Sailing downwind - faster than the wind?

No Horatio, the idea is not that the propeller extracts energy from the headwind to drive the wheels. The power apparently comes from the wheels which are being turned by the road to drive the propeller and provide thrust. Thus keeping the craft moving. Hmmm.
 
pmagowan said:
What we are talking about here is the same as an electric car being powered by an electric dynamo on its own wheels.
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Not quite, there is energy being inserted into the system by the wind.

I think you would agree that the contraption could be blown direct downwind at a speed just less than the wind speed. Frictionless, it could get close to wind speed. At wind speed no more energy would be input. But it has stored energy in it momentum. Can it convert that energy into increased speed for a short period?
 
IAN13 said:
Have a look at this .. HERE .. :eek:
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Ah, I see the prop doesn't acquire energy from the wind at all, except by virtue of it blocking the wind like a sail, as the vehicle accelerates the prop speed builds up gaining inertia as the vehicle reaches wind speed the thrust generated by the spinning prop bumps it over.

It then falls back as the prop loses inertia.

The energy is gained as the vehicle speeds up and stored in the spinning prop.

I suspect an identical vehicle with a jammed prop and disconnected gear would accelerate quicker and cover the ground quicker, albeit with a lower top speed.

They could improve the performance by having a proper fly-wheel geared to the prop. but then the illusion of impossibility would be lost.
 
RichV said:
No Horatio, the idea is not that the propeller extracts energy from the headwind to drive the wheels. The power apparently comes from the wheels which are being turned by the road to drive the propeller and provide thrust. Thus keeping the craft moving. Hmmm.
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I think it works the other way round. The wind drag on the whole contraption pushes it forwards, the wheels turn and turn the propeller. The disc drag of the propeller is increased. The increased drag accelerates the device. The propeller is always "blowing backwards" the faster it goes the higher its disc drag making it effectively a larger sail.

I think this means that it can approach wind speed and, given that the wind is never constant, occasionally exceed it.
 
xxyyzz said:
They could improve the performance by having a proper fly-wheel geared to the prop. but then the illusion of impossibility would be lost.
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One key may be that speed is not energy. So if it can absorb enough energy to release later, it could go faster than the wind.

Imagine a conventional wind turbine charging a battery. A battery can hold a lot of energy after a decent amount of charging and carry a vehicle faster than the wind speed that drove the wind turbine. It doesn't have to defy thermodynamics, it just can't do it for long.
 
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RAI said:
Not quite, there is energy being inserted into the system by the wind.

I think you would agree that the contraption could be blown direct downwind at a speed just less than the wind speed. Frictionless, it could get close to wind speed. At wind speed no more energy would be input. But it has stored energy in it momentum. Can it convert that energy into increased speed for a short period?
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Whoa, hang on a sec. The kinetic energy (i.e. its 'moving energy') of something is 0.5 x mass x velocity squared. The only way you could increase the velocity out of its kinetic energy is if it magically gets lighter. So it's got nothing to do with tempoarily getting an extra burst of speed.

The whole thing is not possible. I'd be interested to get your response to my first post on the subject, where I try to explain how the treadmill test cannot be equated to outside, and how the craft could not operate above wind speed.
 
I cant take it anymore! I am off in my electric dynamo car to drive off the edge of this flat planet and exceed the speed of light relative to a backward blowing windmill so that I can have a chat with Einstein in the past. But before I do I will fry up some bacon in the fridge and get ice from the fire for my Gin and Tonic that I have happily been diving into repeatedly in the mistaken belief that I am a slice of lemon!
 
RichV said:
Whoa, hang on a sec. The kinetic energy (i.e. its 'moving energy') of something is 0.5 x mass x velocity squared. The only way you could increase the velocity out of its kinetic energy is if it magically gets lighter. So it's got nothing to do with tempoarily getting an extra burst of speed.

The whole thing is not possible. I'd be interested to get your response to my first post on the subject, where I try to explain how the treadmill test cannot be equated to outside, and how the craft could not operate above wind speed.
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OK. Actually I agree with your posts. But as the mass looks pretty constant, we have, as you say, 0.5mV2 on one side of the equation. What is on the other side? I guess the energy put in by the wind.
 
RichV said:
The whole thing is not possible. I'd be interested to get your response to my first post on the subject, where I try to explain how the treadmill test cannot be equated to outside, and how the craft could not operate above wind speed.
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How does the machine know it's on a running treadmill with stationary bulk air, or on a fixed surface with air moving past it? Which of the forces on it would be different?

And supposing you put it on a treadmill in a wind tunnel, then slowly decreased the treadmill speed while increasing the fan speed to keep the wind speed relative to the treadmill constant. At what point would the buggy suddenly realise that what it was doing was impossible?
 
RichV said:
Whoa, hang on a sec. The kinetic energy (i.e. its 'moving energy') of something is 0.5 x mass x velocity squared. The only way you could increase the velocity out of its kinetic energy is if it magically gets lighter.
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Or if the energy is moved from the fly wheel to the vehicle.

There are two of these: 0.5 x mass x velocity squared though:-
1. Is the flywheel (blades in the video)
2. Is the vehicle

The only way I can see it working is if during the acceleration phase energy is stored in the flywheel, resulting in a lower acceleration.

The energy stored in the flywheel is then used to accelerate the vehicle past the wind speed. So the energy in 1 is reduced and the energy in 2 is increased.

Like those toy cars you rev up and let go.
 
RichV said:
The idea that the treadmill is the same as outdoors in the wind is not correct. Whilst it's true that an aircraft can be modelled in a wind tunnel, you would still need the engine in the aircraft (or a stick that holds the model in place) to provide the forwards thrust.
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No one said that the aircraft in the wind-tunnel would magically progress upwind without support.

You missed the point. The point was that stationary aircraft and moving air is the same as moving aircraft stationary air. No-one can deny that, and if you did you would be arguing with all aircraft engineers who use wind-tunnels.

Therefore, stationary cart and moving road is the same as moving cart stationary road. How can it be any other way?

Having understood that, (which is unlikely) the treadmill test is perfectly valid.
 
Malo37 said:
But read this carefully
If the balloon lowered a water turbine into the sea, the drag of such turbine would cause the balloon to slow down with respect to the wind - there is no other possible outcome. Consequently the baloon would then be travelling slower than the wind ergo - it is not travelling faster than the wind.
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Yes, but you haven't made the connection with the wind-cart. I thought anyone would have seen this, to be equivalent to the wind-cart the balloon would use the power from the water turbine to move itself upwind.

And, since an air-turbine can make enough power from the air to pull itself upwind by putting its power into the water, (as in the windmill driven boat). Then in this case the water turbine can make enough power to pull itself upwind by putting its power into the air.

Both cases are really different sides to the same coin.
Windmill boats going directly upwind in fact prove that going downwind faster than the wind is possible.
 
No-one seems to be taking any notice of Post 51 from Ubergeekian. I'd have thought that would have put Q.E.D on it!

Someone will be along soon to say you should stop your prop rotating to make the boat go faster!
 
geoffatstanpit said:
No-one seems to be taking any notice of Post 51 from Ubergeekian. I'd have thought that would have put Q.E.D on it!
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No, because in his example of two boats on opposite tacks the resultant vector in the straight line between them will be slower than the boats.

geoffatstanpit said:
Someone will be along soon to say you should stop your prop rotating to make the boat go faster!
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Well, not rotating, but maybe it doesn't need to be a prop at all given that it drives the wheels.
 
Ubergeekian said:
Here's a little thought experiment for you. You accept, I presume, that BMW Oracle can sail downwind - in the sense of have a VMG - higher than the windspeed? OK, now get two of them and join them together with a light beam, exactly perpendicular to the wind direction, along which they can both slide. Now have them tack downwind on exactly mirror image courses, pushing the beam directly downwind, faster than the windspeed, between them.

Voila. You've created a sailing vessel, albeit a rather complicated one, which can sail directly downwind faster than the wind. The principle has been demonstrated - now all you need is a neater implementation.
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Exactly. In the same way an air propellor driving a water propellor to go upwind can be viewed in terms of the two halves of the propellor being on opposite tacks.

I would assume that as the 'boat' approached windspeed, the pitch of the prop would have to increase, then be reversed above windspeed.????

You just have to make your composite sailboat have enough sail area vs weight/drag.
And keep the friction down.
Given that 50% is a fairly good efficiency for a propellor, I see problems achieving the power to weight.
Bethwaite's 'High Performance Sailing' gives some ratios for sail area vs mass that must be exceeded to achieve 'proper' apparent wind sailing. You are talking about around 350 sq ft for a typical 2 man skiff. IIRC.
So a very big very light prop, a very efficient gear train etc etc.
It will get messy when you discover that the wind is sheared an 'dead downwind' ceases to have much meaning, becasue as you approach windspeed, the apparent wind will be hugely sheared.
Also sailing dead downwind is a pretty dubious goal if the water is not completely flat!
A skiff heads up for more power to get over a wave, and steers around the big ones.
Perhaps a hybrid: use the windmill to charge a battery, then have an electric outboard?

I expect Leonardo da Vinci thought of it first.
 
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