Wind powered cart goes faster than the wind, directly downwind

[RAI]

https://www.blueplanettimes.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf
Yes sorry, that link is bust, how about this one?

 

[RJJ]

I think we may see it done on a hydrofoil, but the harvesting of sufficient power from the water may be, as you say, an insuperable problem.

And it's going to need a push start of some kind. Won't accelerate from stationary.

 

[JumbleDuck]

And it's going to need a push start of some kind. Won't accelerate from stationary.

Theoretically it shouldn't be needed not but yes, to get it up on foils almost certainly will. Mind you, a big jettisonable parachute might do the trick ...

 

[mjcoon]

It's exactly the same problem, solution and model as going directly upwind with a coupled airscrew and wheels (or propeller) if you turn it upside down and fix the underneath medium. That article is OK, but makes rather a meal of something which is only counterintuitive, rather than complicated.

No, it is not the same. It is easy to imagine crawling directly upwind, much more slowly than the wind, using appropriate gearing; just use a fraction of the energy available. That is not the case if already being blown downwind.

I take it that the secondary discussion about systems that are not self starting is the sub-class of perpetual motion machine that takes an initial investment of energy and builds on it like a classic Ponzi!

Never hire an engineer to do your physics for you! ;-)

 

[JumbleDuck]

No, it is not the same. It is easy to imagine crawling directly upwind, much more slowly than the wind, using appropriate gearing; just use a fraction of the energy available. That is not the case if already being blown downwind.

The only difference is that one is generally easier to imagine, as you say, than the other. Physically the situation is identical, with a shift of axes.

When we had a mammoth thread about this a long time ago - even before this one started - I did a set of diagrams to illustrate this. I can't find them now (maybe they didn't survive the forum software change) but I did find this, which I think is still relevant:

There is no need ever to have the propeller driving the wheels. It's always the other way round.

Imagine the cart arranged so that as it moves forward the fan blows backwards. Put it at a standstill in the wind. Windage will start moving it forward. As it accelerates, the back draught from the fan increases, effectively increasing the windage, so the net force increases, and so on.

Why doesn't it increase for every? Two reasons.

First of all, there is the windage of the the rest of the structure to take account of. Without the benefit of a fan blowing backwards the force on the rest decreases as the cart speed approaches wind speed and at wind speed it reverses.

Secondly there are drag losses in rotating the propeller and these increase with the square of the speed.

For any setup of this sort, therefore, there is an equilibrium point where the the work done by the wind pushing against the fan exactly balances the work needed to turn the fan and overcome other losses. The big question is whether this occurs above or below windspeed, and that depends entirely on the setup in question.

Once final point. Loads of people think that if the car/boat exceeds the windspeed, the propeller must be doing work on the wind. That's not true! Since the cart/boat is moving downwind, the fan is blowing the air back upwind: reducing its velocity and extracting energy from it. The mechanism for extracting the power is force in the direction of the wind: some of this power is then used to overcome drag losses in the propeller.

And apropos that last point, it is important to realise that two sorts of power are involved with the propeller: the shaft power needed to turn it, which is supplied by the wheels, and the work done by the wind on it, some of which is extract by the wheels and used for the shaft power.

 

[pyrojames]

I'm with Jumbleduck on this one. It is theoretically possible. We know that you can get a VMG downwind higher than the windspeed. The America cup demonstrated this to all and sundry.

Imagine having a boat with a cross bar on it 400m wide, and a slider than ran from the tips back to the boat. Connect an AC yacht to each slider, and then have them tack down wind, sliding in and out on the sliders. The central boat with the cross bar will go dead down wind. The AC yachts will individually travel across the wind, but as a whole "vessel" they all go DDW.

The prop of the cart is effectively the sails on the AC yachts and the wheels and effectively the keels/foils on the AC yachts.

 

[RichardS]

You can theoretically connect two boats sailing mirror images courses into one which does direct downwind faster than the wind.

Errrrr ..... you could, theoretically, connect enough boats together to achieve escape velocity and get into low Earth orbit.

Richard

 

[Buck Turgidson]

deleted due to repetition.

 

[Gary Fox]

I'm with Jumbleduck on this one. It is theoretically possible. We know that you can get a VMG downwind higher than the windspeed. The America cup demonstrated this to all and sundry.

Imagine having a boat with a cross bar on it 400m wide, and a slider than ran from the tips back to the boat. Connect an AC yacht to each slider, and then have them tack down wind, sliding in and out on the sliders. The central boat with the cross bar will go dead down wind. The AC yachts will individually travel across the wind, but as a whole "vessel" they all go DDW.

The prop of the cart is effectively the sails on the AC yachts and the wheels and effectively the keels/foils on the AC yachts.

Yes it's theoretically possible.
Difficult and pointless, but possible.
Difficult because of mechanical and aerodynamic inefficiencies; pointless, well maybe not, perhaps I am just lacking in imagination.
(I'm just listening to Matt Ridley's book, 'How Innovation Works': flights of fantasy, made by unqualified dreamers and widely mocked, have resulted in some modern advances we take for granted today. The Admiralty rejected both screw propellors and turbine engines for years..)

 

[Iliade]

To add to the confusion, there is a wind gradient between the top and bottom of the turbine. So the turbine blades are (could be) effectively dynamic soaring.

 

[Elemental]

So, I've long been fascinated by this project and this thread (and the other huge one from ages ago)...

The well respected Veritasium Youtube channel has a video on this

and has recently claimed a $10,000 bet from an american physicist who was sceptical and attempted to explain the success of the Blackbird project using other effects (wind gradient et. al.)

This video contains the best explanation I've encountered of why a wind powered vehicle travelling DDWFTTW is eminently achievable.

 

[JumbleDuck]

This video contains the best explanation I've encountered of why a wind powered vehicle travelling DDWFTTW is eminently achievable.

"Because it has been done" works for me.

 

[Buck Turgidson]

I don't understand why so many smart people haven't just put a torque sensor on the drive shaft to measure the direction of power.

 

[JumbleDuck]

I don't understand why so many smart people haven't just put a torque sensor on the drive shaft to measure the direction of power.

Why would they do that when it's perfectly clear which way the power is going?

 

[Mark-1]

We know that you can get a VMG downwind higher than the windspeed.

I really think the cart is an insanely complicated way to look at this. If ice yachts/windsurfers/fast dinghies/multihulls are already significantly exceeding the windspeed with DD-VMG then why complicate it with a geared cart?

We know that 90 degs to the wind is the fastest point of sail so clearly turning away from the wind even 1 degree slows the ice yacht/windsurfer/fast dinghy/multihull down for the obvious reason. So simplest way to prove or disprove is to look at the Polars for ice yachts/windsurfers/fast dinghies/multihulls and see what the accepted DD-VMG is for speedy windpowered craft unencumbered by wheels/gears/ratchets/turbines/propellers/bearings.

Just to show willing here's my starter for 10:
40 Knots Or Not

I don't even need to dust of my trig to work out VMG - clearly they're not getting much over wind speed even when you'd think they should at 90 degs.

Here their best effort is 34mph in 20mph of wind at 120 degs, again, that's not a VMG of over wind speed DDW:
Apparent Wind Fun

I don't think the AC boats *are* getting a DD-VMG higher than windspeed, looks pretty conventional to me, thye are blisteringly fast but as soon as the move off the wind enough to make downwind progress they get stuck (I haven't worked it out so maybe I'm wrong.):
AC75 vs F50 and Maxis

Maybe others can find an assortment of polars that demonstrate you can get a DD-VMG higher than the windspeed.

(For comparison we're told Blackbird is going 2.8 times the speed of the wind DDW so we're not looking for something subtle.)

I don't understand why so many smart people haven't just put a torque sensor on the drive shaft to measure the direction of power.

I don't know which way it's going but the 'direction of power' must be known because there's a ratchet:

DDWFTTW: Looking for the least confusing explanation

Make of that what you will.

EDIT: Regarding Polars, they'd nail it but just as good would be a televised Ice Yachting event - they have a dead down wind leg. We'd see the point where the DD-VMG went above windspeed because the sail would change sides and the ice yachts would finish the leg appearing to sail upwind to the leeward mark because no component of the apparent wind on the ice yachts would be from behind.

 

[RichardS]

I really think the cart is an insanely complicated way to look at this. If ice yachts/windsurfers/fast dinghies/multihulls are already significantly exceeding the windspeed with DD-VMG then why complicate it with a geared cart?

None of those can travel directly downwind at faster than wind speed. They are all limited to windspeed. The only way to achieve faster than the wind directly downwind is with gearing, more or less complicated.

Richard

 

[Mark-1]

None of those can travel directly downwind at faster than wind speed. They are all limited to windspeed. The only way to achieve faster than the wind directly downwind is with gearing, more or less complicated.

Richard

They are said to achieve a VMG dead downwind faster than the wind so they addresses the key issue - which is why they are frequently cited in this context (and in this thread).

I've watched some video of ice yachts turning on the windward mark which settles it for me. (I won't say which way). Searching for "Polars" for something that travels on ice was slightly less successful. ?

Plenty of Polars for fast water craft though.

 

[RichardS]

They are said to achieve a VMG dead downwind faster than the wind so they addresses the key issue ...

There's no argument about that which is why they are frequently quoted, but VMG downwind is a different thing.

These videos are about a particular, long-debated, scientific challenge which is "faster than the wind, directly down-wind". It has been thought for decades, probably centuries, that this particular aspect of downwind sailing was impossible as the wind would always be the limiting factor when going directly downwind, just as it is with a goose-winged sailing boat. However, it appears to be possible after all.

Richard

 

[flaming]

I don't think the AC boats *are* getting a DD-VMG higher than windspeed, looks pretty conventional to me, thye are blisteringly fast but as soon as the move off the wind enough to make downwind progress they get stuck (I haven't worked it out so maybe I'm wrong.):
AC75 vs F50 and Maxis

Oh please.
[video]

Here's the full race replay from day 1 of the most recent cup. If you watch the build up they say the legs are 1.85 NM. The wind is average 12kts.
ETNZ go round the top mark for the 1st time at 18:55. They then go downwind at over 40 knots, arriving at the leeward mark at 22:08. So that's 1.85NM downwind covered in 3 minutes and 13 seconds.
If my math's is correct that's a VMG of something like 34knots.

In 12 knots of wind.

If they weren't going faster than the wind downwind, that leg would have taken them over 9 minutes.

 

[Mark-1]

Oh please.
[video]

Here's the full race replay from day 1 of the most recent cup. If you watch the build up they say the legs are 1.85 NM. The wind is average 12kts.
ETNZ go round the top mark for the 1st time at 18:55. They then go downwind at over 40 knots, arriving at the leeward mark at 22:08. So that's 1.85NM downwind covered in 3 minutes and 13 seconds.
If my math's is correct that's a VMG of something like 34knots.

In 12 knots of wind.

If they weren't going faster than the wind downwind, that leg would have taken them over 9 minutes.

2 hours of footage, great, sounds like somewhere in there is the conclusion.

1) Post the polars for any boat of your choice that proves the point.
2) If you think using the footage is simpler, just identify a timestamp of a point where you're saying the boat is going in a downwind direction with a VMG higher than windspeed. If you can pick a point where they're about to round the leeward mark even better - we can see the downwind ~headwind turn into a different ~headwind as it turns. Easy to verify because in the case you describe one component of that apparent wind will be a 22kt wind in the opposite direction to the natural wind and we know where that will put the sail. Zero component will be from behind the boat. So we know where the sail should be and where it shouldn't be.


EDIT: I did 2) myself.

I've started the vid at the start of the Downwind leg:

Firstly, the side the main's on. Clearly the wind is (to a degree) from behind still. The sail would be on the other side if a significant component was a 22kt headwind.

But there's a clincher. The boats Gybe. You can't gybe into a 22 knot headwind, you'd tack. And at the point of the gybe the boat is doing 22kts. So unless we think the boat is slowing to below windspeed foor the gybe there is some wind from behind.

No idea how to explain the displayed numbers given that but the sail can't lie, those boats have some wind from behind so their VMG is not higher or equal to windspeed.