21:21, 16 May 2025 (UTC)

Hence why you should always touch an earthing rod before approaching a helicopter, to avoid the magnetism pulling you into their rotors. Kev (talk) 03:11, 17 May 2025 (UTC)

Is this actually wrong? Wouldn't it still be a force on a sailboat, even if it's not the strongest? Smurfton (talk) 22:20, 16 May 2025 (UTC)

I added some explaination on direction and magnitude of the lorentz force, maybe that will help - sga 172.68.234.227 (talk) 22:33, 16 May 2025 (UTC) (please sign your comments with ~~~~)

Yeah, is it more or less effective than the kedging cannon? StapleFreeBatteries (talk) 23:47, 18 May 2025 (UTC)

The explanation states that of the four forces, only the electromagnetic force operates at the macro level. This is incorrect, as gravity is also directly observable by humans. There should also probably be a link to https://en.wikipedia.org/wiki/Airfoil to provide an explanation for how sails actually allow a boat to sail upwind. I recommend removing the remark about the poles potentially flipping in the future, as this is irrelevant. 172.68.55.124 23:52, 16 May 2025 (UTC)

What i meant was, for 2 objects at scales of humans =, maybe did not prase it well. In this case, it is the wind and the sail. Wind does not have a "mass" (the atoms most certainly do, but) we essentially have a pressure force, or momentum of wind, where instead of using the energy of atoms (and hence the mass) as given by kinetic theory is not used (that is random (as given by boltzman maxwell statistics)) and uniform (in the sense that for any direction, number of particles going against and towards is equal) and what we have is just pressure applied by a effective "group velocity" of the wind atoms. The gravity interaction between wind and boat, or the local waves and boat is negligible, and planetary gravity is not considered because that is not relavant for in plane motion. the pole fillping was added just for future proofing the article. I am sorry for the puns. I have rewwritten some parts, and reduced the part about pole flipping, and also added the average case scenario for the force, hope it is better now. - sga 172.70.143.75 (talk) 02:37+, 17 May 2025 (please sign your comments with ~~~~)

That is one huge rambling paragraph, if it's (mostly) yours. I'm no stranger to writing huge rambling paragraphs, myself, but I gave up only a little way in on trying to make it read better. Grammatically, prosaically and with relevence.

May I suggest that each 'frame' is treated to its own (shorter) paragraph, explaining what effect it tries to convey, what logic it individually tries to follow, but where it fails and what actual forces dominate a true example. (e.g. the hull-shape, including keel, helping convert roughly lateral sideways forces into forward ones against the water; those lateral ones having already been a conversion of largely head-on winds in the first place, thus two "almost up to 90 degree" redirections of force allow very nearly a 180-degree reversal of wind-blown movement. Feel free to discuss the comparisons and differences between 'flappy sail', though blown taught by the air, and an 'upright aircraft wing' solid design. ...See, told you I could ramble, but someone can surely do better at segmenting and summarising the basics of this.) 172.71.178.32 08:32, 17 May 2025 (UTC)

This is super embarrassing to admit, but I came here to verify whether this was a serious thing or not. I had no idea how a sailboat sails against the wind. Catgofire (talk) 23:58, 16 May 2025 (UTC)

You aren't alone - I think I was an adult before I understood tacking in the sailboat sense of the word. 162.158.174.127 02:45, 17 May 2025 (UTC)

I'm wanting to add in some wisdom about "science-y" explanations that appear to be sensible but are completely wrong, segueing into how generative language models appear to be far more reliable than they are. However this margin is too narrow Kev (talk) 03:09, 17 May 2025 (UTC)

To answer the question: sailboats move by using the Coriolis effect. That's why sailboats can't sail directly in the direction of Earth's spin, and why ships often get becalmed at the equator. Modern vessels create their own Coriolis effect by using steam powered turbines as gyroscopes. RegularSizedGuy (talk) 16:52, 18 May 2025 (UTC)

I've been really annoyed with ExplainXKCD in the last few months ever since the initial posting has always been LLM generated. It requires more brain power to make sense of AI slop and edit it, than to contribute to a blank page. 162.158.162.103 162.158.162.103 (talk) 15:44, 17 May 2025 (please sign your comments with ~~~~)

I don't think that LLM has been used for the most troublesome bits. LLMs can 'hallucinate', but tend (unless specifically asked) to make a lot more grammatical sense if you don't look too much further. 162.158.33.240 18:29, 17 May 2025 (UTC)

Any chance we can add an explanation of how it *actually* works? 162.158.216.174 10:03, 17 May 2025 (UTC)

Through judicious angling of sail, wind (from any direction other than fully head-on) is deflected(/uses 'wing-effect') to create a force, trying to push the boat, that might be mostly sideways but also a bit forward. Because of the shape of the hull, any sideways force is resisted by the water, reinforcing the remaining forward component which the hull is far more ready to take advantage of. Enough sail (and enough stability to resist rolling) gives a large amount of movement towards, but not exactly towards, the wind. 172.69.224.72 10:41, 17 May 2025 (UTC)

The joke is that the most commonly used explanation for why flow over a foil generates lift - particles going one way have a longer way to travel than the other, which generates a difference in speed and therefore a pressure differential - is wrong. 172.69.109.91 (talk) 10:36, 17 May 2025 (please sign your comments with ~~~~)

What is wrong with the explanation which you say is wrong? What is the more correct explanation? 172.71.150.33 20:28, 17 May 2025 (UTC)

That explanation usually implies/assumes that the portion of air going above the wing and the portion going below have to arrive at the other edge of the wing at the same time. So if a particle that happened to go underneath took exactly x seconds, then an identical particle that happened to go over would also take exactly x seconds. This turns out not to be true. It is true that (most wing-generated) lift comes from a pressure differential, and it is generally true enough most of the time that most (not all) of that pressure differential is tied to an airstream speed differential. (I say "tied to" because I am not in the mood to argue about how the causality runs.) JimJJewett (talk) 06:41, 18 May 2025 (UTC)

The picture seems to show an axis of rotation (the mast) for the sail being on the end of the sail. Is that correct for a certain class of sailing vessel?~~ 162.158.146.128 (talk) 15:57, 17 May 2025 (please sign your comments with ~~~~)

Yes. Though offhand, I can't think of a good sailboat with only 1 sail where it was true beyond a first approximation. JimJJewett (talk) 06:41, 18 May 2025 (UTC)

There's the catboat class, and a smaller dinghy may not have (or always use) a foresail. Obviously it doesn't look like a square-rigger 'sail hanger' of most larger ships, but a mast with a single outward stretch of sail fabric is a very good way to demonstrate how any given sail sits in the wind, without complicating matters by showing a combination of gaff-rigged, bermuda-style, spritzers, etc. 162.158.74.14 17:50, 18 May 2025 (UTC)

Currently the explanation says "most interaction of physical things at macro scale (humans and boat sized objects) are electromagnetic in nature" I have certainly read that, and have seen examples of electromagnetic interactions between atoms. However, I also encounter explanations that describe interactions in terms of Pauli exclusion principle (see for instance Contact force). This makes me question the view presented in the first sentence. Since my physics is a bit rusty I haven't tried to fix it, but I think it may need clarification. 172.71.150.33 20:28, 17 May 2025 (UTC)

I split up the example calculating Lorentz force on a boat. It still needs some work (I was just untangling it so I could see what it said). The paragraph about one coulomb of charge I left as is - it needs untangling, so be bold. In the example - somebody should recheck the math (I just copied what there, but in changing units to be more familiar, like km/h, I might have introduced errors). I also changed the field strength to the right order of magnitude for Earth's surface, and multiplied the wind speed by 10 to compensate. As best I can figure the numbers for the example may have been chosen to get a force of 1 Newton. (I can't see any other reason for the ludicrous wind speed of thousands of km/h.) Might be better example to use a reasonable wind speed (e.g. dial it back to hurricane force) and a reasonable charge (something like what you could accumulate with an automobile, or when you zap yourself after getting out of a car seat) wind up with an even more negligible force. Then we could dispense with the paragraph explaining why 1 Coulomb is silly. 172.71.151.93 22:15, 17 May 2025 (UTC)

If anybody wants to restore parts of it or play with it, the version with the calculation of Lorentz forces is here [1] I thought it somewhat interesting just as physics problem to show what the effect was.

I got curious about how much charge is involved when one zaps oneself on a car seat. A lighting bolt is a few coulombs. 172.71.142.188 23:40, 17 May 2025 (UTC)

Extreme apologies to an(other) IP editor who may have made several possibly great improvements to the article. I had so many problems with what was already there that I did a massive rewrite and set things up so differently that I'm not sure that (at a very long glance, but maybe not as long as it diserves) much of that effort is really worth feeding back in. Or even capable of being. Obviously, it's up to the rest of the you (including the person/people I overrode) to make your own judgement about that. I will also go back in to carefully check what I may have desecrated. 162.158.74.68 22:37, 17 May 2025 (UTC)

No problem. I remade a few tweaks. The calculation is linked above is anybody wants to restore part of it. 172.71.142.188 23:40, 17 May 2025 (UTC)

Talking of multiple sails (as the explanation does, at least right now), I'm reminded of a children's SF-based book I read when I was... well, a child. It was set on a (mostly) waterworld, as I recall, that had multiple suns (in orbit around it..? ...already we can be sure it was probably not the hardest of hard-SF settings, of course). The boats/skiffs/whatever had multiple sails to move around. Multiple solar sails. Depending upon which ones were deployed (kite-surfing-like), they'd get pushed (and you'd get pulled) by the sun that they were for. So if you wanted to go <- thataway, deploy one particular sail, or thataway -> deploy another instead. As if solar sails even work that way. (Or even would work that way as effectively as a kite might in the... I presume there was an atmosphere... if there wasn't, then that'd explain the need for no-air sail-like solution, but raise significant other questions ...though clearly could not raise kites.) I may have misremembered some of the details, even perhaps some of the 'wrongness', but... I definitely remember I had to suspend quite a lot of disbelief (don't ask me from which sun it is hung!) when I read that. 172.68.205.187 23:25, 17 May 2025 (UTC)

I was just looking at some things about solar sails and similar. This comic got me curious about: Whether one could use a solar sail to sail "upwind"? -- you can - thanks to gravity and orbital mechanics. e.g., Sail in direction of your orbit - shifts apogee out, perigee in. Whether you could build a {{w|magnetic sail))? -- yes - doesn't work quite like the one in the comic (sun provides wind of particles, sail is magnet to redirect them). There is also a version of solar sail using electric fields to redirect charged particles. One question I haven't found anything about is do solar sails (conventional ones, not electric) accumulate charge, and what effects that might have. I just mention here in case anybody thinks way makes sense in comic explanation. 172.71.142.188 23:40, 17 May 2025 (UTC)

Well, if you send charge along a long conductor (which might be what you do if you're sending/receiving the sail-charge through the thing that the sail is attached to the payload with), and pass through any magnetic fields (planetary, solar... galactic?) then you're into the territory of the Electrodynamic tether as also useful for propulsion. 172.68.205.187 00:24, 18 May 2025 (UTC)

Related to the question of how sails might work (against the wind), and the relationship to wing effects, I'm reminded of how a comedy radio sitcom dealt with the wings thing. In case it's not quickly obvious (and with some non-obvious info added), characters are Arthur ('simple' but questioning Air Steward), Carolyn (his mother, bossy owner of the airplane and chief Air Steward/everything else that's not actually flying), Martin (chief pilot/captain, knowledgable but inexperienced) and Douglas (second in command, but senior in years and experience and 'street smart'/air-smart to the point of (usually well-deserved) smugness).

...just thought anyone who hadn't heard this (or had, but liked the comedy involved) might like to read it. 172.68.205.187 00:24, 18 May 2025 (UTC)

Sounds like comic 803. StapleFreeBatteries (talk) 23:21, 18 May 2025 (UTC)
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