Downwind faster than the wind. Poll

[RAI]

The boats are so efficent they can bear away significantly and the apparent wind is still forward of the beam. At some point, however, the boat is travelling too far downwind and there is not enough wind from the side in order to drive the boat. Imagine the boat travelling at 20 knots on a broad reach in 6 knots of wind. There is still some (true) wind coming over the side of the boat. The speed of the boat will bring that apparent wind well forward of the beam. But if the boat bears away to DDW, the vectors simply cancel each other out. The boat is now, in effect, head to (apparent) wind, and can't sail. Just as there is an optimum tacking angle upwind, for these boats there is an optimal gybing angle downwind (they don't tack downwind, they gybe downwind).

Tack/Gybe -A moot point, since the apparent wind for these boats stays ahead of the beam.

These boats do NOT use the mechanism on which the DDW advocates rely - which is going straight DW, and extracting energy from the kinetic energy of the vessel and impart it into the air to produce thrust.

Are the propeller blades on this machine going dead down wind, even if their axle is?

 

[bbg]

So you admit that a vehicle could be designed, in theory, to progress DDWFTW - using reciprocating aerofoils or similar to replace Oracle, or even consider just the 'waterskier case'? It is just this particular design you are unhappy with?

MD

No I don't admit it. Quite a leap isn't it, for me to point out that two mechanisms are different, to you believing I have admitted something? I don't rule out the possibility that such a vehicle could be designed. But it can't be theoretical - I'd have to see how it purports to work exactly. The waterskier is a red herring. The vessel powering her is not moving DDW, so it doesn't fit the design criteria of the case.

I find it interesting that the true believers talk about "faster than the wind", but in fact they are largely talking about the boundary condition of "as fast as the wind". And they never explain how the vehicle gets up to wind speed in order for the "extra boost" to be available.

15 days to go.

 

[halfway]

mobeydick: That is a terrible assertion.

The political tool which is the banned subject has no basis in engineering or science. The destruction we are doing to our planet is not due to burning oil, it is to do with what we do with the energy it provides. In fact the best way to help this planet would be to burn all the oil as quickly as possible! Then the real cause would disappear.

It took nearly 2 days for me to work out what was going on here, mainly due to a lot of bad info. But I have been looking at the world and its destruction, first hand, for 13 years now and I see no evidence to support the other theory.

Anyway I will post a use for this "technique" on the other thread that might interest you if you are into oil free society.

 

[bbg]

Tack/Gybe -A moot point, since the apparent wind for these boats stays ahead of the beam.

No, it isn't a moot point. It is correct terminology. In the same way that port doesn't become starboard when you are facing aft.

 

[xxyyzz]

And leave out Oracle too, tacking downwind is not what is under discussion.

Actually, no, Oracle is an important proof.

It has mass, it has friction against both air and water.

It moves from point A to point B where point B is directly downwind of point A.

It moves this mass faster than wind speed.

If a mass is _OBSERVED_ to have moved from one point to another against friction in a certain time then the energy has been supplied to make it possible.

It may appear that the energy is not there, indeed when this thread started I doubted it as well, but the fact is that the mass has been moved (in actual fact - where theory hits fact, fact wins) and so the energy _MUST_ be available.

It's perfectly reasonable to argue that this machine can't extract that energy efficiently enough, but the energy is available. To argue otherwise is to argue that Oracle gets additional energy from somewhere.

In short, the energy is proven to be available.

 

[Keen_Ed]

These boats do NOT use the mechanism on which the DDW advocates rely - which is going straight DW, and extracting energy from the kinetic energy of the vessel and impart it into the air to produce thrust.

Just depends on your frame of reference. Is the air still and the water moving, or vice versa?

 

[snowleopard]

"..different wind and water velocities to work". That's ridiculous.

A Boat is sitting in a 5 knot tidal current. The wind is blowing at 5 knots in the same direction. Can you make the boat sail? - No.

The tidal current is 5 knots but it's a calm day. Can you make the boat sail - yes. What's more you can sail down tide faster than the tide. That's because there is a difference in the velocities of wind and water.

Now let's take a case where the wind is acting on the sails and we take the velocity of the water out of the equation: put a mast and sails on a hovercraft. There is no way for the vessel to get a grip on the water so all it can do is drift downwind like a balloon.

 

[RAI]

No, it isn't a moot point. It is correct terminology. In the same way that port doesn't become starboard when you are facing aft.

You obviously sail your boat different to the way I do mine. When I have the apparent wind ahead of the beam, I tack to move the apparent wind from port to starboard of the bow (or reverse). When the apparent wind is behind the beam, I gybe to get the apparent wind to change sides across the stern. Somewhere else in the world the wind is blowing in a different direction, that doesn't change my apparent wind or my definition of tack and gybe.

By the way, are the blades of this contraption travelling dead downwind?

 

[snowleopard]

I looked at a GPS the other day. When I took it out of its box I could see it couldn't possibly work because there are no wires to connect it to anything so it can't possibly get information to tell it where it is. There's no point in turning it on and trying it because I know it can't work.

Various people tell me they've seen one in action but they must be lying and all the films and photos of it are obvious fakes. There are hidden wires or it's just a box with numbers printed on to look like the real thing. All this talk of satellites up in space is a nonsense. It's a vacuum up there so how could information be transmitted?

Anybody who buys one of these things must be really stupid. If it were true we'd be able to put two of them together and travel at the speed of light. Hey suckers, phone me when you get to Mars.


Sound familiar?

 

[RAI]

I looked at a GPS the other day.

Me too, doesn't work, stupid thing. Even when you do switch it on. When I'm here, it tells me I'm here, but I know that. When I go away, it doesn't know where I am. Useless. It simply doesn't understand reference frames, let alone relative velocity.

P.S. Are you supposed to take it with you?

 

[Keen_Ed]

Take it with you? Ridiculous thought. How's it supposed to know where it is if you move it?

 

[halfway]

1.

"Tack/Gybe -A moot point, since the apparent wind for these boats stays ahead of the beam."

They are completely different. Just draw the diagrams of the 45 degree cases.

Then draw a chord through the sails.


In the upwind diagram the wind is "pulled", from 45 degrees to the bow, toward the centre line of the boat. Max change about 15 degrees. The sail is basically INLINE with the wind and its generated force must stay in front of the beam.


In the downwind case the wind is "pulled" from 45 degrees to the stern, to a point near the bows. Max change about 90 + 15 degrees. The angle moved is much much bigger than the upwind case. The sail chord is basically at RIGHT ANGLES to the original wind and its generated force must stay in front of the beam.


So that is why wind vanes work to windward but not so well down wind. Unless you are a slow boat w.r.t. the wind.


2.

Please stop using the argument that it is the wind relative to the ground that makes the energy. Because that means there is no limiting speed to the system. Just keep reducing the drag and you will go for ever faster. This can not be the case. The use of this argument is why people are having so much difficulty with the concept. It is wrong.

The theory is just down to apparent wind angles seen by the blades of a forced to rotate prop. Just like the apparent wind angles a yacht experiences when it is forced to sail on a 45 degree gybe.

3.

We already take energy out of our motion to make a boat go down wind. Take the 45 degree case of slower than the wind downwind sailing. The boat WILL slide sideways to our keel direction. So the resultant apparent water flow is offset from the boat motion. The keel will therefore provide lift to oppose the side slipping. You can increase the attack angle by using the trim rudder on the back of a yacht to angle the keel a little more and control the final motion to the desired gybe angle.

The rudder and keel are both using forward energy to change the yacht angle so that the sail sees a nice apparent wind. The keel is not just a weight to make the mast stay upright.

So why can a car not use the power taken from a wheel to cause a prop to rotate so that its blades experiences a nice apparent wind?

 

[RAI]

Well anyway, I was thinking if I get a big enough wind generator on my boat and an electric motor to drive the prop., plus a bit of electronics and some extra batteries, I should be able to power downwind, at least at hull speed. Does that count as DDWFTTW? On a calm day?

 

[colingr]

Well anyway, I was thinking if I get a big enough wind generator on my boat and an electric motor to drive the prop., plus a bit of electronics and some extra batteries, I should be able to power downwind, at least at hull speed. Does that count as DDWFTTW? On a calm day?

No because the amount of energy collected by the generator (turned into electricity) cannot exceed the amount of energy needed to drive the boat so that your batteries will steadily go flat. You could add some solar panels plus an exercise bike to top up the batteries but then you would be cheating.

 

[RAI]

No because the amount of energy collected by the generator (turned into electricity) cannot exceed the amount of energy needed to drive the boat so that your batteries will steadily go flat.

Not if I recharge them every day on a mooring. Afterall, all the energy is coming from the wind. I'm just storing some of it and using it later.

 

[Keen_Ed]

Please stop using the argument that it is the wind relative to the ground that makes the energy. Because that means there is no limiting speed to the system. Just keep reducing the drag and you will go for ever faster. This can not be the case.

Except, that of course, this is exactly true. Reduce drag and your boat will go faster. Why do you think racing sailors spend so much of their time polishing the bottom of their boats? Why do they drysail 45' boats? To reduce drag. To go faster.

You can't, of course, reduce drag to zero, because we live on a planet with things like air and water. Even parachuters in free fall have a terminal velocity.

 

[colingr]

Not if I recharge them every day on a mooring. Afterall, all the energy is coming from the wind. I'm just storing some of it and using it later.

That would work but:

It contravenes the original remit of using the wind to power a vehicle dead downwind at more than wind speed. You are adding in stored energy, albeit originally derived from the wind. You are in fact using the wind to extend the range of a battery driven vessel.

 

[Keen_Ed]

1.

"Tack/Gybe -A moot point, since the apparent wind for these boats stays ahead of the beam."

They are completely different. Just draw the diagrams of the 45 degree cases.

Then draw a chord through the sails.

So what do you call it when a foiling moth gybes - staying foiling the whole time - and the apparent moves around the bow. Just search Youtube for moths gybing to see.

 

[halfway]

"Well anyway, I was thinking if I get a big enough wind generator on my boat and an electric motor to drive the prop., plus a bit of electronics and some extra batteries, I should be able to power downwind, at least at hull speed. Does that count as DDWFTTW? On a calm day? "

It is the other way around. You put a generator in the water and power the air prop. You just need enough energy to rotate the air prop depending on your forward speed.

The air prop then sees an apparent wind that is favourable and will produce lift in the direction of travel.

On a calm day your forward speed would create an apparent wind that had no angle from the bow so could not be used to create lift.

That would be the same as trying to sail down wind on a calm day. As soon as you moved the apparent wind you cause, would be exactly from the direction you are heading.

 

[snowleopard]

Please stop using the argument that it is the wind relative to the ground that makes the energy. Because that means there is no limiting speed to the system. Just keep reducing the drag and you will go for ever faster. This can not be the case. The use of this argument is why people are having so much difficulty with the concept. It is wrong.

There is a finite amount of energy in a column of air travelling at the speed of the wind. It is the mass of that column of air x 1/2 the square of the wind speed. That is what is available to drive the machine. You can extract more energy by using an efficient prop and you can reduce the drag by aerodynamic improvements and reducing friction in the mechanism. There is a limit to the amount of energy available, a practical limit to the amount that can be extracted and a practical limit to how far drag can be reduced. When the drag equals the amount of input energy you can go no faster. That doesn't invalidate the principle.