Sailing downwind - faster than the wind?

pmagowan said:
If the propeller blades all wizzed off in their various directions then they are just small sails. If they are attached to a moving vehicle being proppelled directly downwind then, as has been said on numerous occasions, when the apparent wind hits close to zero they will stop as no air is being driven through them to 'propel' them.

I understand that this is not the method being proposed as the wheels and stuff is involved but as per my previous post this is irrelevent and would only cause the prop to slow quicker.
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Clearly, you still have not twigged the fact that the propeller on this contraption is going in the opposite direction to that which the wind would drive it were it a windmill.
 
fireball said:
But the propeller is moving across the wind is it not? It isn't moving with the wind (ala paddlewheel) ... and as one is moving across one way the otherone is moving across the otherway ... so countering any sideways thrust ... therefore all thrust is directed forward no? It does look like tacking ... just with 2 boats ....
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This thinking is on the right track but not quite there. The propeller is not taking the place of sails tacking. It's taking the place of the keels of the boat.
On a boat sailing close to the wind, the keels or leeway resistance are providing force to take the boat to windward (not directly to windward). They are the equivalent of the the propeller blades which drive the cart through the air.
 
nmeyrick said:
This is where the problem enters. Yes there is relative motion between the vehicle and the road. Yes this could be used to turn the wheels and, via a gearing to drive a prop. But the relative motion is the result of the vehicle's kinetic energy, and since the vehicle is powered only by the wind, its kinetic energy comes from the wind.
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How many times oh lord, how many times?

The energy does not simply come from the wind. It comes from the different velocities of wind and water. Stick a toy sailing boat in a bath on a train and tell me how fast it sails when the air around it is moving at 100+ mph.

You are claiming that the wind makes the vehicle move, which turns the wheels, which powers the vehicle. That is perpetual motion.
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No it's not. There are two types of perpetual motion machines. The first produces continuous motion with no power input and the second produces net power out. Neither of these is the case here: the devices under discussion will leave both wind and water wakes.

But it aint being powered only by the wind.
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And the water skier moving straight downwind at faster than windspeed towed by BMW Oracle. What's powering her?

Here's a related question for you. Can an unpowered vessel being towed in a straight line on a calm day travel faster than the tug?
 
pmagowan said:
It is like driving a car with a preset fuel consumption on a flat surface. It will reach an equilibrium at a certain speed. Ahh, but the ground is moving relative to the vehicle so if we lower a little wheel on to it with a dynamo attached we could get energy from this to make the car go faster??? No you can't, drag slows car, energy gained must be less than energy lost (law of conservation of energy) ergo car slows. The fuel in the previous scenario is wind.
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Sorry, but no. That doesn't apply, because there is no speed differential to exploit.
 
pmagowan said:
Ahh, but the ground is moving relative to the vehicle so if we lower a little wheel on to it with a dynamo attached we could get energy from this to make the car go faster??? No you can't, drag slows car, energy gained must be less than energy lost (law of conservation of energy) ergo car slows. The fuel in the previous scenario is wind.

Nice little thought puzzle tho!
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Not quite right. The wheels of the car turn and drive the car along the road.
But, (and this may sound weird). It's just as valid to say that the car stays still and drives the road past itself.
When you are driving in a car look around you. The inside of the car appears stationary, but the world outside the window is going past! The only thing that counts is the difference in velocities. Road driving car - car driving road - it's exactly the same thing. So it is with the downwind cart.
After the cart reaches windspeed it is stationary in its airmass, and the movement of the road drives the prop to move it through the airmass.
 
Ubergeekian said:
How many times oh lord, how many times?

The energy does not simply come from the wind. It comes from the different velocities of wind and water. Stick a toy sailing boat in a bath on a train and tell me how fast it sails when the air around it is moving at 100+ mph.
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Oh Lord: Water velocity equals zero, wind velocity = x. Wind is the energy source. The bit about bow waves making this not a pepetual motion machine is reverse thinking. Not only is is making more energy than is entering the system and manageing to make the vehicle move forward at God knows what speed it is also making waves which are furthur energy loses. This weakens not strengthens your arguement. I would be happy to assume no waves and a frictionless world because it would still not work
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So what is the energy?

There's been lots of talk about 'energy' and glib remarks about having to create it. Let's have a look at what energy is available in the system and how much is required to push the vehicle along. I will take the case where the vehicle is travelling a minute fraction faster than the wind over the ground.

The energy available in the mass of air passing through the swept area of the prop at the velocity of the wind relative to the ground.

For the sake of argument I will take a velocity of 5 m/sec, a nice force 3.

Using the outdoor version in the videos, the prop diameter is around 1.5m and I will make a generous assumption that it requires a force of 5 newtons or 1/2 kg to overcome rolling resistance.

The mass a cylinder of air 1.5 m diameter and 5 m long is 10.6 kg. When moving at 5 m/sec it has a kinetic energy of 132 Joules. Altering the velocity of that mass of air from 5 m/sec to zero gives us an available energy of 132 joules/sec.

Now how much energy is required to overcome the rolling resistance of the vehicle. Our generous allowance of 1/2 kg resistance = 25 joules per sec.

So the available energy exceeds that required by a factor of 5. No need to call in the laws of thermodynamics.
 
snowleopard said:
There's been lots of talk about 'energy' and glib remarks about having to create it. Let's have a look at what energy is available in the system and how much is required to push the vehicle along. I will take the case where the vehicle is travelling a minute fraction faster than the wind over the ground...



...So the available energy exceeds that required by a factor of 5. No need to call in the laws of thermodynamics.
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I don't even think you have to go as deep as this.
Assuming we take as correct the statement that the wheels are driving the prop.

From start - the wind will move the cart slowly - but given a minimal resistance due to friction should start to build up towards windspeed.

As it builds speed (SLOWLY because the available force to push it is low and the angular momentum of the prop is huge) a column of air thrusting backwards is developing.

This column of air pushes against the wind creating a reaction and increasing the "surface" against which the wind can act. One could visualise it like two columns of air pushing against each other - the propellor-generated one slowly buiilding to its maximum.

Eventually the cart reaches a downwind speed where it is getting maximum thrust out the prop (maximum air column velocity/length) and therefore has reached its final, maximum "sail area".
The system of forces is balanced, (wind against prop air column/"sail area" and friction losses) and an equilibrium is reached and the cart can go no faster.

Because the air column from the prop is now balanced against the wind it cannot go any faster than the wind.

It's a bit like a ping-pong ball sitting on a vacuum cleaner set to "blow", the bit in the video of the real demo is like the bit where the vacuum cleaner is turned on! Eventually it reaches its maximum distance from the nozzle and THATS the accelleration you are seeing on the cart video. What you are not seeing is that it is now only sitting there in the airstream from the nozzle (i.e. from the wind).
Is the ping-pong ball going faster than the blast from the vacuum? no!
Is the cart going faster than the wind? No!

In the more convincing of the vids, with the cart with a flag on a stick behind it it is no surprise that the flag streams upwind - it is in the air column from the prop and out of the real or apparent wind!
If the flag had been further back I suspect it would have streamed downwind or at best hung down.
It would have been acceptable if the flag had been to one side of the cart out of the prop airstream.
As it was, given the flywheel effect of the prop and the tiny forces available to get the whole system going, it cannot be domonstrated that the cart actually reached windspeed or greater in the distance available. particularly in light of the dubious, nay erroneous "windspeed indicator"
 
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Speculating entirely here.

I can understand yachts sailing faster than the wind at various angles to it but dead down wind.

Assuming perfect conditions, if a vessel is sailing at 9 knots in 9 knots of breeze, to go any faster is to then create a negative pressure that a vessel will begin to push against which it will be unable to do so which will keep the vessel at wind speed.

Anyway next subject - as this is way over my head
 
I'm probably going to regret this.

I can'y get the video links where I am so I apologise.

I can't see the cart would accelerate to even reach the constant wind speed as the relevent velocity of the wind over the blades reduces as the cart accelerates, then the force on the blades driving it reduces, until at some point where equilibrium is reached and the cart is travelling at a velocity lower than wind speed. Any extra energy derived from the wheels would then need to be converted to thrust at the propellor which would counteract the thrust already at the propeller! That would only be achievable if the blades could be turned to provide thrust in the opposite direction to which the cart is moving, so that the cart is being pulled along by the propellor . The cart would slow down and continue to oscillate in this manner as one force is trying to overcome the other. If you consider a square sail which could be pushed along at wind speed with a propeller in front of it to be engaged when the cart is at wind speed, any extra thrust generated is immediately counteracted by the drag of the sail. In all circumstances the cart will not go faster than wind speed.
I'm assuming with Oracle and the tacking, the wind speed is constant throughout, whereas the wind speed over the blades of the cart reduces as it speeds up.

I'm probably way off base, but hayho!
Bob
Didn't the guy who invented the gyrocopter have some fancy way of altering the blade angles so he didn't get tipped out.
 
snowleopard said:
The mass a cylinder of air 1.5 m diameter and 5 m long is 10.6 kg. When moving at 5 m/sec it has a kinetic energy of 132 Joules. Altering the velocity of that mass of air from 5 m/sec to zero gives us an available energy of 132 joules/sec.
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Joules/sec are conventionally called "Watts" - the SI unit of POWER - and you need to bring time into it for that.
 
pmagowan said:
Ubergeekian said:
How many times oh lord, how many times?

The energy does not simply come from the wind. It comes from the different velocities of wind and water. Stick a toy sailing boat in a bath on a train and tell me how fast it sails when the air around it is moving at 100+ mph.
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Oh Lord: Water velocity equals zero, wind velocity = x. Wind is the energy source.
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You're almost getting it, I'm glad to say. If you use a reference frame fixed to the water then the wind is indeed the energy source. Use a reference frame moving with the wind and the water is the energy source. Once again it's having a velocity difference which matters. If the water and the wind are moving at the same speed, whether that's zero (pond on a calm day) or a hundred miles an hour (bath in a train) sailing will not occur.

Say it ten times: Kinetic energy is a relative quantity
This weakens not strengthens your arguement. I would be happy to assume no waves and a frictionless world because it would still not work
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What's the problem with bow waves? The wind loses energy.
 
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Energy

mikemonty said:
I don't even think you have to go as deep as this.
Assuming we take as correct the statement that the wheels are driving the prop.
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That's a partial statement, but too many people are getting hung up on it.

Remember that there are two sorts of work associate with a propellor. There's the shaft work required to turn it, and there's the reaction work it does on the fluid through which it moves.

In the case of the trolley it's the wheels which turn the propeller (shaft work), but it's the wind pushing against the propeller (reaction work) which turns the wheels. There's a lot of fuzzy thinking going on about these two sets of work.
 
Ubergeekian said:
You're almost getting it, I'm glad to say. If you use a reference frame fixed to the water then the wind is indeed the energy source. Use a reference frame moving with the wind and the water is the energy source. Once again it's having a velocity difference which matters. If the water and the wind are moving at the same speed, whether that's zero (pond on a calm day) or a hundred miles an hour (bath in a train) sailing will not occur.

Say it ten times: Kinetic energy is a relative quantity


What's the problem with bow waves? The wind loses energy.
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Your own particular straw man.

Pick a reference which does not move - the seabed.
Now you have a energy source from the wind
And if you want to complicate matters, an energy source from the tide or river flow.

But to try to relate the two without reference to the boat and/or the seabed is daft.

A boat travelling along on a tidal stream at the same speed and direction as the wind still has energy in relation to the seabed, the only fixed reference.

Say it ten times I will not argue for the sake of it.
 
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