Friday, November 19, 2010


Is it possible to build a craft that will "sail" dead downwind faster than the wind (DDFTTW)?

Watch the video. It certainly looks like that cart is sailing DDFTTW. How can that be possible? Is this an illusion, a fake, a scam? If it's real, how does it work? What's the science behind it?


Anonymous said...

Maybe the hidden electric motor driving the rotor? There is clearly already some electrics on board for the RC steering.

Dan said...

You must have seen this blog:
They built a full size manned vehicle where they proved this concept.

Tillerman said...

So it can be done Dan? But how? Someone please explain the theory behind this in simple words and short sentences with no math.

Dan said...

This has been discussed(and cussed at) at great length in Sailing Anarchy.

The guys that built and tested the vehicle are very talanted individual that put a lot of thought into it.

As far as I know, the prop linked to the wheels accelarates the cart to a speed where it can cross the zone where apparent wind drops to zero and generate apparent wind from dead ahead where the prop starts providing power to the wheels agin and accelarating the cart again. I believe they got to a speed of nearly three times the true wind speed.

Sort of sounds like perpetual motion, doesn't it?

Tillerman said...

So does the prop reverse direction as it goes through the zone when apparent wind is zero?

And what makes you think the prop drives the wheels? Couldn't it be that the wheels are driving the prop?

And does it even need any wind? If you just gave it a little push on a zero wind day, would it just accelerate away?

Yeah it does sound like perpetual motion. I must admit I still don't get it. More explanations please.

Bursledon Blogger said...

Perhaps DDHFTTW - dead down hill faster than the wind!!

Intersting stuff, an engineer friend of mine built a vertical axis windmill powered catamaran when he was at Uni, it would travel directly to windward (converted the windmill power to turn a propeller) but I don't recall it going faster than wind speed

Anonymous said...

I hang my cynical head in shame :(

I still don't understand the physics though.


Anonymous said...

A simple way to think of this is that the propeller blades are effectively reaching (their apparent wind is not from dead behind/ahead owing to their rotational velocity). Some sailing boats can achieve a velocity made good downwind which exceeds the true wind speed by gybing back and forth, much the same as the propeller blades are doing here.

It is relatively straight-forward (certainly should be a trivial exercise for Mr Tillerman as a former Cambridge NatSci) to write down the relevant conservation of energy and momentum relationships for this device.

Tillerman said...

Anonymous said, "Some sailing boats can achieve a velocity made good downwind which exceeds the true wind speed by gybing back and forth."

I didn't know that. I learn something new every day. My readers are so smart.

But I'm still confused about this cart. Are the wheels driving the propeller or is the propeller driving the wheels?

Ed F said...

The wheels are driving the prop. There's a ratchet systems - the prop can never drive the wheels.

Mark Drela's analysis:

i.e. - relatively simple to achieve on land. More difficult to achieve on the water.

Tillerman said...

So the wheels drive the prop which creates the forward force to drive the cart which turns the wheels which drive the prop which creates the forward force to drive the cart which turns the wheels which....


Have I got that right?

O Docker said...

There's an easier to digest (if somewhat murky) explanation here (thanks for making me curious enough to google).

Apparently, the wheels are geared to drive the propellor, but that gearing may only engage above a certain threshold speed. And, it looks like the contraption may actually be moving upwind in its apparent wind.

Curiously, there is a connection to a well-known SF bay area sailor in the explanation I linked to.

So, sailing downwind faster than the wind may well have languished in relative obscurity for years and would probably remain there today had it not been perfected by an enterprising and dedicated San Francisco engineer and then brought to the world's attention by passionate San Francisco writers.

Dan said...

Go to this link for a video and it will link to many other videos:

Watch a few and you can get an idea about what is going on.

The propeller is linked to the wheels with bicycle chain and sprockets. It is like a bicycle with a fixed gear, so when they push the cart the wheels drive the prop, but once underway, the prop drives the wheels. I believe their is video of the cart starting totally by itself without being pushed. It took some stronger winds for that to happen.

There is a rachet mechanism in their drive train, but if you explore their blog, I think you will see where they talk about how the the rachet is only used to disconnect the wheels from the prop when they are moving the cart around.

David said...

I was at El Mirage when they set the record. Call me and I can talk to you about it. four oh five 830-642zero The guy that built it is my best friend. The main reason it works is the differential between ground speed and airspeed, the wheel is turning across the ground the propeller is working in the downwind air.

David said...

The builder said I could have taken out the workd "main" because it is "the reason", and like others said the wheels are geared to and turn the propeller, not the other way around.

David said...

Ratchets, gears, wheels, propellers, etc. aside, the key think to think about here is the fact that the sails on this craft are rotating at a speed much much faster than the speed of the wind, or the cart for that mater and they aren't exactly moving in the same direction, either. For us sailors who all totally understand the principles that make "normal" boats go (right? something about squeezing a watermelon seed between your fingers. wink wink), we need to block out of our minds the paradigm that the rig has to move through the air at the same speed and in the same direction as the rest of the craft. Freed from this limitation, it shouldn't be too hard to imagine a craft that is headed in one direction while its rig is working in another and at a much higher speed (and coupled back together by gears, wheels, belts, etc.) So, I guess that watermelon thing needs to go since we can decouple drive from motive force (OK, I have no idea what I just said.) Note that these same "wackos" think they can also build a craft that will sail directly into the wind faster than the wind is blowing the other way. I say, "power to 'em."

Anonymous said...

12:41pm anonymous here:
The prop is driving the wheels at all times. To see this, consider from where the kinetic energy and momentum of the cart comes ... it's from the air not the ground beneath the cart. As long as the prop extracts KE and momentum (by whatever means) from the air the relative wind speed between cart and air is irrelevant.

Joe said...

It's called magic! The god's are appeased by sacrificing 2 pizzas (no anchovies, please), a bottle of Gran Patron, and a wash cloth.

Tillerman said...

Wow. Thanks guys.

So Bluevark says there is an electric motor.

Dan says the prop drives the wheels.

Ed says the wheels drive the prop and because of a ratchet the prop can never drive the wheels.

O Docker says the wheels drive the prop, but only at a certain threshold speed.

Dan says that when they push the cart the wheels drive the prop, but once underway, the prop drives the wheels.

David says the wheels are geared to and turn the propeller, not the other way around.

Another David says it's all about squeezing the watermelon.

Another Anonymous says the prop is driving the wheels at all times and the relative wind speed between cart and air is irrelevant.

Joe says it's magic.


Now, I'm totally confused.

I think I'll go to bed now.

David said...

I hope you enjoyed yourself today, T-man, baiting us all into sounding like idiots. Sleep well.

Tillerman said...

Not at all David. You all sound like very smart people to me. It's just that, like all the smart people running the country, you can't seem to agree about anything.

I think I will have to ask my grandchildren to explain it to me.

David said...

Too true.

Joe said...

You guys are smart. I'm the simpleton of this group. I just re-read my comment. Hokey smokes, I wrote God's instead of Gods. Possessive, plural ...plural, possessive... Put me in the corner, I'm a dunce.

breezetrees said...

For downwind the wheels drive the prop. The prop always turns so it blows air out the back of the vehicle. For upwind sailing the prop would drive the wheels.

The best explanation I've heard of why it's not perpetual motion is something like:

If you sail DDW in a laser, you leave a zone of slower wind ahead of you. That is where the energy to move your boat came from: slowing the wind down.

The DDWFTTW cart also leaves a zone of slower wind. If it's moving faster than the wind the zone is behind it, if it's moving slower than the wind DDW it's in front like the laser, but the energy comes from the same place-- slowing the wind down.

If there is no wind to slow down there is no motion. Mark Drela's papers are the best references on the details like forces and power.

Tillerman said...

Geeze, now we have a real professor chiming in and explaining it to us. Thank you sir.

I think, having read some of the websites on the topic, I'm beginning to understand how this thing works. I've just about convinced myself that it's not really a perpetual motion machine and can understand how the prop is extracting energy from the wind to propel the vehicle forwards.

I still haven't really got my head around all the issues though. The more I think about the details of what's happening the more confused I get. I suspect this will be a subject that I return to in the coming weeks for some good old geeking out like we did on earlier posts like Why Do Sails Work?

Watch this space. If you dare.

Baydog said...

Joe, you're not a dunce. But I can't believe you don't like anchovies! Marco Pierre White uses white anchovies.

Someone had to have a comment not pertaining to the topic.

I tried to follow all of the explanations, but I got a headache. I think I'll go to bed now.

O Docker said...

Baydog, you're right.

I count something like 25 on-topic comments. That must be some kind of record for this blog.

I was waiting for someone to explain that the oil companies have known about this for years but have conspired with the auto manufacturers to suppress it. We could all be driving wind-powered cars and saving a fortune on gas.

I think this explains the mystery of the black helicopters, too. They're all flying around checking that no one shows up driving a wind-powered car. Now that this has come out in Tillerman's blog, we're all in danger.

Eeyore the old grey donkey said...

You guys are smart. I'm the simpleton of this group. I just re-read Joe's comment. Washcloth?

Tillerman said...

25 comments on topic? A record? Wow.

There must be a message there somewhere. Maybe I should always right about subjects on which everyone has an opinion but nobody can agree. The National Debt? Who invented Irish coffee? The lee bow effect?

Anonymous said...

In the words of ill educated engineers like me, "Yer wha?"

Baydog said...

Right about subjects? You're so damn good with grammar, I had to call you on that. I know, get a life Baydog.

But I'm sure you'll be able to turn it around and make me look like the dunce that I am. I'm ready

Tillerman said...

Damn. You got me there. But it passed the spell-checker so it must be write.

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