DDW Faster Than The Wind
- Thread starter Maine Sail
- Start date
that's only half the picture... the key is that you have to factor in how the apperant wind shifts relative to the propeller blades, and speed of the car.
I believe it will be a USCG mandate soon, you're well ahead of the curve!Maine, I have a placard on my boat advising to stay at least 3 nautical miles away from the horizon.
Thus the need for variable pitch. It is hard to tell from the video, but it would appear, (and make good sense to me), the the pitch is flat at the beginning providing max surface area to get the vehicle moving. As it accelerates the pitch becomes more angled creating the lift.
Actually Rick and JB's original prototype/model used a fixed blade and still went DDWFTTW which is what lead to all the hoopla..
http://www.youtube.com/watch?v=_fBDcchw5nw
The blades change pitch to decrease drag and convert rotational energy to drive the wheels.... making it go faster than the wind. It can't go indefinately. Drag (parasitic and induced) will increase as its speed increases. Energy must come from somewhere to overcome this drag. Notice it slows down at end of video.
The wheels are driving the prop - not the other way around. There is a direct linkage to the prop. To some extent, the prop becomes a sail that is continually being corkscrewed backwards even as the boat moves forward.
What I think.. could be wrong. Say there is no wind and the boat is moving at 10 mph. The wheels cause the prop to rotate at such a speed that the air being blown backwards by the prop is also at exactly 10 mph.
In this case, the air mass exiting the boat prop does not move with respect to the ground - ie, there was no actual acceleration of any air mass- so no energy input. This would assume the craft and prop have zero drag - not exactly true but probably close.
If a 10 mph wind is now added behind the boat, its pressure acts on the 10 mph wind exciting the back of the prop. Since the wind exciting the prop is actually stationary with respect to the ground, the tail wind can still put pressure on the prop (ie, sail) even though the boat is going exactly the wind speed. Ie, the prop sort of acts like a sail which is continually being corkscrewed backwards - and the vessel can exceed the dead down wind speed.
What I think.. could be wrong. Say there is no wind and the boat is moving at 10 mph. The wheels cause the prop to rotate at such a speed that the air being blown backwards by the prop is also at exactly 10 mph.
In this case, the air mass exiting the boat prop does not move with respect to the ground - ie, there was no actual acceleration of any air mass- so no energy input. This would assume the craft and prop have zero drag - not exactly true but probably close.
If a 10 mph wind is now added behind the boat, its pressure acts on the 10 mph wind exciting the back of the prop. Since the wind exciting the prop is actually stationary with respect to the ground, the tail wind can still put pressure on the prop (ie, sail) even though the boat is going exactly the wind speed. Ie, the prop sort of acts like a sail which is continually being corkscrewed backwards - and the vessel can exceed the dead down wind speed.
correct, I had posted a comment about that a few minutes prior, but it isn't showing up now that I look back through the thread.
my original comment was something to the extent that his basic idea is correct, he just needs to expand on it to include the apparent wind etc....
I am having great fun reading all of these explainations. I suspect that the answer lays in the very low rolling resistance and the rotational energy stored on the prop and the innards of that nacelle since it is apparently capable of only relatively short sprints. It would be nice to see it turn about and return up wind on its own.
your explanation was a little hard to follow, but it I think you understand what is going on quite well.
If you look through the information they have wrote about the car, there is a comment somewhere that the driving force is actually the fact that they are slowing / stopping the moving air directly behind the car, so your interpretation that the tail wind can still put pressure on the prop because the wind exiting the prop is stationary etc, is spot on!
actually, if you read up, it was able to accelerate from a dead stop, and reach a SUSTAINED 3.4x the speed of the wind, using only the force of the wind.
that's because they hit the brakes, at the end of the test. the car would go on indefinitely if they let it, just like your sailboat will sail to wind close hauled indefinitely, provided there is always wind, and nothing to run aground on.
the fixed blade was "only" able to acheive ~2.5 times the true wind speed from what I could tell, the later variable pitch prop was able to hit a sustained 3.5 times true wind!
Yea, true, but isn't this induced tail wind just a effect of the prop getting sucked forward from the rotationally induced lift? The prop is attached to the car, the movement of the car turns the wheels which rotated the prop and creates the lift. I wonder what M.C. Escher would think of this phenom?
I think I get it.
I was sceptical at first, but that was when I was thinking sailboat. You have to think airplane. The wheels drive the prop, which pulls the vehicle through the air. Now, in zero wind, this arrangement would slow down and stop due to under 100% efficiency. BUT, we have the energy of the wind behind us. So, if the car is going, say, 30mph, it only needs the energy to go 20mph if there is 10 mph wind behind it.
It does need a very efficient system with little drag apart from air resistance, but it can be done!
druid
I was sceptical at first, but that was when I was thinking sailboat. You have to think airplane. The wheels drive the prop, which pulls the vehicle through the air. Now, in zero wind, this arrangement would slow down and stop due to under 100% efficiency. BUT, we have the energy of the wind behind us. So, if the car is going, say, 30mph, it only needs the energy to go 20mph if there is 10 mph wind behind it.
It does need a very efficient system with little drag apart from air resistance, but it can be done!
druid
We sail off the wind because the sail produces a forward lift vector(I don't remember moving forward in irons). This thing is going directly downwind and accelerates......using stored energy from that rotating mass driving the wheels. How big is the flywheel inside that nacelle? Once you accelerate past wind speed, and becomes a headwind, there is no move wind energy to more you forward. I didn't see a sustained speed faster than the wind in that video. It will eventually slow down and start the cycle all over again.
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The prop does not pull the boat forward in a traditional sense - also doesn't provide lift in a traditional sense.
It just provides something for the rear wind to push against even when the boat is traveling at wind speed or faster. No momentum "trick" involved.
I think it likely works. Also, the Nalsa guy ratifying things (Bob Dill) ain't going to be tricked.. He held the world land sailing speed record for about 10 years before Richard Jenkins took it several years ago.
It just provides something for the rear wind to push against even when the boat is traveling at wind speed or faster. No momentum "trick" involved.
I think it likely works. Also, the Nalsa guy ratifying things (Bob Dill) ain't going to be tricked.. He held the world land sailing speed record for about 10 years before Richard Jenkins took it several years ago.