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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Wind powered vehicle speeds much faster than wind; downwind!
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Thread: Wind powered vehicle speeds much faster than wind; downwind! Reply to Thread
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Topic Review (Newest First)
07-19-2011 10:53 PM
ziploc Actually, our blade pitch is not terribly small. We use variable pitch, but run about an 18 foot pitch at our design point (prop advances 18 feet in one rotation). That puts the prop tips about 20 degrees out of plane - giving them an angle of attack around 5 degrees at speed.
11-09-2010 12:13 PM
RichH For those of you who have a working knowledge of trigonometry the explanation for this is 'astoundingly simple' and elegant .... you (the fan blades) are essentially dividing by the sine of 'close to zero' which by mathematical solution approaches 'infinite force'.

In common laymans terms you can do the same thing with a slippery 'pumpkin seed' between two fingers: you press your thumb and forefinger together onto the slippery pumpkin seed, and the pumpkin seed 'rockets' out between your fingers as a result. In the case of the fan blades, the pitch angle of the blades is soooooo close to zero that any input/energy from the moving wheels gets 'magnified' by literally hundreds of times as an effect onto the 'blades' (less the friction of the transmission system), and the greatly magnified speed of the blades coupled with precise aerodynamics .... moves the vehicle MUCH faster than the speed of the wind .... all because the blades pitch angle is 'dividing the energy input by the sine of the pitch angle of the blades'.

Its all due to the BRILLIANT 'application of elemental trigonometry' (aka: *pumpkin seed effect*). If you would have actually understood your high school trigonometry teacher, the valid explanation of why this vehicle is covered in the early session of a simple HS trigonometry class.

The folks who did this are profound MATHEMATICIANS. Sadly, understandable mathematics isnt taught anymore in the USA; hence, the misperceptions and doubts that are being stated about this brilliant and yet simple accomplishment simply by the application of very basic engineering and mathematical fundamentals. Elegantly profound, extremely simple !!!!!
11-09-2010 11:53 AM
eyytee
Quote:
Originally Posted by LarryandSusanMacDonald View Post
This continues until the vehicle reaches the speed of the wind at which time the apparent wind is zero and the only thing propelling the vehicle is the propeller, powered by the wheels. Now this is where I start to get confused - the vehicle continues to accelerate. What causes this acceleration?
Acceleration is caused by a non-zero net force. In this case:
F_thrust_prop > F_braking_wheels

Quote:
Originally Posted by LarryandSusanMacDonald View Post
Now the vehicle is moving faster than the true wind and the apparent wind is from the front of the vehicle. At this point the propeller becomes a turbine,
No, it never becomes a turbine. A turbine.going DDWFTTW would push the air forward and accelerate the true wind. To extract wind energy you have to slow down the true wind : push the air backwards in the downwind case. This is similar to tacking downwind with VMG > windspeed:
YouTube - DOWNWIND VMG GRATER THAN WINDSPEED (STREAM LINE)
From there it is just a small step to the DDWFTTW propeller:
YouTube - Sail_to_Prop.mov

Quote:
Originally Posted by LarryandSusanMacDonald View Post
It seems to me that if the vehicle will accelerate with an apparent headwind, that you could turn the vehicle around initially and face it into the wind. Turbine from dead start turns the wheels and it would accelerate dead into the wind. Am I missing something or would this work?
Not if you just turn it around (it would go downwind backwards in that case). But if you change the gear ratio or the blade angle, the propeller becomes a turbine, and you can go directly up wind. This case is shown here:
YouTube - Directly Down & Up Wind Faster than the Wind
11-09-2010 11:32 AM
eyytee
Quote:
Originally Posted by AdamLein View Post
Okay this is a pretty awesome illustration. 100 points for "worth a million words".

Where'd you get it?
I did that one myself. Here the second part that works continuously :



You will find more visualizations of here:

YouTube - eyytee's Channel
YouTube - coolaun's Channel
YouTube - axelerat3d's Channel
11-09-2010 10:19 AM
LarryandSusanMacDonald Still munching on my shoe leather. I remain unconvinced, but not unconvincible.

These are the points I'm having trouble wrapping my old balding head around:

The only wind a moving object is affected by is apparent wind. The True wind will affect apparent wind - but a moving object is only affected by apparent wind. If true wind and apparent wind are equal, the object in question is at rest. Am I correct so far?

I was confused when I initially looked at the video as I thought the propeller was acting as a turbine and turning the wheels. But now I understand that the wind is moving the entire vehicle causing the wheels to turn which in turn cause the blades to turn and thus it is a propeller. Correct so far?

The vehicle continues to accelerate with the wind propelling it and the blade also propelling it. This continues until the vehicle reaches the speed of the wind at which time the apparent wind is zero and the only thing propelling the vehicle is the propeller, powered by the wheels. Now this is where I start to get confused - the vehicle continues to accelerate. What causes this acceleration?

Okay, I'll bypass that point by assuming that the wind, which has been in the past known to fluctuate, slows down slightly. Now the vehicle is moving faster than the true wind and the apparent wind is from the front of the vehicle. At this point the propeller becomes a turbine, i.e., it is driving the wheels, rather than the wheels driving it. Is my understanding correct so far? And the vehicle continues to accelerate. I do not completely understand why it should continue to accelerate - maybe someone can explain this in terms the more simple minded (that is, those of us who eat shoes) can comprehend.

It seems to me that if the vehicle will accelerate with an apparent headwind, that you could turn the vehicle around initially and face it into the wind. Turbine from dead start turns the wheels and it would accelerate dead into the wind. Am I missing something or would this work?

Two more unrelated questions:
Can anyone suggest a good wine which would go well with shoe leather.

Can anyone tell me the correct spelling of 'unconvincible"?
11-08-2010 07:16 PM
GaryHLucas The catamaran with the windmill is actually quite crude. About 25 years ago there was mention of a monohull with a windmill, and a magazine writer from Sail or Cruising World actually found the guy who built it. He got pictures and went out for a sail on it. The windmill drove a standard inboard prop shaft, and it easily went directly upwind. It was estimated that the power was about equal to a sail of area equal to the swept circle of the windmill blades.

Gary H. Lucas
11-08-2010 01:37 PM
AdamLein Also if it can work on a cart with wheels, why not on a boat with an underwater propeller or paddlewheel?
11-08-2010 01:21 PM
AdamLein
Quote:
Originally Posted by eyytee View Post
Okay this is a pretty awesome illustration. 100 points for "worth a million words".

Where'd you get it?
11-06-2010 01:47 PM
gwmeadows Capt. Larry,
I was browsing Sailnet for a different reason and came across this post today. Indeed, you're gonna be tasting some shoe leather here - whether you ever admit to it or not.

I'm a cruising sailor in FL who happens to be really good friends with the guys who've set this DDWFTTW record. Believe me, they have no interest in creating any hoax, however they are both extremely interested in proving things like this.

I noticed that your current location is 'permanent vacation'. If you are cruising in FL, let me know. We'll get together and I'll explain it to you - using my hands for reference. I'm a very average guy in intelligence and was finally able to fully understand what's going on with the directly downwind cart.

If you just don't WANT to believe that it could happen, or maybe it doesn't fit into YOUR definition of a sailing vehicle, I can understand your take on it, but if you're just gonna stick to you guns on it being a hoax, well you're just wrong.

Let me know if you're in FL.

Capt G.
11-06-2010 04:57 AM
JomsViking Any sailor have tried to go from A to B faster than the wind by tacking effectively, right? (unless sailing an old shoe) So there's more energy to it than the velocity component of the wind, E.g. a more efficient keel-shape on a boat will make it go faster and/or higher.
Wind Turbines wouldn't produce much if there weren't - and it's not perpetual motion, if the wind stops, the vehicle (or boat) stops.
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