# Thrust greater than drag

Discussion in 'Hydrodynamics and Aerodynamics' started by Solario, May 18, 2023.

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### SolarioJunior Member

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### jehardimanSenior Member

Welcome to the forum; but is this going somewhere? You can use the search function and see that this is well plowed ground.

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### baeckmoHydrodynamics

Directly into the wind is perhaps not as mind-blowing (!) as the "faster than the wind downwind"-concept, I'd say.....

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### SolarioJunior Member

If you fitted a larger version to a sailing boat you could go upwind without tacking.

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### jehardimanSenior Member

Aaaahhhhhhh....maybe. There are issues with the square-cube law and the realities of "anytime, anywhere" sea conditions. As I said, you are walking over well trod ground.

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### SolarioJunior Member

Wouldn't the square-cube law make it work better in a larger size? It would not have to be at sea, could be on a river or a lake. Walking over well trod ground does not mean stop walking.

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### jehardimanSenior Member

No, the resistance increases by 8*sf, the available energy by 4*sf , and the thrust by sqrt(sf).

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### SolarioJunior Member

Can you help me out here. I can see that if we increase the area of the wind rotor it us subject to square law and since the power of a wind turbine is determined
by the swept area then that is a good advantage. But where does the cube law come in? What is the resistance?

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### jehardimanSenior Member

The resistance is the amount of force necessary to propel the hull. The difference in resistance of two geosim hull is roughly proportional to their displacements. A hull twice as big in all dimensions is 8 times the displacement.

Nobody is disputing that those toys work. But many things that work on small scale don't work scaled up. Otherwise we would have pig-sized bumble-bees.

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### SolarioJunior Member

Is not the resistance determined by the wetted surface area and not the displacement?

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### jehardimanSenior Member

No. Total resistance is the sum of the wave making resistance (a function of the length and displacement), the skin friction (a function of the wetted surface), and for some vessels, like what you propose, the air drag.

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### SolarioJunior Member

You say it would not work in larger sizes. But it must work in all sizes because of this: If the boat is sitting in still water and the wind blows the wind-rotor around, then since the resistance
is zero (because the boat is not moving) it will work until the power available equals the resistance and then the boat will be in an equilibrium speed.

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### jehardimanSenior Member

No, increase in size is not endless. Depending on the windmill design and hull form, it will eventually reach a size where the induced drag and windage is greater than the propulsive thrust at zero advance. Also the environmental conditions necessary for that energy balance are unrealistic and other concerns, such as induced motions and stability, render the cross-over point significantly below what a conventionally spared sailing vessel could make to weather in the same conditions. Really, go learn the physics. If it was such an outstanding idea there would be hundreds of them operating now. There aren't. And it is not because of some Illuminati conspiracy. Several have been tried, all have been found lacking.

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### SolarioJunior Member

Can you clarify your meaning here. In the case we are talking about with zero advance there is no water resistance. So you mean induced aerodynamic drag and windage of the wind-rotor, support structure and any superstructure and you are saying that there will come a point where all the above water parts will no longer be able to provide enough thrust to counteract their drag?

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### rwatsonSenior Member

I think this may be of interest

" Blackbird, a wind powered cart, has shown it can travel upwind at more than twice the speed of circulating air. Rick Cavallaro, an aerodynamicist focused on designing record-setting vehicles that challenge wind speed, first made news two years ago when his Blackbird vehicle raced downwind at 2.86 times the speed of the wind. This time, after modifying his cart, he and his Blackbird team went out to the New Jerusalem airport in Tracy, California, where the vehicle achieved a top speed 2.01 times faster than the wind speed when headed upwind."

https://phys.org/news/2012-07-blackbird-cart-fast.html

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