In this clip we can see a model of a car-boat with a sail.

(Explicit links. Whole version: https://www.youtube.com/watch?v=zp1KzGQdouI. Parts: https://www.youtube.com/watch?v=jhem8Z9ujPE, https://www.youtube.com/watch?v=7g1Gz-62dHQ)

Facts are (for the first video):

  • sail is flat (thus both sides are equally long)
  • sail is at 35º respect motion line
  • wind comes from ahead: the model sails close-hauled
  • the wind impacts only on the winward side of the sail
  • leeward side has 0 wind (telltales are down)
  • the car moves forward

My hypothesis is that the effect propelling the car is only "air deflection on windward side".

Because there is no wind on leeward, no net effect occurs on that side (Bernouilli, differences of pressure, difference or airflow speed, whatever).

Although, of course, "leeward side" effects occur in real situations and they do contribute to planes lift / boat propulsion, their contribution is not necessary for motion/lift as the video shows.

Am I correct?

Does anybody know of any video demonstrating wing lift / sail propulsion in realistic models (not just lightweight papers) exclusively due to effects occurring on the leeward side?

  • $\begingroup$ I take objection to the claim that "there is no wind on leeward side." $\endgroup$ Commented Sep 3, 2020 at 13:08
  • $\begingroup$ More germane: please explain how you could have a situation where there's no effect on the sail/vehicle from the incoming wind on the windward side. $\endgroup$ Commented Sep 3, 2020 at 13:09
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    $\begingroup$ If you are talking about a thin cloth sail (i.e., not a thick, rigid wing as seen on some high-tech racing yachts), then both sides are "equally long" even when the sail is not flat. Also, the idea that air moving over the longer side of a wing must move faster because it has farther to go is completely false. When a wing splits a parcel of air, nothing compels the two halves of the parcel to meet up with each other again after the wing has passed. $\endgroup$ Commented Sep 3, 2020 at 13:10
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    $\begingroup$ @cibercitizen1 Did you seen this page komorebi-yacht.com/en/propulsion-wing.cfm ? $\endgroup$ Commented Sep 5, 2020 at 21:24
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    $\begingroup$ @GuyInchbald youtube.com/watch?v=jhem8Z9ujPE $\endgroup$ Commented Sep 6, 2020 at 22:36

2 Answers 2


I give you a possible explanation of the experiment in the video. If you look at the picture below, I have drawn the trajectory of a gas molecule which hits the sail. How you can see from the force scheme, when the molecule hits the sail then a force (N) acts on the sail and of course the same force acts on the molecule. The x-component of the force N on the sail accelerates the sail on the x axe, while the y-component is balanced by the constraint. You should also consider the impacts of the molecules on the other side of the sail (leeward) that generate a force in the opposite direction. If $n$ is the (constant) number of molecules which hit the sail on leeward, it will depend only on temperature $n=n(T)$. Calling $m$ the number of molecules which hit the windward, you have the net force on the sail on x axe: $$F_{x}=mN_{x}-n(T)$$.

enter image description here

  • 2
    $\begingroup$ While correct, what you say is incomplete. It's crucial in that analysis to show that there are fewer such molecular impacts per square centimeter per second on the leeward side of the sail than on the windward side. $\endgroup$
    – S. McGrew
    Commented Sep 6, 2020 at 19:53
  • $\begingroup$ Yes, you are right, I have edited my answer including what you said but I haven't proved anything. @S.McGrew $\endgroup$
    – Landau
    Commented Sep 7, 2020 at 8:23

You are wrong in your statements about the videos:

  1. It is misleading to say that the "wind comes from ahead", it comes from the starboard (right hand) bow at a marginally steeper angle than the sail; that angle is critical.

  2. The air blast is aimed at the leading edge; it is passing both sides of the sail.

  3. What telltales? I see none.

The sail is acting just like an aeroplane wing; air passes around both sides and is deflected backwards. There are several principles involved.

First is the simple "bouncing molecules" model given in one answer. This model can only account for well under half of the lift generated. The leeward path is longer than the windward path so Bernoulli does apply to some extent, but again not enough. Circulation theory gives a fuller description of how wings work, but it's complicated. Ultimately, by deflecting the air backwards and slowing its sideways movement, Newton's laws require an equal and opposite reaction on the sail, pushing it forwards and sideways. The wheels resist the sideways force, but not the forward one.

  • $\begingroup$ Hi. Thanks for your interest. 1. It's clear that boats can't sail face to wind. "Come from ahead" means that the wind angle is lesser than 90 degrees to the bow-stern axis. This is known as a "close-hauled course" (en.wikipedia.org/wiki/Point_of_sail#Close-hauled). 2. & 3. Telltales are the black threads hanging down on port tack (leeward). Because they are down, it shows there is no air flow on the leeward side. $\endgroup$ Commented Sep 7, 2020 at 11:21
  • $\begingroup$ Oh, OK, I got a better image now. The tufts look like they are deflecting to me, especially the rear one in the first video. Really, one needs more of them, plus a demonstration of what it takes to deflect them, some sense of calibration, for example similar tufts on the windward side: tufting in a boundary layer can yield all sorts of surprises. Right now, the blower is just too close to the leading edge for me to accept there is no airflow round it. Of course, if there really is none then the sail is not close-hauled either - you certainly cannot have it both ways! $\endgroup$ Commented Sep 7, 2020 at 12:49
  • $\begingroup$ I edited the videos: check this out youtube.com/watch?v=EdKzESGzVZ4 $\endgroup$ Commented Sep 8, 2020 at 9:59
  • $\begingroup$ Different position of blower, different airstream, different aerodynamics. What your new video shows is what the other answer stated. The wind now direct from the side, not "from the front". Emphatically not close-hauled, which by definition has wind flowing in the same direction on both sides. So you need to make up your mind which question you are really asking, undo the inconsistent change/s and maybe fix any remaining inconsistencies that cropped up during all your messing around. $\endgroup$ Commented Sep 8, 2020 at 14:59
  • $\begingroup$ The videos show air coming from two spots. 1. clouse-hauled (wind to the boat at aprox. 45º) and 2. beam-reach (wind to the boat just at 90º). In both cases, wind is deflected to back. In both cases, there is no wind on leeward side, thus no Bernouilli effect, if I am right, hence the question. $\endgroup$ Commented Sep 8, 2020 at 16:58

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