FAO Ubergeekian.

[Mark]

There's a thread on TOP about you which lead me to your website.

Firstly, congratulations on escaping TOP. As an occasional TOP poster I predict you won't regret it. Secondly, I see you've met/know Johnny Ball! I couldn't be more impressed if Jesus had washed your feet. (Has he?)

It seems clear from you website that you know a thing or three about maths and don't mind sharing it. That sparked in my mind a memory of the DWFTTW debate. Until now I just assumed it was simple, I thought that to travel downwind faster than the wind you'd need to accelerate in an apparent wind of zero, therefore it wasn't possible. I was so certain I didn't even bother checking.

However, I do remember you saying the maths of DWFTTW was simple. (I don't remember if the maths proves it possible or impossible!) I didn't know your background so I just assumed it was bull****.

Anyway my maths goes no further than K.A. Stroud, can you spoonfeed me the maths I need to work out a definitive answer for myself so I can amaze and delight my friends? (and I do mean spoonfeed!)

[ubergeekian]

There's a thread on TOP about you which lead me to your website.

Yes, I know about that thread. Ho hum.

Firstly, congratulations on escaping TOP. As an occasional TOP poster I predict you won't regret it. Secondly, I see you've met/know Johnny Ball! I couldn't be more impressed if Jesus had washed your feet. (Has he?)

Neither Jesus nor Johnny Ball has, of the best of my knowledge, washed my feet. But yes, meeting JB was a great thrill, and not just for me: everybody on-set wanted to talk to him. He's a delightful chap.

In case anyone is wondering what this is about, by the way, watch BBC2 at 6pm on Friday 23rd December and all will be revealed ...

Anyway my maths goes no further than K.A. Stroud, can you spoonfeed me the maths I need to work out a definitive answer for myself so I can amaze and delight my friends? (and I do mean spoonfeed!)

Ooer. Quite a time since all that stuff came up. If you PM me an email address, I'll try to dig something out. The basic principal is a lot simpler than the sums, by the way. Here's one way of looking at it ...

1. Are you happy with the idea that a boat with turbine and propellor, suitably linked, can go directly into wind on a calm sea? If so, think of this as a device sitting at the interface between two mediums. M1 (the sea) is doing 0, M2 (the air) is doing -v and D (the device) is doing +u ... ie D is making headway at u into a wind with absolute speed v.

2. Now, change to a set of axes moving at -v (with the wind). In the new axes system, M1 is doing +u, M2 is doing 0 and D is doing u+v.

3. Now turn the whole thing upside down (or move to Australia). You now have a fixed medium (M2) with a moving medium (M1) doing u and a device (D) sitting in between and using the velocity difference to do u+v.

4. Relabel M2 as "the sea", M1 as "the wind" and "D" as "a device which is moving directly downwind faster than the wind"

Oh dear, this would be much easier with diagrams. However, my point is that, at a theoretical level, building a device which can go DDWFTTW is exactly the same as building a device with can go DIW. They both exploit the velocity difference between two fluids.

Where most people go wrong with the DDWFTTW idea is to cling to the idea that the wind blows the air screw which turns the wheels/propeller which pushes the thing into the wind. Once you think of it as the road/sea turning the wheels/propeller which operate the air-screw to push the cart along, it all becomes clearer. At least it did for me, though it took a fair amount of sitting around with a damp towel on my head to get it straight.

Or, to put it yet another way, it doesn't matter that there comes a point when the vehicle sees no apparent wind, because the airscrew can still produce thrust then, just as the airscrew on a Cessna 150 can produce thrust when the aircraft is sitting on teh ground in a calm day.

Oh, sorry to have gone on so long. My only defence is that I find this sort of thing fun.

[marisca]

That's all very well but what I want to know is where the bumble bee that passed me heading east when I was between Islay and Rathlin came from. And how does it manage to produce sufficient lift to do it?

[Mark]

Yes, I know about that thread. Ho hum.

90pc flattering, I thought. I doubt many other forumites would have fared better.

But yes, meeting JB was a great thrill, and not just for me: everybody on-set wanted to talk to him. He's a delightful chap.

A hero to a generation for so many reasons. I even remember him explaining the physics of sailing with a broomstick which wsa responsible for a pretty direct improvement in my performance on the water.

Anyway my maths goes no further than K.A. Stroud, can you spoonfeed me the maths I need to work out a definitive answer for myself so I can amaze and delight my friends? (and I do mean spoonfeed!)

Ooer. Quite a time since all that stuff came up. If you PM me an email address, I'll try to dig something out. The basic principal is a lot simpler than the sums, by the way. Here's one way of looking at it ...

1. Are you happy with the idea that a boat with turbine and propellor, suitably linked, can go directly into wind on a calm sea? If so, think of this as a device sitting at the interface between two mediums. M1 (the sea) is doing 0, M2 (the air) is doing -v and D (the device) is doing +u ... ie D is making headway at u into a wind with absolute speed v.

2. Now, change to a set of axes moving at -v (with the wind). In the new axes system, M1 is doing +u, M2 is doing 0 and D is doing u+v.

3. Now turn the whole thing upside down (or move to Australia). You now have a fixed medium (M2) with a moving medium (M1) doing u and a device (D) sitting in between and using the velocity difference to do u+v.

4. Relabel M2 as "the sea", M1 as "the wind" and "D" as "a device which is moving directly downwind faster than the wind"

Oh dear, this would be much easier with diagrams. However, my point is that, at a theoretical level, building a device which can go DDWFTTW is exactly the same as building a device with can go DIW. They both exploit the velocity difference between two fluids.

Where most people go wrong with the DDWFTTW idea is to cling to the idea that the wind blows the air screw which turns the wheels/propeller which pushes the thing into the wind. Once you think of it as the road/sea turning the wheels/propeller which operate the air-screw to push the cart along, it all becomes clearer. At least it did for me, though it took a fair amount of sitting around with a damp towel on my head to get it straight.

Or, to put it yet another way, it doesn't matter that there comes a point when the vehicle sees no apparent wind, because the airscrew can still produce thrust then, just as the airscrew on a Cessna 150 can produce thrust when the aircraft is sitting on teh ground in a calm day.

Oh, sorry to have gone on so long. My only defence is that I find this sort of thing fun.

Thanks.

[mm5aho]

1. In the boat scene above, is there any assumption about fluid density and slippage of the relevant propellor?

2. Who is Johnny Ball?

[ubergeekian]

1. In the boat scene above, is there any assumption about fluid density and slippage of the relevant propellor?

No, and that's the Big Fat Elephant in the room. The theory of extracting energy from the interface between two moving fluid is one thing and fine: actually doing it is quite another. It's much easier if one of the fluids is actually a solid, which is why the Youtube DDWFTTW demonstrations (which if I recall correctly got up to about 3x windspeed) are on salt flats. It may be doable on water, but my gut feeling is that it will be a lot more marginal, and possibly need some sort of fancy variable-ratio transmission between the propellers.

Of course BMW Oracle has already done it, sort of. All you need are two of them tacking down wind in perfect mirror image unison and joined together by a bungee with a bloke in a canoe in the middle...

2. Who is Johnny Ball?

Are you under 40, by any chance?

Johnny Ball was the presenter of a series of children's TV shows from the 60s to the 80s. He started - well, I first saw him - on Play School, but his best were the science and maths shows, starting with Think of a Number. He had a lovely enthusiasm and a way of talking to children as a non-patronising adult which has almost wholly disappeared from TV these days, now that all children's TV presenters have to be 26 year olds pretending, in a slightly creepy way, to be about 15.

He's also the father of Zoe Ball, who is married to Fatboy Slim.

[wully]

just as the airscrew on a Cessna 150 can produce thrust when the aircraft is sitting on teh ground in a calm day.

Oh, sorry to have gone on so long. My only defence is that I find this sort of thing fun.

Define 'calm' if there is no wind then there can be no thrust?

And doesn't the whole thing depend on the force acting on the power turbine, the friction loss in the generation system and the force required to overcome the inertia and , and ,and.

My head hurts.

[Mark]

2. Who is Johnny Ball?

A sort of 80's Brian Cox/Karl Segan for kids.

http://www.youtube.com/watch?v=NVPwYCcsAr0

As some b@5t@rd newsreader said the other week about Jimmy Saville, "Your Dad may have heard of him."

[mm5aho]

Are you under 40, by any chance?

No, over 50, but I'm an immigrant. Only been here 20 years so missed 1980's UK TV.
And I watch very little (perhaps under 30 mins/month) of 2000's UK TV, so perhaps ignorant of these things!

I once saw a wind turbine to water propellor powered small boat, but ut could not go directly into the wind. Too much loss in the transmission system perhaps, it was pretty crude.

[ubergeekian]

just as the airscrew on a Cessna 150 can produce thrust when the aircraft is sitting on teh ground in a calm day.

Oh, sorry to have gone on so long. My only defence is that I find this sort of thing fun.

Define 'calm' if there is no wind then there can be no thrust?[/quote]

With no relative wind a sail can't produce thrust, but a propeller rotated by some external agency (like a water screw and some gears) can.

And doesn't the whole thing depend on the force acting on the power turbine, the friction loss in the generation system and the force required to overcome the inertia and , and ,and.

Yup.

My head hurts.

If it didn't you wouldn't be thinking about it hard enough. tricky stuff. Now my head hurts as well.

[ubergeekian]

Are you under 40, by any chance?

No, over 50, but I'm an immigrant. Only been here 20 years so missed 1980's UK TV.
And I watch very little (perhaps under 30 mins/month) of 2000's UK TV, so perhaps ignorant of these things!

I don't even have a television. What is this "X Factor" of which people speak?

I once saw a wind turbine to water propellor powered small boat, but ut could not go directly into the wind. Too much loss in the transmission system perhaps, it was pretty crude.

Quite a lot of these have been made. There's one in the Scottish Maritime Museum at Irvine, which was made at a university (Strathclyde?) and I think used to live at Tighnabruiach in the 70s/80s. Carefully made they do OK, but not as well as something with sails. BMW Oracle is, I suspect, doing about as well as anything will. Aerofoil efficiency increases with span, so a towering rig and deep keel are always going to be more efficient and effective than comparatively short propeller blades.

[mm5aho]

"I don't even have a television."

ditto. Incredible consumer of time and productivity! (internet fast catching up!)

[Booby Trapper]

Johnny Ball was the presenter of a series of children's TV shows from the 60s to the 80s. He started - well, I first saw him - on Play School, but his best were the science and maths shows, starting with Think of a Number. He had a lovely enthusiasm and a way of talking to children as a non-patronising adult which has almost wholly disappeared from TV these days, now that all children's TV presenters have to be 26 year olds pretending, in a slightly creepy way, to be about 15.

He's also the father of Zoe Ball, who is married to Fatboy Slim.

I believe he actually started out as a stand up comedian.

[Bodach na mara]

Uber, My brain is now nipping, but at step 2 I wonder if you have mixed up the u s and v s. As I see it, if you change the frame of reference to a stationary wind (M2, 0) and moving sea (M1, +v), the device is moving at u + v but as u = -kv then the device vector is v(1 - k)

I am not sure where I am going with this and it is rather late so I will cop out at this point.

[Gordonmc]

That's all very well but what I want to know is where the bumble bee that passed me heading east when I was between Islay and Rathlin came from. And how does it manage to produce sufficient lift to do it?

It has been proven mathematically that a bumble bee cannot fly.
(!)