The Wind Powered Sail-less Boat

Those videos were done by a friend of ours (Michael C) on another forum discussing this same topic. He has a gift for boiling a concept down to its essentials and demonstrating it very clearly.

No question about it. When I first started flying R/C helis, the electrics were fragile and anemic. Now they are quite astonishing, and it seems the power to weight and capacity to weight ratio will continue to climb. I'm eager to see ultra-capacitors possibly replace batteries altogether one day.

 

A thought about smoke & streamers. Logical thinking is not a natural activity for the human brain - you have to train it, and keep checking. For this reason, it is quite easy to say of the streamer on Goodman's cart "it's blowing backwards because it's in the slipstream of the prop". It would have been better to mount it on the front of the cart, or much further off the axis of the prop..

 

I think the sreamer is to the side, beyond the prop by at least a full prop-radius. Unfortunately, this isn't obvious in most of his video.

 

Rick.
The problem I'm having difficult with is how to overpass the stagnation point (how can we extract enough energy to do it). Once it overpassed, maintain an speed faster than the wind could be possible. As an example, if the tacking iceboat within the frame were already going at an speed which's projection in the wind direction is greater than the wind speed, then the boat would be 'cutting' through not perturbed wind (so at the original wind speed), and could take energy from it to keep on going faster (projected) than the wind.

Of course if we can take all the energy from a wind of mass M and velocity V and transfer it to a mobile of mass m<M the mobile will move at v>V. If we have losses all we need is to make the difference M > m big enough.

But I still do not see how to overpass the 'breakeven point' if you allow me the expression.

I'm not afraid of not (or yes) being embarrassed. I'm not even afraid to be wrong. What I'm afraid of is stop thinking, stop from trying to understand things. Even if I look stupid or stubborn to your (or whoever) eyes in the process.

Cheers.

 

Wait a moment....If the width of the theoretical cart within which the iceboat is tacking downwind, is long enough as to allow the boat to extract enough energy from a bigger mass of wind than the mass of the cart, then the cart can accelerate to an speed greater than the wind's.
I think I've got it now.

Cheers.

 

I will try to explain regarding your notion of the stagnation point.

To picture something that will actually work on water look at the attached. Do not be bothered about building this. The idea is to show the proportions that are needed to make it work on water at relatively low windspeed.

This boat will not exceed windspeed if the wind velocity is much below 3m/s. I have not found the exact point but it will achieve windspeed if wind is above 3m/s, roughly 6kts.

So we are not in a borderline condition, I am going to give you the numbers for 4m/s windspeed. If any of these numbers do not seem to match your expectation then please advise.

Windspeed at 4m/s boatspeed at 3.5m/s (almost 7kts)
The water turbine is doing 48rpm. I have attached a close-up of the turbine I have designed to add more meaning. It has a large pitch of 3.5m. It will have a near field velocity of 3.43m/s meaning it is slowing the water past it a tiny amount. There is a lot of power in a water stream at 7kts through a diameter of 650mm.

The gear reduction to the airprop is 5.1 so the prop is doing 9.5rpm. It sees a far field airflow of -0.5m/s but the near field flow is 1.05m/s. So the airflow away from the prop is going forward but the prop is dragging air inwards and backwards. In reality there is a large circulation loop around the prop but it is still applying forward thrust by drawing air though it. If a propeller could not do this then you would never be able to get forward thrust if you were still coasting astern.

Under these conditions I have the situation where the net thrust is 5N and the excess power from the turbine is 93W. In my model these interchange but as long as both are positive it will keep accelerating.

Going up to boat speed of 4m/s (8kts) the following conditions exist:
Turbine is now up to 54rpm. Its nearfield velocity is 3.98m/s. Power output is 144W. It has a drag of 47N.

The prop is doing 10.6rpm with pulling near field velocity of 1.1m/s. Note that the far field velocity is now at your so-called stagnation point. Its input power is 136W and thrust is 107N. This condition can be likened to the static thrust of any propeller. If it is spinning it will be drawing fluid through it.

The boat drag is 50.7N. So I have a net thrust of 9N and excess power of 8W. I have not allowed for any transmission losses because I was intending to use a chain and these get 99% efficiency. (After drawing it I realised a chain would be ridiculous.)

Conclusion is that it has no problem getting through your stagnation point. It is the same reason any prop can generate thrust at "stagnation" or static conditions. It actually imparts velocity to the fluid in its proximity although the velocity of the fluid at some distance is zero.

Hope this makes it clearer. Nothing like some realistic numbers to help work out what actually happens.

Rick W.

 

Rick: I don't know if Guillermo understands what near filed and far field velocities are. I don't, although I can guess from the context.

 

I really appreciate that, Rick.

What I call stagnation point (probably the name is not adequate) does not have to happen when the boat/craft is at wind speed, but when the air rotor has developed a vortex ring. Is there enough energy from the turbine to overpass that point?

Thanks for your efforts.

 

Do you?

 

PS I think it was Spork who mentioned some way back the airflow associated with a helicopter rotor as it changes back and forth between powered and autorotating states. Does anyone have a link to diagrams (or, better still, a computer simulation) of this? I've made (free flight, again) helicopters with rotor blades hinged so that they autorotate when power gives out, but never studied propellor/rotor aerodynamics.

 

Guillermo: I can only make a vague guess, so I'd like a proper definition from anyone who can give me one.

I only mentioned your name because Rick's post was directed to you, and I was back in 'technical editor' mode, critiquing Rick by playing the dumb reader. =>

 

Guillermo's comment about vortex rings triggered a thought: has one of you boffins got access to a low speed wind tunnel - the old smoke trail kind? It would be wonderful to put a cart on a short treadmill inside one and film it, so that we could see what was happening to the air around the prop.

 

This way of thinking is not going to get you far. It would work, if there were no losses, but losses are MUCH more important that the kinetic energy needed. So you need MUCH more air mass than your mass AND you don't want to take all the energy out of the air (= slow it down speed of water), since that is likely unefficient.

Think of Rick's new design. It has a propeller of 15 m diameter, thus the disk area is about 180 m2. Even a 1 m long "tube of air" would have a volume of 180 m3 and a mass of over 200 kg. But this propeller will affect the air flow on a much bigger volume, thus we are "touching" easily 10 000 kg of air even when the apparent wind is zero or the net flow through the propeller is zero.

Joakim

 

Unlike the cart the boat will drift downwind under the influence of the wind until the turbine bites. The turbine has mechanical advantage over the prop. Respective pitches, from memory, are 3.5m and 10.6m with mechanical ratio of 5.1.

Once the prop starts to turn it is drawing air backwards through the blades and producing thrust.

I have not done precise calculations on the very low boat speed condition. There is likely to be reverse rotation of the prop when the boat is stopped because the prop has a good reverse flow regime under this condition while the turbine is in a poor regime. But the downwind force will cause the boat to progress downwind such that the reverse flow regime on the prop gets worse and the forward flow regime on the turbine improves. Once this happens the mechanical advantage of the turbine takes over and away it goes, accelerating until there is a force and power balance somewhere higher than windspeed.

Rick W

 

This rather stunning footage is of an RC sailplane dynamic soaring at, apparently, speeds up to 357mph.

http://www.youtube.com/watch?v=Vi0hrjqU15I

it was posted a long way back in this thread. I've dug it up again, because it shows, if I understand correctly, the sailplane extracting energy from a differential in the velocity of a single medium, to achieve extraordinary performance, very well in excess of the wind speed, differential or absolute.

I would be grateful for any (not too mathematical! - I can handle simple lift, centrifugal and centripetal forces etc.) explanation of what is going on; I understand that there is wind shear, i.e. layers of faster moving air higher up, slower lower down, as the wind flows over the back of the hill. The plane dips into the slower air, then the orbit pops it up into the faster air. Does it rely on the upper near vertical wing being in faster air than the lower near vertical wing, akin to the sail and the keel on a yacht, or is the mechanism to do with the whole plane moving from faster to slower air? The upper part of the orbit provides the acceleration and the lower part of the orbit provides the vertical component of lift to prevent it sliding into the hillside?