Talk:Forces on sails

Link to sandbox containing experimental or legacy material

A sail is not "like a wing"

I have prepared the linked article which analyses the use of the "like a wing" analogy from the perspective of confusion and necessity to present an argument for removing all references to aviation and aeronautical terms and concepts from the Article "Forces on sails".

I am brutally aware that I am emulating the boy in the "The Emperor's New Clothes", and of the risk of appearing to claim Copernican wisdom when challenging this universally accepted truth which is supported by the eminences of Fossati, Flay and Marchaj. Gpsanimator (talk) 05:34, 14 February 2021 (UTC)

Gpsanimator, I read your article with interest, since this is a topic that is difficult to follow, as you suggest. What I see in your suggestion are two choices:

1. Recognize that including every logical step of derivation works only for a more knowledgeable audience, per WP:AUDIENCE.
2. Omit the steps that explain the translation from lift and drag into F_T and just have it appear without explanation. Simple, yes, complete, no.

We must remember that we are to avoid original research and imparting our personal point of view. Instead, we must paraphrase and cite Wikipedia:Reliable sources, which the article currently does. If you have some more elegantly and simply presented reliable sources that explain the matter more clearly, then let's look them over and emulate them. Cheers, HopsonRoad (talk) 13:55, 14 February 2021 (UTC)

I have to echo what User:HopsonRoad has said. We cannot accept original research and have to go with what reliable sources say, regardless of our own opinions. Now if you can find a reliable source that agrees with your point of view, then we can enter that in the article and explain that sources disagree, giving both perspectives. - Ahunt (talk) 14:47, 14 February 2021 (UTC)

Your objections are based on the Wikipedia requirements to avoid original research and imparting our personal point of view. I know this is not a forum, but I would like to offer linked explanationof why my proposal does not transgress these principles.

If we choose to omit all references to Lift and Drag, then there's no need to resolve the complicated problems associated with your two choices:

1. Recognize that including every logical step of derivation works only for a more knowledgeable audience, per WP:AUDIENCE.
2. Omit the steps that explain the translation from lift and drag into F_T and just have it appear without explanation. Simple, yes, complete, no.

Gpsanimator (talk) — Preceding undated comment added 20:28, 14 February 2021 (UTC)

The main issue there is that this interpretation is yours. To go that route and at least provide an alternate explanation to the existing reliable sources, you need to show a new reliable source that explains this, the way you have done. Basically your own Google Doc, while fascinating, is not a reliable source and doesn't give us any option to ignore the existing reliable sources and include your interpretation. - Ahunt (talk) 21:51, 14 February 2021 (UTC)

Gpsanimator, you have shown us some thoughtful arguments, based on reliable sources, which suggest that a previous editor may have overlaid some of the vectors in some of the diagrams that you suggest are unnecessary to explaining the topic and may not even be in the sources cited. You have also demonstrated a facility for editing diagrams. I could suggest that you mock up key elements of your proposal for an improved presentation in Talk:Forces on sails/sandbox, which currently contains the previous state of the article, before it was overhauled to make it more understandable and can now be deleted. If you can make this article more accessible to our readership, I'm all for it!

I note in one of the texts that you reproduce it says, "A sail is a wing". This causes me to puzzle over your objection to this often-used analogy.

One key point of interest to me would be to see what language you would substitute in the lead image showing upwind and downwind points of sail, which you feel is POV. Cheers, HopsonRoad (talk) 23:34, 14 February 2021 (UTC)

Thanks for your words of encouragement. I have to report that it's a breath of fresh air after weeks of fielding abuse, scorn and derision on various sailing forums (sailingAnarchy and boatDesign as Sailor Al). This is my first foray into Wikipedia and I have to admit to being apprehensive about the thought of such a major rewrite. Would you be able to put the current article into the sandbox, so I could at least start with the latest version?

I'm off to the State Library tomorrow to study Eiffel's much quoted work on cambered foils to see just how relevant his polar diagrams really are to sails, in particular to see if he looked at wind angles above the 25° we see in sailing and what sort of Reynolds number applied. I would like to see if his results ever generated a Thrust as opposed to a Drag component.

And I quoted the "A sail is a wing" reference to point out how entrenched the analogy is, but then to point out that it wasn't for another 8 pages before he ever used any aspect of the wing analogy! Gpsanimator (talk) — Preceding undated comment added 00:26, 15 February 2021 (UTC)

I'd be happy to, Gpsanimator. Doing so will obliterate the tortured previous iteration of the article. I tried to bring some order to it with what you see here. I look forward to whatever improvements that you will make.

BTW you may not be familiar with signing your posts on a Talk page with the four tildes (~~~~) that you can find at the bottom of the edit window. Cheers, HopsonRoad (talk) 03:01, 15 February 2021 (UTC)

 Done I'll look forward to watching your progress, Gpsanimator! Cheers, HopsonRoad (talk) 03:04, 15 February 2021 (UTC)

Thanks. As I said, it's my first venture into Wikipedia although I am quite an experienced documenter, used to Google Docs, Word, HTML, SVG and drawing tools. Can you give me any advice on avoiding the newbie pitfalls please? Gpsanimator (talk) 03:18, 15 February 2021 (UTC)

Coanda Effect

This sentence: "The tendency of the air to stay attached to the outside convex sail surface is explained as the Coandă effect" should be removed or changed.

Referring to the description of the Coanda Effect page in Wikipedia (https://en.wikipedia.org/wiki/Coand%C4%83_effect) is the tendency of a fluid jet to stay attached to a convex surface. With a sail there is no jet. A very good description of the misconceptions concerning the Coanda effect is given by Doug McLean in Understanding Aerodynamics: Arguing from the Real Physics(pages 275-281). Another way to look at is that even a flat plate can produce lift and at appropriate angles of attack the air flow stays attached. Msachse (talk) 16:59, 5 January 2022 (UTC)

 Done The sentence didn't help explain the topic, even if it were correct. HopsonRoad (talk) 18:32, 5 January 2022 (UTC)

Source of the pressure differences?

The effects of the aerodynamic force F_T, are thoroughly explored and explained in the Article using classical mechanics and vector algebra to explain how  the force operates on the boat at varying points of sail.

The source of the force is correctly identified as the pressure differences around the sail using basic physics:  force = pressure /area.

The origin of these pressure differences is identified as:

"These pressure differences arise in conjunction with the curved air flow. As air follows a curved path along the windward side of a sail, there is a pressure gradient perpendicular to the flow direction with lower pressure on the outside of the curve and higher pressure on the inside. "

However, there is no explanation of how the curved air flow causes the pressure differences. Simply stating " in conjunction with the curved air flow" is incomplete.

Where is the explanation of how the curved air flow generates a pressure difference?

This is a serious shortcoming of the article and requires its own section at the start of the Article. Gpsanimator (talk) 23:40, 3 January 2024 (UTC)

The effect of a curved surface is explained in Lift (force), which is cited as the main article at the top of the section. Also, the references given suffice for purposes of the article. This article is about the sail's interaction with the wind and the resistive forces coming from the sailing craft's contact with the surface. HopsonRoad (talk) 03:52, 4 January 2024 (UTC)

But it doesn't explain what causes the pressure differences. Is it magic? How does the curved airflow cause the pressure differences? Gpsanimator (talk) 05:03, 4 January 2024 (UTC)

The quotation supplied by Gpsanimator can be found in the section Components of force: lift vs drag etc. Dolphin (t) 04:14, 4 January 2024 (UTC)

Yes, I know.

The point is that the source of the pressure difference is not explained. Just saying "arises in conjunction with" doesn't explain what's happening. Gpsanimator (talk) 05:04, 4 January 2024 (UTC)

Don't forget: Wikipedia articles must not contain original research. On Wikipedia, original research means material—such as facts, allegations, and ideas—for which no reliable, published source exists.

All I'm saying is that none of the cited references explain the source of the pressure differences. Gpsanimator (talk) 05:11, 4 January 2024 (UTC)

For anyone who understands the concept of centripetal force it is easy to grasp the reason why curved streamlines are always associated with a pressure gradient. When fluid particles are following a curved streamline they must be experiencing a centripetal force which manifests as a pressure gradient - lower pressure where the radius of curvature of the streamline is smallest; and higher pressure where the radius of curvature is greatest. This is compatible with a lower pressure on the upper surface of an airfoil.

This explanation of lift has been published by Holger Babinsky. His work is one of the sources cited at Lift (force) - footnotes 49 and 62. We need to insert these ideas into the article in the place you have indicated. Dolphin (t) 06:07, 4 January 2024 (UTC)

I'm not looking for you to explain it in this Talk, but to see the explanation in the Article. 2001:8003:2C0C:C701:3041:F336:3E7:9C3 (talk) 09:46, 4 January 2024 (UTC)

Whoever wrote “But it doesn't explain what causes the pressure differences. Is it magic? How does the curved airflow cause the pressure differences?” is asking for an explanation here on the Talk page. I have given the beginning of a very rudimentary explanation - there is no point retracting it, is there? Dolphin (t) 11:10, 4 January 2024 (UTC)

That was posed as rhetorical question, (note the use of "magic") to accentuate the absence of an explanation in the Article.

(I have had a long and spirited conversation with Babinsky. He makes no reference to centripetal forces and falls back on the application of Bernoulli's principle. )

What I'm pointing out is that this Article is about sailing, and will be read by sailors to learn about how a sailboat works.

It correctly identifies pressure as the mechanism through which the air (a gas) creates a force on the sail (a solid), but the core question of where the pressure differences arise is absent.

I have read dozens of sailing books and scores of physics, thermodynamics, fluid dynamics, aerodynamics, gas dynamics, and haven't found the answer. Gpsanimator (talk) 20:13, 4 January 2024 (UTC)

Thank you for these, observations, Gpsanimator. It seems like the question needs to be resolved elsewhere and then linked back to here. Cheers, HopsonRoad (talk) 20:42, 4 January 2024 (UTC)

"It seems like the question needs to be resolved." indeed!

It seems to me that everybody assumes that the question must have been answered so there's no point in worrying about it. Yachts do sail upwind, planes do fly, propellers do work, wind turbines abound. "The scientists must know".

But if they did, we could read about it in the textbooks.

I worry about it because none of the books address the question: What is the source of the low pressure to leeward and the high pressure to windward of a sail?

Doesn't that worry you as well? Gpsanimator (talk) 21:14, 4 January 2024 (UTC)

Gpsanimator: It will be helpful to me and others if you clarify what you mean by “Babinsky ... falls back on the application of Bernoulli’s principle.

For example, do you consider BP to be appropriate or unhelpful or something else? Dolphin (t) 21:19, 4 January 2024 (UTC)

Whose application of "BP"?

All the ones I have found end up in a circular argument: the pressure difference is caused by the accelerating airflow which is caused by the pressure difference!

Babinsky explained the lack of evidence of airflow acceleration in his now-famous video as:

"Friction (which is another thing we often ignore, but which matters very close to the wing - more so in this lab experiment because it is low Re)"

i.e. experimental error! Gpsanimator (talk) 21:31, 4 January 2024 (UTC)

No, you haven’t responded to my request. What do you mean or intend when you write that Babinsky falls back on Bernoulli’s principle. Dolphin (t) 21:46, 4 January 2024 (UTC)

He stated in our email exchange:

"And yes, on my video the air pressure and velocity follows Bernoulli's equation." Gpsanimator (talk) 22:18, 4 January 2024 (UTC)

What you meant when you wrote your comment about Bernoulli’s principle; and what Babinsky stated in his email to you, are two very different things.

I think this discussion has achieved about all that is possible. Dolphin (t) 04:14, 5 January 2024 (UTC)

OK, I understand. You are enquiring about the phrase "falls back on", not the body of the comment.

It might have better been phrased:

"He makes no reference to centripetal forces and declares that his video demonstrates the application of Bernoulli's principle." Gpsanimator (talk) 22:56, 5 January 2024 (UTC)

Oops: force = pressure x area (not pressure /area) . Glad to have avoided the embarrassment of having someone else picking this up! Gpsanimator (talk) 22:09, 4 January 2024 (UTC)

Apparent wind

In many diagrams, the vectors representing the relationship between Va, Vb and Vt are confusing. Va and Vt represent wind velocities whereas Vb doesn't. I believe that the Vb vector should be replaced by a velocity vector (let's call this vector Vh) representing headwind experienced by the boat moving in still air. This way, the part of every diagram showing the relationship between Va, Vb and Vh would truly reflect the fact that Va is the vector sum of Vt and Vh. Félix Langlois (talk) 13:12, 1 December 2024 (UTC)

Thank you for your suggestion, Félix Langlois. Two points to make here:

1) Your suggested Vh is really −Vb and is not actually a wind velocity, since it holds true at Vt = 0. This is described in the article as, (V_A = −V_B + V_T) and with this illustration.

2) The direction schema is from published sources. We Wikipedia editors should avoid creating ways of describing things, not employed in our sources.

Cheers, HopsonRoad (talk) 14:15, 1 December 2024 (UTC)