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Author Topic: can you make a wind-driven boat/vehicle go faster than the wind 'pushing' it?  (Read 9789 times)

Offline yor_on

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Heh, better admit that I stole this one from Sapos joint, and yes, it was worth stealing (sorry Sapo). It's too sweet to ignore, especially to those remembering the discussion of a plane on a conveyor band. I know you guys won't settle just for a discussion here though. You're too a inquisitive lot :)

Take a look. Can a vehicle be built which can go directly downwind, faster than the wind (DDWFTTW), powered only by the wind, steady state?
=

Better add that I'm not as sure about a boat though? Although I think it should be possible if you minimize the resistance relative its 'surfaces'? Still, so da**ed cool.
« Last Edit: 19/03/2013 23:15:01 by yor_on »


 

Offline dlorde

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The answer is 'yes' for both vehicles and sailboats. For a vehicle, the wheels must be coupled to a prop to leverage the wind speed relative to the ground, and for a boat, it must have a device to leverage wind speed relative to the water (e.g. a keel).

See Sailing Faster Than The Wind.
 

Offline yor_on

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Sweet stuff, although, I want it more esthetic. A sailingboat consisting of propellers may not be the stuff of my dreams :) Ah well, I do like the speed though..
 

Offline dlorde

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You'd use a prop on a land yacht (e.g. sand yacht). For a water sailing boat you could just use the sail and the keel.
 

Offline graham.d

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Not sure you need theprop. Wouldn't the wheels have a similar effect to a keel in that they stop sideways movement.

Doesn't work on my boat sadly as I don't get much propulsion when getting closer to the apparent wind than about 40 degrees. It may be interesting to see if I could get more speed "tacking" downwind rather than using a spinnaker, especially if the wind is light. It's hard to test though unless there was another identical boat with the same rig to race.
 

Offline David Cooper

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In the last America's Cup they used multihulls which could travel at three times the speed of the wind (e.g. 24 knots in 8 knots of wind). The next America's Cup may see even higher speeds as the technology is developing fast. On land the difference between the speed of a land yacht and the wind should be a lot higher due to a huge reduction in friction compared with dragging a hull through water, and the same applies to ice yachts where I think I remember something about one doing over 100mph (and I think it might also have been about a century ago).
 

Offline yor_on

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You know, I would really like to see that original paper suggesting this effect, and how the guy formulated it. It's referred to in the link I gave, but i didn't find the paper.
 

Offline graham.d

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David, with respect to the multihulls in the Americas cup, was it stated they travelled 3x the windspeed going  down wind? It is not so hard to exceed windspeed when going across the wind for example. The best speed is usually when the wind is just forward of the beam. The problem is trying to get a net speed in a windward direction (even with tacking) that is faster than a balloon released into the wind. It can be done but most yachts will not achieve it. 3x the wind speed would be very impressive.
 

Offline David Cooper

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Sorry - I didn't read the original post carefully enough and my error was compounded by the second post which talks of the necessity of a keel (to stop the boat slipping sideways).

So, the question is much more interesting than I'd realised, and I don't know the answer. If these boats "tack" downwind (technically gybing) at 135 degrees to the wind they'll have to cover 1.41 times as much distance as the direct distance downwind, so they need a boat speed just a little higher than that, which sounds as if it may be possible. The fastest speed is probably achieved on a beam reach (at 90 degrees to the wind). There will be a range of angles and speeds in between those two which all need to be looked at, the speed of the boat reducing as the distance that needs to be covered comes down.

I don't know if this can be worked out through software. The best bet would be to send a question direct to the people who actually use the fastest kinds of boats to ask them, because they'll certainly know. Search terms to try to find a way to make contact: AC45, AC72, Extreme 40s, moth sailing dinghy. There is a complication with hydrofoils which I think make the boats faster downwind while slowing them down upwind.

Edit: And I now realise I'm still not answering the direct question which is about a boat going directly downwind, which should rule out zigzagging, though you could theoretically design a rig to zigzag while the boat under it goes directly downwind.
« Last Edit: 21/03/2013 18:39:23 by David Cooper »
 

Offline graham.d

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Yes a Moth may be a good one. I have seen these go extremely quickly on their hydrofoils. They don't look easy to manage though! I have a relatively lumbering 35' keel boat; the hull length limits its speed to around 7 knots although in a really strong wind and a beam reach I have had a bit more.

It may well be possible to make headway downwind faster than the wind by tacking but probably not in a keel boat. A cat or hydrofoil would be much better but I don't know what could be achieved. It is hard to see how it could be achieved with the wind directly behind but maybe there could be some cunning variable keel design.
 

Offline yor_on

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Or turbines maybe, on a catamaran? If you can get it to lift, maybe? Wasn't there some very fast new Australian sailing boats? Don't remember where I read about them but they were really fast.
 

Offline David Cooper

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I'm beginning to doubt that it's possible. If you put a wind turbine on a boat, for example, the turbine will stop turning altogether as soon as the boat's up to the speed of the wind, and the turbine blades are essentially tacking downwind, so it looks as if a hot-air balloon flying just above the water will always win a race downwind against any kind of wind powered boat (or anything on land using wheels or skates). It's probably only by eliminating all friction with the ground that you can keep up with the balloon.

I've finally got round to looking at the link in the original post. I think the stuff about going downwind faster than the wind is all bogus. As soon as you reach the speed of the wind you lose all power, and from then on your turbine will do nothing but add to the drag - it's worse than a perpetual motion machine.

The next interesting question is how fast it can go upwind though, and maybe they've been putting out misinformation about downwind speed to make people think they're nutters so that they have a clear run to go for the upwind speed record without any competitors.
« Last Edit: 22/03/2013 18:29:40 by David Cooper »
 


Offline yor_on

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I'm very proud of this thread, and will now make a statement.

It will work :)

Prove me wrong.
=

Damn :) Thought Wired said it was not possible Imatfaal? the obvious conclusion must be to not read wired ::)) We can make this one biiig too, if we just refuse those magazines. I got to admit that I really miss the plane on the conveyor belt. Had so much fun there.
« Last Edit: 23/03/2013 11:19:34 by yor_on »
 

Offline MarkV

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I think the stuff about going downwind faster than the wind is all bogus.

Gaunaa, Mac; ye, Stig; Mikkelsen, Robert (2009). "Theory and Design of Flow Driven Vehicles Using Rotors for Energy Conversion". Marseille, France: Proceedings EWEC 2009.
newbielink:http://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content [nonactive]

"It is theoretically possible to build a wind
driven car that can go in the downwind
direction faster than the free stream wind
speed (using a propeller in the air)."


The next interesting question is how fast it can go upwind though

newbielink:http://www.nalsa.org/ [nonactive]

"The NALSA Board of Directors has ratified the following two records achieved by Rick Cavallaro on New Jerusalem Airport near Tracy California on June 16, 2012, with the wind turbine driven sailing craft, Blackbird. Mr. Cavallaro achieved a maximum boat speed to wind speed ratio of 2.1:1 while sailing directly into the wind and a maximum speed in a wind turbine driven sailing craft of 22.9 mph on a different run.
On July 2, 2010 on El Mirage Dry Lake, Blackbird sailed directly down wind at a speed of 27.7 mph in a 10 mph wind to set a first record for the ratio of Boat Speed to true wind speed of 2.8. BlackBird was designed and built by the Thin Air Designs team (Rick Cavallaro and John Borton) and sailed by Rick."
« Last Edit: 23/03/2013 12:16:16 by MarkV »
 

Offline dlorde

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I've finally got round to looking at the link in the original post. I think the stuff about going downwind faster than the wind is all bogus. As soon as you reach the speed of the wind you lose all power, and from then on your turbine will do nothing but add to the drag - it's worse than a perpetual motion machine.

My understanding is that this is only true if your craft is free-wheeling (land) or floating free (water). In that case, you can only approach the speed of the wind because you're relying on the relative difference between your speed and the wind speed.

That's why the prop has to be coupled to the wheels for a land sailer and why you need a keel or similar device to couple a boat to the water. This way, you can use the relative difference in speed between the wind and the land or water to provide additional propulsion. For a boat, the keel doesn't just reduce sideways movement and yaw, but couples the sail to the water; the net result is much like shooting out a lemon pip by squeezing it between the fingers.
« Last Edit: 23/03/2013 12:09:33 by dlorde »
 

Offline graham.d

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Yes but look at the vectors in the lemon pip analogy. The maxium velocity is achieved when the direction of the force (wind) against the immovable, slippery plane (keel) are nearly at right angles. If you are pushing your pip (wind dierection) in the same direction as the slippery plane (keel) you only get the pip going as fast as you can push (as fast as the wind).
 

Offline MarkV

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Yes but look at the vectors in the lemon pip analogy. The maxium velocity is achieved when the direction of the force (wind) against the immovable, slippery plane (keel) are nearly at right angles. If you are pushing your pip (wind dierection) in the same direction as the slippery plane (keel) you only get the pip going as fast as you can push (as fast as the wind).
That's why conventional sailboats can outrun the wind only when sailing at an angle to the wind, not directly downwind. Here is the analogy to a squeezed wedge (lemon pip):


Note that the sail-craft outruns the air-mass along the directly downwind axis (downwind VMG > wind speed). Only ice-boats, land-yachts and the most efficient sailboats can achieve that ( see: newbielink:http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good [nonactive] ). They can beat a balloon to a directly downwind point, by tacking. On the DDWFTTW cart only the rotating propeller blades perform a long continuous tack, while the rest of the vehicle moves directly downwind, faster than the wind:

 

Offline David Cooper

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Thanks for finding all those fabulous links, people.

I was hoping I might somehow be wrong about this one, and fortunately it turns out that I was. If it's true that boats moving at perhaps 135 degrees to the wind make actual downwind progress faster than the wind (I don't know which angle's best), then the answer's very clear that they will be able to beat a balloon downwind. Eliminating the tacks (gybes) isn't particularly important as they can be made completely insignificant simply by making the course larger so that the turns take up an infinitesimal proportion of the time. In practice you'd need to do a lot of experiments to prove the point as wind speed can vary a lot on different parts of the course so you wouldn't be sure the boat wasn't picking up more wind than you were recording, but the statistics would soon add up to prove the point beyond reasonable doubt.

Unfortunately, it's hard to visualise the mechanism by which this can work, but the case where wheels are connected to a turbine is much easier to think your way around because it's easy to see where the power is tapped and how it is unleashed. I had been wondering how the wheels were connected to the turbine, but it appears to be a simple connection without variable gears, which fits in with the extremely slow acceleration: the wheels may initially hold the turbine back.

I had thought originally about the case of a turbine on a balloon. Because the balloon travels along at the speed of the wind, the turbine simply won't turn. What happens though if a wheel on a pole is lowered to the ground to try to pick up energy from the moving land below? You could have both a dynamo and a motor in the wheel: the dynamo generates power which you can then feed into the motor to power the wheel: the result will inevitably be drag, so the balloon will be slowed down, though if you could eliminate all friction there would neither be loss nor gain. That initially looks like the final say on the whole business, but it isn't. The balloon is sitting in moving air, so any power taken from the wheel can be sent to the turbine to make it serve as a propeller where it has a much better chance of tapping power out of air which is stationary relative to it than it would if the air was moving past the propeller at the same speed as the ground - putting the energy back into the wheel through a motor doesn't have that advantage, though there would be an equivalent if you had a long treadmill next to a road with the surface of the treadmill moving at the same speed as the balloon: you could then use two wheels, one with a dynamo in it tapping energy from the stationary road and the other with a motor in it putting out that power down onto the moving surface of the treadmill which is not moving relative to the balloon. That setup would be more efficient than the turbine and is a lot easier to get your head around.

Let's assume 100% efficiency. The wheel with the dynamo will generate drag, so there is a clear cost, but because we're assuming 100% efficiency we can put all the generated energy into the other wheel with the motor. If this wheel was also on the road, there would be no gain or loss for the balloon as it would directly cancel out all the drag caused by the dynamo, but of course the wheel with the motor isn't supposed to be on the road: it's going to be on the treadmill surface. If the balloon is moving at 10mph (the same speed as the wind and the treadmill surface), the energy captured from the wheel with the dynamo can be transferred to the wheel with the motor to make it try to run at 10mph on the treadmill surface. This will necessarily apply an acceleration force to the balloon - there's absolutely no doubt about it.

Suppose the balloon now reaches 15mph - we can design it to be shaped like an airship to reduce drag, so this should be possible. We now have the wheel with the dynamo moving at 15mph on the road and the wheel with the motor on the treadmill trying to go at 15mph to match, so we're clearly going to go on getting higher amounts of power generated all the time until the drag catches up with it and we hit a maximum speed.

If you could eliminate all friction, it looks as if it could go on accelerating without limit, continually tapping more and more energy from the difference in speed between the the ground and the treadmill surface. Imagine building two tracks in space with one moving at 10mph relative to the other. Now attatch a vehicle to both tracks using wheels which grip the tracks from two sides such that they are forced to remain in contact at all times. The wheels with the dynamo are initially rotating at 10mph because the vehicle can lock itself to the moving track to get up to that speed. Once at the speed of the moving track, the system is unlocked and the power generated in the dynamo is fed to the motor, thereby making the other wheels try to move at 10mph along the moving track. The result will soon be that we have a vehicle moving at 20mph with one set of wheels rotating at 20mph and another set of wheels rotating at 10mph and wanting to rotate faster. It does look as if it has to go on accelerating forever. In the real world, of course, there will be losses, but losses can be reduced for a long time, so the maximum speed will continue to creep up towards some theoretical maximum where it's impossible to improve the efficiency any further.

One thing puzzles me about the vehicle with the turbine though - my internet connection is too slow to watch the videos so the answer may be in those. To begin with when the vehicle is moving slower than the wind, the turbine is being blown round. Once the vehicle starts moving faster than the wind, the turbine is from that point on moving the wrong way to act as a propeller. How do they deal with that? I've checked this with a little turbine - when you blow it it turns anticlockwise, but if you power it anticlockwise it blows air away from you. Do they flip the blades to a different angle to get round this problem?

I now want to try to understand how a boat or ice/land yacht can make progress downwind faster than the wind, but it's going to be a lot harder to think through as it's not so easy to see energy being picked up from the water and then thrown out from the sail. I'll take my time and try to model this in my head too, but I don't expect to be able to crack it.

Quote
Their friendship started as an argument over an aerodynamics riddle that hinged on whether you could know the true direction of the wind while hang gliding without looking at the ground. (You can, though Cavallaro has never fully conceded the point.)

Looks as if another thread needs to be started on that.
« Last Edit: 23/03/2013 22:07:16 by David Cooper »
 

Offline MarkV

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my internet connection is too slow to watch the videos so the answer may be in those.
You should really watch those first. This will spare you a lot of guesswork (pay attention to the blade angle vs. rotation).

The downwind runs:

The upwind runs:

If it's true that boats moving at perhaps 135 degrees to the wind make actual downwind progress faster than the wind
It is definitely true. Check the GPS measured vectors for an ice-boat on page 4:
newbielink:http://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf [nonactive]

I had been wondering how the wheels were connected to the turbine, but it appears to be a simple connection without variable gears, which fits in with the extremely slow acceleration: the wheels may initially hold the turbine back.
In the downwind case it is a propeller (turned by the wheels) all the time (below and above windspeed). In the upwind case it is a turbine (turning the wheels). The  slow acceleration in the downwind case is because the propeller creates a retarding force at the wheels, as it turns against the aerodynamic torque.

Once the vehicle starts moving faster than the wind, the turbine is from that point on moving the wrong way to act as a propeller.
The pitch and transmission are chosen such that it always acts as a propeller. Nothing changes at wind-speed about that.

Do they flip the blades to a different angle to get round this problem?
No. The prop pitch is always positive. On the prototype the pitch is even fixed, and it still accelerates from zero to  >2 windspeed.

I now want to try to understand how a boat or ice/land yacht can make progress downwind faster than the wind,
This might help:



but it's going to be a lot harder to think through as it's not so easy to see energy being picked up from the water and then thrown out from the sail.
You have to go the a reference frame that moves directly downwind faster than the wind, at the same speed as the downwind VMG of a boat going at TWA 135. In this frame the boat slows down the moving water via keel, and accelerates the air via the sail. This frame is analogous to the rest frame of the DDWFTTW cart.
« Last Edit: 23/03/2013 23:11:47 by MarkV »
 

Offline David Cooper

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my internet connection is too slow to watch the videos so the answer may be in those.
You should really watch those first. This will spare you a lot of guesswork (pay attention to the blade angle vs. rotation).

It simply isn't possible to watch them from where I am, but I will try to view them some time when I am in a better location. My internet connection (3G, but it feels more like GPRS today) is cutting out after each page load and it's taking forever to get anything done online - I suspect the weather's the problem because it isn't normally quite this bad.

Quote
Once the vehicle starts moving faster than the wind, the turbine is from that point on moving the wrong way to act as a propeller.

The pitch and transmission are chosen such that it always acts as a propeller. Nothing changes at wind-speed about that.

Do they flip the blades to a different angle to get round this problem?
No. The prop pitch is always positive. On the prototype the pitch is even fixed, and it still accelerates from zero to  >2 windspeed.

It's all very well saying that, but I've just repeated my experiment and it appears to go directly against that. When I blow at the turbine it turns anticlockwise and when I power it anticlockwise (with it facing in the same direction) it blows the air away from me. When the vehicle's moving faster than the wind, the turbine cannot be being turned by the wind without turning in the opposite direction (if someone else blows the turbine from the other side I see the blades turn clockwise from my side), and if it's being powered anticlockwise it should push air forwards. I'm not trying to be difficult - I'm just trying to understand why my experiment directly conflicts with theirs on this point.

EDIT: I've just worked it out and switched the computer on again to add this, and I also see by rereading what you said that you already know the answer, but I misunderstood what you'd said because I was still seeing it as a turbine. There is no turbine - it never acts as a turbine, and that's the real reason the acceleration is so slow rather than being down to torque. I had thought that it was acting as a turbine to begin with, but it isn't - the whole vehicle is actually acting as a very ineffective sail, but as it gets blown along faster it turns the propeller faster and faster as a propeller, so instead of having air moving downwind through a turbine there is air being moved upwind through it.

Quote
You have to go the a reference frame that moves directly downwind faster than the wind, at the same speed as the downwind VMG of a boat going at TWA 135. In this frame the boat slows down the moving water via keel, and accelerates the air via the sail. This frame is analogous to the rest frame of the DDWFTTW cart.

I'm still struggling to visualise it, and I can't see how a keel can slow down the water as an ice runner isn't going to be able to slow down the ice, but I'll take my time over this and keep studying the diagram you've provided.

This whole thing is a fabulous overthrow of parts of my model of reality though, and that's exciting. It's a pity it takes five years for something like this to get through - makes me wonder what other revelations I've been missing out on.
« Last Edit: 24/03/2013 02:35:47 by David Cooper »
 

Offline MarkV

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There is no turbine - it never acts as a turbine, and that's the real reason the acceleration is so slow rather than being down to torque. I had thought that it was acting as a turbine to begin with, but it isn't - the whole vehicle is actually acting as a very ineffective sail, but as it gets blown along faster it turns the propeller faster and faster as a propeller, so instead of having air moving downwind through a turbine there is air being moved upwind through it.
Yes. To increase the intial acceleration you could use the rotor as a trubine below windspeed. This is what Andrew Bauer did on his DDWFTTW cart in the 60s:
newbielink:http://projects.m-qp-m.us/donkeypuss/wp-content/uploads/2009/06/Bauer-Faster-Than-The-Wind-The-Ancient-Interface.pdf [nonactive]

From what I heave read this guys opted for a one way transmission for several reasons:
- To prove steady state you want to exclude the possibility that stored momentum in the spinning rotor can drive the wheels.
- Installing ratchet hubs at the wheel axle allows differential wheel speeds in turns. A ratchet hub at the propeller shaft allows to brake the vehicle, without having to brake the propeller rotation (quick emergency stop).
- When you switch the direction of the power transmission, the chain slack has to go from one side to the other. Bauer had problems with the chain jumping off here.
- It is simpler: You don't need to build a variable pitch mechanism (although they added one later on, but it doesn't go negative)
- It makes a much better brainteaser if it starts in a tailwind, with the rotor turning the opposite to what most people assume.
Quote
I'm still struggling to visualise it, and I can't see how a keel can slow down the water as an ice runner isn't going to be able to slow down the ice,
If you apply a force to the earth (or the ice) there is always some tiny acceleration. In a reference frame where the earth moves this can slow down the earth a bit and extract energy. You don't notice the acceleration, because the huge mass of the earth. With fluids only a small mass is affected, so it is more obvious where the energy comes from or where it goes to, because the acceleration is noticeable.
« Last Edit: 24/03/2013 14:38:02 by MarkV »
 

Offline David Cooper

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More things are beginning to fall into place for me. In my thought experiment with the two tracks in space where the vehicle can move without any drag, it can accelerate up to the speed of light in either direction by tapping into energy from the relative motion of the two tracks to each other. In our Earthly experiments we're working with two tracks of a different kind though where one is air and the other is either land or water, and because one of the tracks is air, we are going to have trouble putting the power down against it efficiently, added to which there will be more drag going upwind than downwind, so this suggests to me that the speeds should always be higher downwind, thereby explaining why there's more advantage in using a hydrofoil downwind while it costs speed through added drag going upwind if the boat can't go fast enough to rise up on the foils.

Another thing I realise now from looking at the diagram you provided (with four boats and lots of vectors shown) is that the trailing edge of the sail is actually upwind of the leading edge (going by the true wind), just as the propeller blades on the wheeled machine are set to act as propeller blades rather than as turbine blades. In the case of the wheeled vehicle it's easy to see how the wheels are being forced round and are forcing the propeller to turn to push air backwards, but it isn't at all easy to see how it works with the boat and I'm still not there.

With a beam reach (90 degrees to the wind), it's easy to work out what's going on - a continual acceleration force is being applied by the wind by pushing against the sail and being deflected to the side, so if all friction could be eliminated the boat could keep on accelerating to the speed of light.

Another way of looking at it is like a wet bar of soap. If the two opposite sides are exactly parallel, it can slip out of your hands, but pressing harder won't accelerate it, however if the sides aren't parallel the pressure will cause an acceleration, and the harder you press the faster it'll accelerate. The keel/daggerboard is one of the surfaces being pressed and the sail is the other, and they are angled such that the pressure on these two surfaces will clearly accelerate the boat forwards.

In the case of the boat travelling at 135 degrees to the wind, this bar of soap analogy still fits, but because the boat is making downwind progress faster than the wind, it's hard to see how it can still be being squeezed from both sides. The diagrams clearly show that it is still generating a forward force, and that's all it needs to accelerate beyond the point where progress downwind is faster than the wind, but I'm still struggling to see how the power can really still be on in this way - it still feels as if it should be impossible, even though it clearly isn't. The sail is not backing - it's still being pushed out in a downwind direction by the wind (even though the wind is moving slower downwind than the sail) while water is still pushing against the keel and making the whole thing act as a wet bar of soap, so clearly it is still undergoing a force that will try to accelerate it. The sideways movement of the boat across the wind is what keeps the sail from backing, and that keeps the acceleration force active, but it seems strange that the boat's existing movement can generate this additional acceleration force. This is the final barrier to me understanding this properly.

With the wheeled vehicle we have a similar situation, but it all adds up more clearly: the wind is the first part of the chain of causation, pushing the vehicle forwards, making its wheels turn which makes the propeller turn, and it goes on working beyond the speed of the wind because the propeller is always pushing against air which is moving downwind rather than starting out with the speed of the ground. With the boat it must be the same: the sail is being driven through air which is moving at the speed of the wind and not the speed of the stationary water, so it's very different from a sail pushing stationary air where there would be no gain of energy - that must be the key to how this force is still being tapped.

Yes, I think I may be getting there. If you look at the diagram with the four boats and the vectors, imagine holding the boat in still air and moving the boat directly sideways through the air: you apply a sideways force to generate a forwards force and lose out overall to drag. Now do the same with the keel in water and you generate more forwards force while losing out to more drag. In both cases it isn't sustainable and the boat would slow to a halt if you let go of it. Now try it again though with the water moving while keeping the air still. The water is flowing towards you from ahead while the air is still. You now push the boat sideways again with the keel in the water. This time the keel is being pushed from the other side which you'd think would make things worse, and actually it does in a way, because the boat is now going to use the force it generates just to maintain it's position relative to you (in upwind/downwind terms, with the air being stationary) - previously the boat was being deflected ahead of you at high speed when the water was stationary with the air. That is one major difference, but the other is that the effect of the moving water keeping the boat level with you is that its sail is being driven through air which is moving relative to a boat in a very different way from in the case where the water wasn't moving relative to the air, because in that case the boat was deflected forwards and its sail was hitting air which was moving fast and backwards relative to it. In this new case with the water moving, more energy is necessarily tapped from the relative boat and air speeds, ensuring that an acceleration force remains in place continually.

That is it - a plain words explanation without any confusing abstract maths. I can expand further upon it by comparing it to what happens when you are swimming. When you start the length, you push away from the wall and it's easy to gain a lot of speed by doing so, but if the wall started to move away from you at a fixed speed just as you started to push against it, you'd get less energy from the push. If the wall moved with you as you pushed off, you'd get more energy from it. That is the mechanism by which the boat can continue to accelerate beyond the speed of the wind, because it is still tapping that extra energy that comes from the fact that the air is moving at a different speed from the water, keeping its speed closer to that of the boat.

Okay - I'm happy with it all now, though I wouldn't be surprised if I'm still missing something. What I'd particularly like to see now though are diagrams of tables showing different angles to the wind and boatspeed, ideally for many different kind of craft (including slow monohulls). If anyone stumbles upon anything of that kind, please could you post a link to it here.
« Last Edit: 24/03/2013 18:57:16 by David Cooper »
 

Offline MarkV

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Once you have faster internet you should check the animations. They pretty much show what you are trying to grasp intuitively.

In the case of the boat travelling at 135 degrees to the wind, this bar of soap analogy still fits, but because the boat is making downwind progress faster than the wind, it's hard to see how it can still be being squeezed from both sides.
The squeezed wedge for downwind VMG > WS is shown here:

Now try it again though with the water moving while keeping the air still. The water is flowing towards you from ahead while the air is still.
This animated diagram shows downwind VMG > WS from 3 different reference frames:
Note how that air is still being slowed down in the frame of the surface. To extract wind energy you have to slow down the true wind (reduce the velocity difference between air and surface).

Here is an article explaining this as well:
newbielink:http://rightnice.blogspot.de/2010/08/racing-wind.html [nonactive]

What I'd particularly like to see now though are diagrams of tables showing different angles to the wind and boatspeed, ideally for many different kind of craft (including slow monohulls).
The diagrams are called "polars". You can search google images for them.

Here is one for slow mono hulls:
newbielink:http://www.sailonline.org/static/var/sphene/sphwiki/attachment/2011/06/25/TallShipPolars.png [nonactive]

Here for skiffs that achieve downwind VMG > WS:
newbielink:http://www.yachtblick.de/www.yachtblick.de/wp-content/uploads/2012/02/BoatAndWindSpeed-custom-size-660-440.jpeg [nonactive]

Didn't find any for ice-boats or land-yachts, which easily achieve downwind VMG > 2WS.
« Last Edit: 24/03/2013 22:34:30 by MarkV »
 

Offline David Cooper

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Once you have faster internet you should check the animations. They pretty much show what you are trying to grasp intuitively.

Will do.

Quote
Here is an article explaining this as well:
http://rightnice.blogspot.de/2010/08/racing-wind.html
I see there are links to videos at the bottom of that page too - if they're different ones I'll check them too when I can get better Web access.

Quote
The diagrams are called "polars". You can search google images for them.

Thanks. That's the vital word I lacked, and this is one of the particular things I was looking for:-

http://www.thebeachcats.com/OnTheWire/westnet/_lpm/hobie/archives/v1-i3/feature3.htm

The top two are catamarans, and I particularly wanted to see how well the Tornado (until recently an Olympic class boat) races the wind - the answer is that it just about achieves the speed of a ten-knot wind at 135 degrees and will therefore fall far short overall, though at 130 degrees it goes 10% faster than the wind and actually makes slightly faster progress downwind. The lowest part of the line is the point where progress downwind is greatest.

Actually there's a serious flaw with those diagrams which makes them very misleading. If you look at the one for the Soling (Olympic class monohull), it looks as if it can gain by tacking downwind, and the text below claims that all six of these boats do gain by doing that, but if you apply pythagoras/trig to the diagrams you should be puzzled at how a point where the wavy line hits 8 knots on th 135 degree line can be further down the diagram than the point at 6 knots on the 180 degree line. The answer turns out to be that the numbers appear to have been put on the wrong lines with 4 actually being 3, 5 being 4, etc., so either the speeds are wrong or the graph is distorted, and I suspect the former because if it was the latter there would be no advantage for the Soling in tacking downwind. Clearly you need to keep your wits about you when looking at polars.

Edit: I've just seen the links you've added. It's interesting to see the Class C ketch in the tall ships one - in 3 knots and 20 knots of wind it should tack downwind, but at 12 knots and 30 knots of wind it should go directly downwind. You really need to know your boat's polars.
« Last Edit: 24/03/2013 23:29:02 by David Cooper »
 

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