# Vertical Windmills...

Discussion in 'Boat Design' started by cstretten, May 26, 2007.

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### GravioJunior Member

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### mitchgrunesSenior Member

Is what you are talking about more efficient than sailing more or less with the wind, or tacking into the wind on alternate tacks, using conventional sails?

Some of these pictures look a bit like "rotor sails", which you could look up for fun - though they use external energy. I wonder if the rotating turbine could directly drive a rotor sail on the same boat. You would only need one gear linkage, and nothing at right angles. But I can't do the engineering calculations. I wonder if the turbine sail would create more drag than the rotor sail would compensate for. It feels like cheating.

Another way that feels like cheating would be to have two slightly non-parallel plane surfaces pointing into the wind, but closer together in the rear. The idea would be that the air stream is compressed, and goes out faster behind the surfaces, creating a net forwards thrust. Simple, no moving parts. But again, I suspect the drag would be greater than the generated thrust.

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### mitchgrunesSenior Member

Wait a minute - I think none of these things can work. Consider a boat that starts moving with the wind, at the speed of the wind. There is no force on the sails, rotor(s) or plane surfaces. Hence there is no power for the drive to generate forward thrust. So you can't sail directly upwind, from that state, no?

EDITTED:
Too bad the video clip of the models has been removed from Youtube. Did it actually show a boat moving directly upwind for a sufficient time period to demonstrate it wasn't slowing down?

If so, I can't figure out what is wrong with the argument I just made above.

Why does the o.p. think flywheels are the ideal energy storage media? That's a lot of complexity and steps in between, to waste energy.

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### GravioJunior Member

But there is also a gearbox (on a screw). It is that which is decisive in this matter. The boat can go

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### keroseneSenior Member

Such apparatus can work both directly downwind (faster than the wind) or directly upwind.
Directly upwind is easier to understand. Say wind is 5 knots and boat is still. The turbine starts spinning and geared to a prop starts to push the boat upwind. The drag from the turbine can be small because the speed difference it experiences is large (compared to the speed difference between the boat and the water).
At say true wind of 5 knots and boat speed of 1 knot, we have 6 knot apparent wind but only 1 knot of apparent boat speed (boat/water). Different gearing ratios and prop blades optimized for this condition it is easy to take “small force” from high speed difference (turbine/air) and gear it to higher force at lower speed (prop/water).

now directly downwind is bit funkier. You can google FTTWDDW (faster than the wind directly downwind) Blackbird. I have spent probably hundred+ hours on why this cart works. Boat is more complex due to the nature of the resistance being so complex but in principle such boat is possible. It does require different gearing however than the upwind setup. And you need to flip the thinking around, now the propeller in the water needs to become a turbine and the propeller in the air is a thrust device (not energy harvesting turbine).

Imagine a boat going 15 m/s downwind at true wind speed of 10 m/s. (I switched to metrics as its easier to do the power math with these units)

so the boat will face apparent headwind of 5m/s, and experience apparent water speed of 15m/s.
Put a turbine in the water that causes a drag of 100N. Theoretical maximum power to be achieved is 15m/s * 100N = 1500Watts. (P=FV)
Lets ignore inefficiencies as this is about principle and pretend we really do get that 1500W.
Now we put that 1500W into a thrust propeller operating in 5m/s air flow. 1500W at 5m/s can produce thrust of 300N (F=P/V, 1500W/5m/s=300N).

In the example we have 300% thrust vs drag. So room for ample losses and beating boats resistance.

it is vey counter intuitive, UCLA physics professor “recently” called the downwind cart a hoax only to have to eat his words a little later.

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### mitchgrunesSenior Member

I guess my argument had many obvious flaws in any event - I didn't consider the forces in the water, or flow graidents. It is often the case that the surface water flow is not at the same speed or direction as the wind, nor is the surface air flow the same as that at elevated heights, nor is the water flow the same at all heights. There are also up and down flows. There are probably ways to take advantage of all those flow gradients.

I can't help but still think that it is likely to be very inefficient, and it might be better and simpler and maybe cheaper to sail at an angle to the wind. But I can see how it could be fun to design and build something that took advantage of these things. Mainly because it at first seems so implausible.

But then again, tacking against the wind is at least a little non-intuitive to me. As best I understand it, it works because the wind exerts a force on the sail in approximately the direction the sail fills, not the direction of the main wind body, by changing the local air flow directions. But that change in air flow occurs because of interaction with the sail. It is non-intuitive that the interaction that modifies air flow directions doesn't create a greater downwind force component than the upwind force component created by filling the sail. But centuries of maritime practice have demonstrated that it works beautifully.

I wonder if a submarine could somehow take advantage of underwater flow gradients to move against the main current... (I don't just mean in extremely turbulent conditions, like hydraulics, and upstream eddies, where some levels and/or places flow upstream)

What an intriguing idea.

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### mitchgrunesSenior Member

By the way, I knew from college days an engineer at Dupont Chemical, Charles (Chip) Wilker, who tried to get Dupont to make vertical windmills (or wind turbines) for power generation, I think mostly for home use. Last I knew, Dupont management wasn't interested.

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### GravioJunior Member

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### mitchgrunesSenior Member

I'm sorry, but that video does NOT prove this is viable in the long term.

In particular, the propeller stores energy from the treadmill, while the person creates a breaking force that prevents the vehicle from moving with the treadmill. As the vehicle moves upwind the propeller clearly slows down. It's very much like charging up a battery using the motion of the treadmill, or stretching a rubber band on a sling shot with a geared mechanism, and using its energy to temporarily drive the vehicle faster than the treadmill.

To be truly viable in a legit sense, the vehicle (or sailboat) must continue to move forward without dissipating energy - i.e., it should keep up its speed, without slowing down the propeller.

It is interesting that in the video, the physics geek needs an engineering geek to actually build the vehicle. That's the problem with undergraduate physics (I was an undergraduate physics major too.) It doesn't teach anything practical.

But I'm sure it was a fine engineering job. The young lady deserves credit for building what she was asked to build.

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### keroseneSenior Member

There are numerous of videos of similar gadgets accelerating - it's not her design as it's been done over and over again. It is perfect analogy for the faster than the wind directly downwind cart. One was built to full scale with a driver and all and it did 2.8x wind speed directly downwind.

Powered only by the ground

Here is the cart

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### keroseneSenior Member

and here same cart propped and geared differently goes upwind faster than the wind.

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### mitchgrunesSenior Member

Hmmm. Do I have this right: the pitch of the blade is such that the propeller is initially rotating AGAINST the direction the wind would tend to drive it as a turbine.

So at the beginning, the propeller is mostly acting as a sail, that drives the craft downwind. The gears and the reaction with the ground initially drive the propeller in the "wrong" direction". But that means it is fighting the turbine action, so the propeller initially turns slowly.

But once it passes windspeed, tailwind becomes headwind, the headwind drives the propeller around much faster.

In either phase, it only works because the ground stands still, so there is a relative motion with between the wind and ground, that the turbine can take advantage of.

In the water, there has to be a relative motion between the wind and the surface water. If wind sheer over the water were to drive the surface water forward at about the same speed as the wind, it wouldn't work. In short, it is a contest between the forces of wind sheer and the internal viscosity of the water, which tends to resist letting the surface water move forwards. That suggests that whether it would work might depend somewhat on the currents in the water. It might work if the currents are upwind, but may not if the currents are downwind, especially not if the currents are comparable in speed to the wind.

I still find it unintuitive, but it clearly works, unless the video is fake.

Are there any videos of it working for boats, where the propeller does NOT slow down at the end?

BTW, it seems to me that if the pitch of the blades reversed around the time there is no apparent wind, it would work better, because the tailwind could initially drive the propeller in the right direction, and switch to driving it right when there is a headwind.

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Your 80% efficiency is true of induction motors which are the worst. A permanent magnet motor is around 95% efficiency. Generator to motor direct connection is then 91% efficient. That's the same as the average manual transmission. A radial piston hydraulic pump motor combo has that same efficiency. So geared, electric, hydraulic, doesn't matter in efficiency terms. Hydraulic would be smaller lighter and more rugged then electric while not requiring any gears unlike mechanical. Hydraulic is continuously variable by default.

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### GravioJunior Member

This is a windmill on a boat.

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### gonzoSenior Member

Can you explain how the boat started going faster than the wind to start with? If you graph the speed of any sailboat going straight downwind, there is a vertical asymptote where the speed goes to zero. You say it magically goes to 5??

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