This video blew my mind.
It makes the idea of going down wind on angles with VMG greater than wind speed seem so much more plausible. Mechanically its different , but ultimately there are similarities. That is exploiting the gradient between moving wind and stationary water to capture energy, and then using angles, gears , levers etc to go faster than the gradient you are exploiting.

I think this will get you all thinking.

Sorry heavy, we've been here a couple of times before, and I think most of us know what's going on.
Apart from the few that still deny it's possible.

You just have to get your mind around, the wheels driving the prop, not the prop driving the wheels.
With a different gear ratio, the prop would drive the wheels and it would go upwind.

It's already been done. Plenty of boats can already do downwind and upwind vmg's greater than windspeed. I think in some cases more than double. Wouldn't suprise me if the windfoilers are doing VMG downwind greater than windspeed.

en.wikipedia.org/wiki/USA_17

Race 1. Wind speeds were 5-10 knots (9.3-18.5 km/h; 5.8-11.5 mph) during the first race. USA 17 reached the windward mark in 1h29, so her velocity made good was about 13.5 knots (25.0 km/h; 15.5 mph), or about 1.8 times wind speed. USA 17 took 63 minutes to reach the downwind mark, so her velocity made good downwind was about 19 knots (35 km/h; 22 mph), or about 2.5 times wind speed

Race 2. Wind speeds were 7 to 8 knots (13 to 15 km/h; 8.1 to 9.2 mph) during the second race. USA 17 reached the windward mark in 59 minutes, so her velocity made good was about 13.2 knots (24.4 km/h; 15.2 mph), or about 1.65 times wind speed. The course was a triangle, so the velocity made good downwind was only 11.5 knots (21.3 km/h; 13.2 mph), or about 1.4 times wind speed. USA 17 averaged 26.8 knots (49.6 km/h; 30.8 mph), or about 3.6 times the wind speed, on the faster first triangular leg

AC72s are hitting above 30kts both up and down wind in under 15kts of wind speed so it's very possible for a foiling craft.

Nice examples guys. Still seems to be a bit of a mystery in less well informed people. I thought it was a cool example.
Yes the wheels are driving the prop. But it only works because the air is realatively still allowing the prop to get more purchase on the air. Compared to a zero wind situation . In zero wind the apparent wind would make it harder for the wheels to get any value out of pushing the prop. Hence avoiding a perpetual motion machine.

I must admit... I can understand broad reaching faster than the wind. I also know vmg can be faster than the wind. But getting your head around why is interesting. I felt like this example helped that a bit.

Another question ???
Racing to bottom mark... on plane or foil... what strategy gives best vmg.
1... go as deep as you can... staying certain not to drop off plane or foil.
2... go progressively deeper until you start to slow down slightly... then tighten up... concentrate on speed ... not depth.

I've heard both arguments and no doubt it depends on the craft... but what do you think?

Foils regularly outpace the wind speed upwind and downwind.

Select to expand quote

Maddlad said..
Foils regularly outpace the wind speed upwind and downwind.

But not directly downwind!. Once you're heading directly downwind at wind speed, there's no force in the sail!
It's easy to beat windspeed at an angle to the wind, that's a different matter.

Select to expand quote

Heavy1 said..
Yes the wheels are driving the prop. But it only works because the air is realatively still allowing the prop to get more purchase on

I'm not sure about that. I think the faster the wind, the faster the craft. There's no chance it can work in 0 wind.
The drag on the craft has to be greater than the torque from the prop, to start it moving downwind. If the prop torque is greater than the vehicle's drag it will go upwind. Which way it goes depends on the gear ratios.

Mind you this is only what I think, I really should build one and test this out.

Select to expand quote

decrepit said..

Maddlad said..
Foils regularly outpace the wind speed upwind and downwind.

But not directly downwind!. Once you're heading directly downwind at wind speed, there's no force in the sail!
It's easy to beat windspeed at an angle to the wind, that's a different matter.

Well, not forgetting apparent wind angle change. As you accelerate off breeze you start generating apparent wind angle change, which can see you round to straight downwind given enough breeze.

Select to expand quote

Subsonic said.. Well, not forgetting apparent wind angle change. As you accelerate off breeze you start generating apparent wind angle change, which can see you round to straight downwind given enough breeze.

I'd like to see that done!
Apparent wind becomes more head on, the faster you go, that's why you can sheet in going deep.

Select to expand quote

Heavy1 said..
Another question ???
Racing to bottom mark... on plane or foil... what strategy gives best vmg.
1... go as deep as you can... staying certain not to drop off plane or foil.
2... go progressively deeper until you start to slow down slightly... then tighten up... concentrate on speed ... not depth.

I've heard both arguments and no doubt it depends on the craft... but what do you think?

It depends on the gear tuning and the pilot. When I was competing in Formula, option 1 always worked best for me personally. Plus being lightweight I could push deeper than others and overtake people that way.

Select to expand quote

decrepit said..

Subsonic said.. Well, not forgetting apparent wind angle change. As you accelerate off breeze you start generating apparent wind angle change, which can see you round to straight downwind given enough breeze.

I'd like to see that done!
Apparent wind becomes more head on, the faster you go, that's why you can sheet in going deep.

I think maybe because im so light i get to see it happen foiling (on course race foils) maybe not so common amongst heavier foilers. Its generally on a spanking gust that it happens.

I'm also trying to get my head around it . I understand and do it , going across wind at any angle . But going directly downwind eludes me . It's like the old roadrunner cartoon with the fan blowing on to the sail thing . I can see how the model on the treadmill works . The fan is on an angle pushing down on to the belt using the belt energy to go forward . So it's not going directly downwind . Tip it sideways and it's going cross wind . I'd bet if the fan was parallel to the belt it would not work .
If it does supposedly work , does this not claim perpetual motion ?

Select to expand quote

Subsonic said..
I think maybe because im so light i get to see it happen foiling (on course race foils) maybe not so common amongst heavier foilers. Its generally on a spanking gust that it happens.

So are you sure it's not you pumping the foil?
And are you sure you're going at 0 degrees to the real wind?
That's a bit hard to judge when you're going fast and the apparent wind is coming from in front of you.

Select to expand quote

Imax1 said..
I'm also trying to get my head around it . I understand and do it , going across wind at any angle . But going directly downwind eludes me . It's like the old roadrunner cartoon with the fan blowing on to the sail thing .

I can go directly downwind, but it's slow and awkward, standing behind the rig with feet square. Much easier and faster to tack downwind.
Think back to the last America's cup, with a downwind course, non of them went the shortest route straight down wind, they all zig-zagged

Select to expand quote

decrepit said..

Imax1 said..
I'm also trying to get my head around it . I understand and do it , going across wind at any angle . But going directly downwind eludes me . It's like the old roadrunner cartoon with the fan blowing on to the sail thing .

I can go directly downwind, but it's slow and awkward, standing behind the rig with feet square. Much easier and faster to tack downwind.
Think back to the last America's cup, with a downwind course, non of them went the shortest route straight down wind, they all zig-zagged

Yeah , I reckon this going absolutely directly downwind faster than the wind is not a thing .
But for science , I want to be proved wrong .

Select to expand quote

decrepit said..

Subsonic said..
I think maybe because im so light i get to see it happen foiling (on course race foils) maybe not so common amongst heavier foilers. Its generally on a spanking gust that it happens.

So are you sure it's not you pumping the foil?
And are you sure you're going at 0 degrees to the real wind?
That's a bit hard to judge when you're going fast and the apparent wind is coming from in front of you.

Definitely no pumping involved. As to whether im sure its direct 0degrees downwind, i suspect you already know the answer to that. The point of reference is wave direction(wind ripples at times on flat water). Yes waves don't always follow wind direction, but the majority of the time they do.

I'd hazard a guess that when it happens it's actually a knocking gust, which is a temporary change of wind angle in favour of going deeper, So whilst it gets me downwind at "0 degrees downwind" on the true wind, technically its probably cheating.


keep in mind that course race foiling you run much deeper than you'll ever go on normal gear full stop. Thats just how it is to ride. Sailing normal windsurf angles on it can be very disconcerting. Overall speed wise, its slower. But you'll certainly get to a direct downwind mark faster owing to better offwind angles.

I think the science concluded that you cant go faster than than true wind directly downwind with a fixed sail, but you can with a rotating sail (propellor). It's all about the apparent wand angle seen by the foil/sail.

Select to expand quote

Imax1 said.. Yeah , I reckon this going absolutely directly downwind faster than the wind is not a thing .
But for science , I want to be proved wrong .

It's certainly a thing for the wheel powered prop, but I have the biggest doubt possible kit's a thing for fixed sails.

Select to expand quote

Subsonic said.. keep in mind that course race foiling you run much deeper than you'll ever go on normal gear full stop. Thats just how it is to ride. Sailing normal windsurf angles on it can be very disconcerting. Overall speed wise, its slower. But you'll certainly get to a direct downwind mark faster owing to better offwind angles.

So do you wear a GPS?
Can we see some tracks?
A polar plot would be great.

Select to expand quote

Subsonic said..

decrepit said..

Subsonic said..
I think maybe because im so light i get to see it happen foiling (on course race foils) maybe not so common amongst heavier foilers. Its generally on a spanking gust that it happens.

So are you sure it's not you pumping the foil?
And are you sure you're going at 0 degrees to the real wind?
That's a bit hard to judge when you're going fast and the apparent wind is coming from in front of you.

Definitely no pumping involved. As to whether im sure its direct 0degrees downwind, i suspect you already know the answer to that. The point of reference is wave direction(wind ripples at times on flat water). Yes waves don't always follow wind direction, but the majority of the time they do.

I'd hazard a guess that when it happens it's actually a knocking gust, which is a temporary change of wind angle in favour of going deeper, So whilst it gets me downwind at "0 degrees downwind" on the true wind, technically its probably cheating.


keep in mind that course race foiling you run much deeper than you'll ever go on normal gear full stop. Thats just how it is to ride. Sailing normal windsurf angles on it can be very disconcerting. Overall speed wise, its slower. But you'll certainly get to a direct downwind mark faster owing to better offwind angles.

No disrespect intended but not sure you are on a consistent basis - reason is that when you are faster than the wind offwind, the apparent wind is coming more and more from the front ie you are really close hauled. Just like going traditionally upwind, there is a point where there just isn't enough forward lift to overcome the drag forces which is why you can't go straight downwind.

if I understand things (I spent an hour last night in the rabbit hole), the gearing change between the wheels and prop on the wind car make it possible and remember that while the car is still headed straight downwind, the tips of the props are still sweeping across the wind ie tacking. The limit of the car is both drag, which while lower than a sailing vessel still catches up with you, and the prop eventually, like us, is too much into the wind. He has, in bursts, gotten to 3x the speed of wind but his official record as of the video below was 2.8x. This is all in wind speeds of 10-15 kts.

A couple of good videos - the "originator of the car" (he does say that at least two people prior to him had done it as well) at the St. Francis Yacht Club He shows a number of different animations that are helpful in wrapping your head around things



How to build your own that works on a treadmill

It certainly not a consistent ride direct downwind, never said it was. Gusts don't last forever. But when riding that gust.


it's the apparent wind angle shift (the wind coming from further ahead) that allows/ causes a dive further downwind to keep the sail in the power zone. It's not limitless, and as I said it's more than likely a knocking gust that allows the direction change to direct downwind. Which probably quantifies to technically not actually pointing directly downwind.

i could stick the gps on next time it's decent, post the seabreeze wind direction at the time from the closest location with it, but that's around about as good a reference point as me taking reference of wind direction from wind ripples on the water ahead of me, Probably worse. I've been meaning to buy a go pro. Maybe now's the time.

Just make a polar plot (like the one I attached ealier) and you'll see that you'll never be able to go straight downwind.

At 160 degrees it can certainly feel like going straight downwind though.

Dont think its possible with a fixed sail.
Rotating wing(prop) is NEVER actually directly downwind. Always at an angle of attack.

Select to expand quote

olskool said..
Dont think its possible with a fixed sail.
Rotating wing(prop) is NEVER actually directly downwind. Always at an angle of attack.

exactly, my guess is that the tips of the propeller are rotating fast so they are getting an apparent wind that is not directly downwind

Select to expand quote

powersloshin said.. exactly, my guess is that the tips of the propeller are rotating fast so they are getting an apparent wind that is not directly downwind

This is probably true, but misses the point.
The prop isn't driving the wheels!
So angle of attack is irrelevant.

This looks like your classic perpetual motion machine. the wheels drive the prop, and the prop pushes the vehicle forwards.
But it isn't, in no wind it won't work.
it's the wind force on the vehicle itself that initially pushes it forward.
Then the prop turning is a positive feedback loop, the faster it goes the more force from the prop, the faster it goes, until as paducah says drag catches up with it.

Select to expand quote

SurferKris said.. It depends on the gear tuning and the pilot. When I was competing in Formula, option 1 always worked best for me personally. Plus being lightweight I could push deeper than others and overtake people that way.

Wow, consistent speed from 45 degrees to 150.
I guess that's race tuning, you need that.
I tune for speed, so there's a huge difference over that range.

I'm actually keen to get out and get some gps data and see what it looks like as opposed to what I'm seeing. Just need some wind that aligns with not being at work. Id also like to make a polar plot of it like yours (SurferKris) but I've not a clue how to begin. get some data first I guess.

Select to expand quote

Subsonic said..
I'm actually keen to get out and get some gps data and see what it looks like as opposed to what I'm seeing. Just need some wind that aligns with not being at work. Id also like to make a polar plot of it like yours (SurferKris) but I've not a clue how to begin. get some data first I guess.

Easiest way is to download GPSSpeadreader. It's only "beer money"
ecwindfest.org/GPS/GPSSpeedreader.html
Here's my PB day at Lilacs Albany


The polar plot is under the view menu.

See how my speed is much less consistent with angle to the wind. I think the few spikes downwind are gybes.
I think wind angle is assumed to be a bisect of the upwind angle. In this case probably not accurate as I was running a starboard assy fin

Apparent wind. Formula racers in the old days, and foil course racers now, all went downwind much faster than the static wind. Always at a VMG, never dead downwind. AC boats, too.

If you do a vector analysis of the wind and its forces, you see that downwind the apparent wind hits you from the side, not the front. This is why you release the clew with adjustable outhaul to curve the leading edge to make it catch the sideways apparent wind with little drag.

Upwind, the apparent wind hits you from the front. You tighten the adjustable outhaul to flatten the leading edge to knife into the apparent with with little drag.