Sailing faster than the wind?

Get your science fix here: research, quackery, activism and all the rest
Post Reply
User avatar
Boustrophedon
After Pie
Posts: 2026
Joined: Mon Nov 11, 2019 3:58 pm
Location: Lincolnshire Wolds

Sailing faster than the wind?

Post by Boustrophedon » Thu Jun 03, 2021 5:52 pm

I have only sailed a little bit, but even I know that you can sail across the wind and can do so at speeds faster than the wind is blowing.

But can you make progress down wind faster than the wind in blowing? My first thought suggest not, but...

So there's a video from the usually reliable Veritasium on YouTube which claims that you can make downwind progress in a boat faster than the wind is blowing. Not only that he rides in a wind driven car that does the same.

There's the car with a big windmill/propeller on the back that is linked to the wheels via sprockets and chain. OK so far. I am happy with the idea that such a car can make progress into the wind, that's fine and the inventor on his channel shows just that. Indeed I have made a similar model that did the same way back in my teens.

But Veritasium is going downwind, there's a static wind sock blowing downwind in the same direction that the car is going and a pennant mounted on the car showing the relative wind blowing in the opposite direction, showing that the car has overtaken the prevailing wind and is now going faster.

WTF?

First when the car reaches wind speed there is no relative motion between the wind and the car, so how can the prop extract any power from the wind.
Second when/if the car goes faster than the wind, then relative to the propeller the wind has changed direction and the prop would go the other way.

The explanations in the film are handwavy, incomplete and make little sense.

Is it fake or is there summat I am missing?

I obviously am because there's a Wiki Article.
Remember it's only a coup if it's from the coup d'état region of France, otherwise it just sparkling white terrorism.

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Thu Jun 03, 2021 6:12 pm

I watched that yesterday and its subject, the Blackbird, was the heart of a very, very long but entertaining thread and massive argument on then-JREF/now-ISF forum.
http://www.internationalskeptics.com/fo ... p?t=173124
(That's a continuation but has a link straight to the original.

It took me a while to get it, but yes, it can go directly downwind faster than the wind which propels it.

One thing that strikes me from the comments is that a lot of people had a lightbulb moment seeing the animation of the two yachts tacking around a cylindrical sea and realising that yes, that was basically a propeller, but that many fewer had grasped the vital point made just before that: that a tacking yacht can not only travel faster than the wind, but the downwind component of its velocity can be faster than the wind. A yacht can genuinely travel downwind faster than the wind, provided it is also tacking across the wind. The point that was perhaps elided was how can the yacht do that?

User avatar
shpalman
Light of Blast
Posts: 4485
Joined: Mon Nov 11, 2019 12:53 pm
Location: One step beyond

Re: Sailing faster than the wind?

Post by shpalman » Thu Jun 03, 2021 6:19 pm

Boustrophedon wrote:
Thu Jun 03, 2021 5:52 pm
First when the car reaches wind speed there is no relative motion between the wind and the car, so how can the prop extract any power from the wind.
The propeller is being driven by the wheels, or if you like by the ground moving beneath the wheels (in the rest frame of the car), so it can turn in order to move air backwards.

The driver has control of the pitch of the propeller blades, and it seemed in the video that it's non-trivial to know what to do with it except try, and see what happens.
molto tricky

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Thu Jun 03, 2021 6:19 pm

The way I convinced myself it could work was to imagine it from the point of view of a small amount of air pushing on a spot in the middle of one of the propeller blades. As the cart moves forward, its wheels turn and the chain drive forces the propeller to turn too. That means that the point of contact of our little bit of air slides across the face of the prop blade. And the pitch of the blade means that the point of contact moves backward as the blade turns. So the air interacting with the blade remains in contact even though the cart itself is moving faster than the air.

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Thu Jun 03, 2021 6:27 pm

Having watched the YouTube vid, I suppose I can also see what I think is their own broader description: that the turning propeller pushes air back, and that essentially creates a cushion of air behind which fills the gap behind the prop. So the wind pushes on that constantly replenished cushion of air, and that pushes the propeller forwards. Part of the energy the wind provides is expended in accelerating the cart and part in turning the prop.

monkey
Snowbonk
Posts: 592
Joined: Wed Nov 13, 2019 5:10 pm

Re: Sailing faster than the wind?

Post by monkey » Thu Jun 03, 2021 7:05 pm

I watched this the other day, and meant to post it here because I thought it was fun. I was fine with the explanation, but I think it needed more work to be clearer and I also assumed that you could make it more complicated just because it involves wings.

The problem I had was that the prop didn't look big enough to be a suitable "sail", but I was obviously wrong.

IvanV
Stargoon
Posts: 101
Joined: Mon May 17, 2021 11:12 am

Re: Sailing faster than the wind?

Post by IvanV » Sun Jun 06, 2021 2:24 pm

Martin Y wrote:
Thu Jun 03, 2021 6:12 pm
... but that many fewer had grasped the vital point made just before that: that a tacking yacht can not only travel faster than the wind, but the downwind component of its velocity can be faster than the wind. A yacht can genuinely travel downwind faster than the wind, provided it is also tacking across the wind. The point that was perhaps elided was how can the yacht do that?
The basic way that it works is to imagine the sail as a reflector of the wind. It changes the direction of motion of the wind. Imagine what is the force required to change the direction of the wind by reflection. Then an equal and opposite force acts on the sail. That force can have a forward component by arranging the angle of the sail to the wind suitably.

In more detail:

Imagine a yacht sailing perpendicular to the wind, so that the component of the wind parallel to the yacht's direction of travel is zero.

For simplicity, let us imagine the sail is a rigid board. Position this board so that it is, for sake of argument, about 45 degrees to the wind and 45 degrees to your direction of travel. The sail/board is on the side of the boat opposite to the side the wind is coming from. Maybe not the most efficient angle, but it will do for illustration.

If you imagine the wind as a stream of solid particles, then they will hit the sail/board, and reflect off it - symmetrically if we assume a perfect Newtonian collision closely approximated by a ball hitting a snooker cushion. So in that ideal sitution, they come off the sail/board in a rearwards direction. The sail/board and the particles undergo an equal and opposite exchange of forces at the collision point. We can see that the force produced by the sail/board required to change the particles' motion from sideways to rearwards must have a significant backwards component. Thus the equal and opposite force on the sail/board must have a forwards component.

In reality, it is made more complicated by the fact that the sail, when taut, is curved, and the boat is moving forward so the relative direction of the wind is no longer exactly sideways. And the reflection of the wind off the sail is probably not a perfect Newtonian reflection either. It also depends on the rigging of the sail.

Square-rigged ships, with spars at the top and bottom of the sail, go very well when the wind is behind them, but not very well at all with winds at a significant angle. This is because such sails fill nicely with the wind behind them and you get an efficient reflection, but don't fill so well with the wind at an angle and the reflection is less efficient. But fore-and-aft rigged ships, like typical modern dinghies and yachts, which have spars at the side and bottom of the sail, don't go so well when the wind is behind them, but much better than square-rigged with oblique winds. As far as I can tell, from the limited amount of sailing I have done, with a wind straight behind a fore-and-aft rigged sail, the location of the spars makes it difficult for the sail to become taut and act as an efficient reflector. Such a rigging is designed to reflect the wind well when the wind is oblique. Using Patrick O'Brian novels as another dubious source, it seems to me that in the great age of sail later ships increasingly had a mix of square and fore-and-aft rigging to balance the advantage. A junk with its numerous spars across the sail at a variety of angles, is another kind of intermediate arrangement.

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Mon Jun 07, 2021 7:54 am

I think that explanation wouldn't settle the minds of people confused that the wind which is supposed to be pushing the yacht is advancing more slowly than the yacht itself. The puzzle is how do you shove something which is already moving away faster than the hand reaching out to shove it.

User avatar
Woodchopper
Stummy Beige
Posts: 3371
Joined: Sat Oct 12, 2019 9:05 am

Re: Sailing faster than the wind?

Post by Woodchopper » Mon Jun 07, 2021 8:15 am

Martin Y wrote:
Mon Jun 07, 2021 7:54 am
I think that explanation wouldn't settle the minds of people confused that the wind which is supposed to be pushing the yacht is advancing more slowly than the yacht itself. The puzzle is how do you shove something which is already moving away faster than the hand reaching out to shove it.
Yes, as Martin Y mentions upthread the key thing to think about is that a yacht can travel faster than the wind when it is travelling at an acute angle into the wind.

User avatar
shpalman
Light of Blast
Posts: 4485
Joined: Mon Nov 11, 2019 12:53 pm
Location: One step beyond

Re: Sailing faster than the wind?

Post by shpalman » Mon Jun 07, 2021 9:08 am

For the example given, with a 45° angle, the (vector) difference between the incoming and outgoing momentum of the "wind" would be sqrt(2) times larger,* and perpendicular to the surface of the rigid board. But the component of this vector in the direction of the boat would be the same as the momentum of the "wind".**


* - by Pythagoras, imagine the diagonal of a square.

** - by trigonometry, because the cosine of 45° is 1/sqrt(2).

sqrt(2) is about 1.4 by the way.
molto tricky

IvanV
Stargoon
Posts: 101
Joined: Mon May 17, 2021 11:12 am

Re: Sailing faster than the wind?

Post by IvanV » Mon Jun 07, 2021 11:41 am

Martin Y wrote:
Mon Jun 07, 2021 7:54 am
I think that explanation wouldn't settle the minds of people confused that the wind which is supposed to be pushing the yacht is advancing more slowly than the yacht itself. The puzzle is how do you shove something which is already moving away faster than the hand reaching out to shove it.
So let's think of another situation where you can use a force in one direction to power something in another direction, and at a faster speed than the incoming motion. So suppose you have a watermill which produces the rotation of a beam about a horizontal axis. You convert that to rotation about a vertical axis, to turn a horizontal grindstone, by use of a gear. You can also change the speed of rotation with a gear. So the grindstone can go faster than the rotation of the millwheel, indeed needs to, provided its momentum compared with the millwheel is not too high - the faster the speend you need the grindstone to turn, the smaller the grindstone has to be.

So energetically, this isn't a problem. All we need is a gearing system to redirect the momentum and consider the amount of momentum harvested and applied to the mass of the object being moved.

Putting a sail at an angle to the wind has some characteristics similar to using a gear wheel to redirect a force. I used the example of a wind coming from the side precisely because it has no motion at all gained in the direction of travel. Yet you can harvest momentum from that wind and redirect it to travel in a direction that the wind has no component at all, perpendicular to the wind. If you are light enough and have a large enough sail area, you can harvest sufficient momentum to go faster than the sideways wind, what does the sideways windspeed matter, all that matters is the amount of momentum you can harvest from it and the resistance to travel of your boat. Though go too fast and your ability to harvest momentum is eventually reduced because of the simplicity of the sail mechanism. Similarly, energetically there is nothing wrong with going faster than the wind when it is directly behind you, provided you have a mechanism that works as a gearing system, harvest enough momentum and are light enough. A simple sail isn't a sufficient mechanism in that case, hence the device of using a land-yacht's wheels to power a propeller.

User avatar
Woodchopper
Stummy Beige
Posts: 3371
Joined: Sat Oct 12, 2019 9:05 am

Re: Sailing faster than the wind?

Post by Woodchopper » Mon Jun 07, 2021 12:01 pm

IvanV wrote:
Mon Jun 07, 2021 11:41 am
Putting a sail at an angle to the wind has some characteristics similar to using a gear wheel to redirect a force. I used the example of a wind coming from the side precisely because it has no motion at all gained in the direction of travel. Yet you can harvest momentum from that wind and redirect it to travel in a direction that the wind has no component at all, perpendicular to the wind. If you are light enough and have a large enough sail area, you can harvest sufficient momentum to go faster than the sideways wind, what does the sideways windspeed matter, all that matters is the amount of momentum you can harvest from it and the resistance to travel of your boat. Though go too fast and your ability to harvest momentum is eventually reduced because of the simplicity of the sail mechanism.
Its interesting to note that in that example the sailor is not just harvesting momentum. The sail is shaped like an aerofoil and the wind passing over the convex surface generates lift (though moving the boat forward rather than upwards as in an aeroplane).

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Mon Jun 07, 2021 12:09 pm

"Provided you have a mechanism" etc is the point where people, including people who have a pretty good grasp of physics, become convinced the Blackbird cannot work and it must be a fraud or a mistake.

At first glance it's like an undershot watermill where the paddle wheels are somehow turning faster than the flow of water that drives them. It's not of course but it takes people into a place where they have to consider how a yacht works (or a wing or a propeller blade) in detail they haven't considered before. The sail of the tacking yacht (sailing close to an apparent headwind somewhat to the downwind side of its course) changes the direction of the air it interacts with as it extracts momentum; it diverts it in the opposite direction to the true wind. And I think that diverted air "fills the gap" which would otherwise occur as the sail retreats from the true wind faster than that wind.

IvanV
Stargoon
Posts: 101
Joined: Mon May 17, 2021 11:12 am

Re: Sailing faster than the wind?

Post by IvanV » Mon Jun 07, 2021 2:05 pm

Woodchopper wrote:
Mon Jun 07, 2021 12:01 pm
Its interesting to note that in that example the sailor is not just harvesting momentum. The sail is shaped like an aerofoil and the wind passing over the convex surface generates lift (though moving the boat forward rather than upwards as in an aeroplane).
Lift also works by changing the direction of motion of the fluid you are travelling in, towards the direction opposite the one you want to go in. It's a more subtle way of redirecting the fluid motion than slanting a barrier across the fluid motion, but does the same thing, but to a greater cross-section of the fluid motion.

User avatar
Martin Y
After Pie
Posts: 2013
Joined: Mon Nov 11, 2019 1:08 pm

Re: Sailing faster than the wind?

Post by Martin Y » Tue Jun 08, 2021 9:13 am

IvanV wrote:
Mon Jun 07, 2021 2:05 pm
Lift also works by changing the direction of motion of the fluid you are travelling in, towards the direction opposite the one you want to go in.
Exactly. Blackbird is powered by air pressure on the windward side of its prop blades pushing the cart forwards, and it uses a portion of that power to drive the propeller to divert air behind it to make up for its outrunning the wind that provides the energy.

User avatar
nekomatic
Snowbonk
Posts: 531
Joined: Mon Nov 11, 2019 3:04 pm

Re: Sailing faster than the wind?

Post by nekomatic » Tue Jun 08, 2021 9:06 pm

IvanV wrote:
Sun Jun 06, 2021 2:24 pm
As far as I can tell, from the limited amount of sailing I have done, with a wind straight behind a fore-and-aft rigged sail, the location of the spars makes it difficult for the sail to become taut and act as an efficient reflector.
That’s when it’s time to get the spinnaker up, a.k.a. ‘fly the kite’.

Post Reply