Blowhards Sailing Downwind

Gulp!

I read too much of that thread and became addled. What John said is so amazingly simple and obvious, and the people that were arguing against it so confused, that it seems to have rubbed off on me.

John merely said that an object could be wind propelled directly into a headwind by way of say, a windmill turning a gearbox and some wheels.

As long as the widmill is allowed to orient itself into the wind (they usually swivel) the wind powered wheeled vehicle can go in *any* direction.

--- Joe

Yes, I did say that. It's fairly obvious. What I don't know is how fast it could move windward, and whether it could actually move faster upwind than the wind speed. Apparently people have hit numbers like

The straight downwind, faster than the wind, case probably works too. It is sure counter-intuitive.

Yup.

John

Light sailing boats can sail faster than the wind but not downwind or with the wind abaft the beam. They are not reliant on the wind pushing the concave side of the sail but the combined effect of low pressure on the convex side of the sail, (Bernoullis Law) and the centrifugal force of the wind rotating in the concave curve of the sail.

Feel free to go f*ck yourself, you ostentatious windbag!

inst

So, what did you conclude about sailing downwind faster than the wind? I read quite a few posts but still didn't figure it out. It seems the propeller prop extracts kinetic energy from the moving car and propels it faster, but that doesn't seem to provide any gain of kinetic energy. How does the car accelerate faster than the wind going downwind? Eventually, there would be a headwind if the car moved faster than the wind, which would provide a drag. How do you explain it in simple terms?

-Bill

Imagine you're in the car and the groundspeed of the wind is 10 MPH. The car is moving downwind at 10 MPH. The relative wind, what you feel, is zero. You have a big lazy pusher propeller on a pylon, and it sees zero relative wind.

So connect a generator to one wheel and make electricity. And use that to turn a motor to spin the prop, in the push-the-car-downwind direction. (Or use a mechanical linkage, same thing.) The wheel is spinning fast, so generates a decent amount of power. You only have to turn the prop a little to make thrust. The result is acceleration downwind.

The trick is that the prop is pushing against the tailwind, and against a zero relative wind. It doesn't have to work very hard to make a goodly chunk of thrust... less than the wheel drag needed to make the power.

Another way to look at it: instead of a tailwind, imagine a truck behind you, going 10 MPH and pushing your car. You pull out a big hydraulic jack and arrange to mount it on your car and push against the truck. Get the hydraulic pressure to pump the jack off one of your spinning wheels. Wheel spins, pumps jack, pushes the car and the truck apart. Now the car is moving faster than the truck, ahead of the truck. Same idea: push against the tailwind instead of the truck, use a prop instead of a jack.

You can do this in an electronic circuit, too:

Imagine the 12 volt battery is equivalent to a 12 MPH wind. Suppose the DC/DC converter inputs 17 volts (on the right) and outputs 5 volts (on the left). The stacked 12v battery and 5v dc/dc output make 17 volts, namely V+. The 17 volts (17 to ground!) powers the input of the dc/dc converter. The voltage stepdown allows a current step-up, so there's plenty of V+ amps available to power the converter input. Tricky, but no paradox, no violation of conservation of energy. You could buy a $6, 12v to 5v isolated dc/dc converter from Mouser and build this.

You can push that around a little

and simulate a car that uses a propeller to drive the wheels and move into the wind.

Cute trick, but not worth getting obsessed about. As some people are.

John

Really??? Rather than a prop, use another wheel, a 5th one, driving against the pavement. Why not?

Is this a joke?

Imagine a calm, no-wind day, and a propeller, powered by a generator attached to one of the wheels, pushes a car along.

Sailing faster than the wind in 4 lines:

P=F.V landspeed is greater than airspeed. for the same ampunt of power less force (drag) is exerted on the ground than is produced (as thrust) at the propellor

P=F.V

Vacuum motors :)

Because the pavement is moving 10 mph relative to the car. So it takes a lot of power to push the cart using a driven wheen, obviously more power than you can extract from a different wheel. But it's easy to push against the tailwind because it is moving AT THE SAME SPEED as the car. It's like pushing against the truck in my example.

It's just an impedance matching problem.

To you, apparently.

Wind-powered cars tend to not work when there's no wind.

The car is WIND POWERED. It extracts energy from the wind. It violates no conservation principles.

You snipped my truck and electronics examples.

John

But so what?

You jumped that conclusion out of nowhere.

easy to

Your truck is a towtruck. It has a long flat bed, and it's carrying the car. Now the car sees it's speed over pavement as zero. A generator on a wheel of the truck can power the car's propeller, and the car will help push the truck faster. You can probably see that the car really isn't needed, the prop can be fixed directly to the truck. Now you're going to become rich selling this miracle invention that produces extra power and increases fuel mileage out of nothing.

Mine is going to work just like yours, it just goes 10mph slower.

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Yes, good ideas. But it the case of the truck pushing the car, there seems to be some energy storage prior to using the jack to push the car away from the truck and go faster than the truck. You could use the same idea and run a generator to charge a battery for a couple hours (while being pushed by the truck), and then use a motor to go much faster than the truck. So, it appears possible to go much faster than the wind downhill, using some energy storage technique supplied by the wind. You might use smaller increments of energy storage, say charge the battery for 1 second, and then run the motor for 1 second and work your way down to the point where the battery and motor are not needed and energy just flows from the wind directly into the car with no storage. That's the hard part to visualize, but seems possible.

-Bill

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I can't see any storage, beyond the momentun of the car itself. The wheel drives a hydraulic pump which operates the jack. It will work for as long as the jack can extend.

You could use

Certainly. You could park for a while, raise a windmill, and store energy. Then haul it down, tuck everything in, and run like hell on the stored energy. That could go much faster, on average, than the wind, and probably faster than the beanie-propeller go-kart thing. But the BeanieMobile can do it steady-state with no stored energy. *Why* do it is a whole nother issue.

John

The jack.. what's it really doing?

(1) Speeding up car + (-1) Holding back truck

----------------------------- *ZERO*

Pushing the car to go faster than 10 MPH. Using power from a wheel.

I assume that the truck can maintain 10 MPH because, well, it's a truck. And because I said it was going 10 MPH.

Same thing with pushing against a 10 MPH tailwind. The only problem there is that the prop isn't as efficient or as stiff as a hydraulic jack, but it seems to work anyhow.

John

The "power from a wheel" is from either the car engine or the truck engine.

Well, a Bentley outweighs an S-10, so you goofed.

You're tapping off power at the front end to re-apply it at the back, and some gets wasted along the way.

Naturally. It's from the truck engine. Or in the other case, from the tailwind.

I said the truck was going 10 MPH. You don't think an S-10 can be driven at a steady 10 MPH, even pushing a Bentley? Use the cruise control!

Besides, the BeanieMobile is small and very light, and it has a big prop so is pushed by a lot of wind cross-section.

Sure, a little in the linkages. But you still come out way ahead.

You don't seem to be getting it:

You get more power from the wheel than you need to run the prop to accelerate past 10 MPH. That's because the prop is pushing against the tailwind, so it doesn't need much shaft power to make a lot of push. My electronic examples work on exactly the same math.

OK, I've done my best.

John

Now that power isn't delivered to the truck's rear axle.

You imagine the car is pushed to a full 10 MPH in a 10 MPH tailwind. That's impossible. Matching the wind speed at 10 MPH, the car feels no wind at its back. No push from behind, no wind blast up front. It's in still air.