Wind powered cart goes faster than the wind, directly downwind

[flaming]

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.

What in the world are you talking about?

Very clearly I've just used that video to demonstrate an AVERAGE downwind VMG at approx 3x wind speed.

Why are you questioning this?

 

[pyrojames]

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.

Are you suggesting that the true wind direction changes every time they round the mark? What about when one is still on the upwind leg and the other is on the down wind? The observed sail set doesn't give you any useful information on wind direction when the SOG is significantly higher than the windspeed.

VMG was frequently shown during the footage for both up and down wind and was invariably higher that the true wind speed.

The AC boats clearly show downwind VMG higher than the windspeed, why on earth are you talking about polars, when there is clear evidence in plain record?

 

[Mark-1]

I edited my original post here it is as a new post:


I did 2) myself.

I've started the vid at the start of the Downwind leg at 19:15:

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 wind is ~22kts on the nose. So unless we think the boat is slowing to below windspeed for 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.



But Polars would nail this and are much easier to interpret.

 

[pyrojames]

snipped...
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.

But Polars would nail this and are much easier to interpret.

OK here are polars for the AC 72 (AC72 Polars & Penalties - Catamaran Racing , News & Design) not the AC75, but still downwind VMG greater than wind speed.

 

[Mark-1]

OK here are polars for the AC 72 (AC72 Polars & Penalties - Catamaran Racing , News & Design) not the AC75, but still downwind VMG greater than wind speed.

Yeah, if we're talking about the diagram at the top I found that last night but couldn't work out what it was telling me, axis aren't labelled and I can't see the angle to the wind. But yeah, if you can tell me (and maybe offer a sample offwind angle from the diagram that gives a VMG over windspeed) then it looks like we have our answer. (Much easier to do this with Polars - they're ubiquitous now, not so much 12 years ago.)

 

[flaming]

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 wind is ~22kts on the nose. So unless we think the boat is slowing to below windspeed for the gybe there is some wind from behind.

No, the apparent wind through the gybe is ahead, as far as the sail is concerned it tacked.

This is very clear from the footage. You only have to look at the footage of a gybe, where the speed of the boat never drops below 30 knots, and remember that the true wind was about 12 knots to understand that.

Look, I get that this can be a bit of a mind messing subject, but you're being presented with very clear evidence. If you want more, pause the video at 21:07. There's VMG figures for you to look at. Of circa 35 knots. In 12 knots of breeze.
Or do what I did, time the boats from top to bottom. And work out how fast they VMG on average.

I really don't know how to lay it out any more simply than that.

 

[pyrojames]

No, the apparent wind through the gybe is ahead, as far as the sail is concerned it tacked.

snipped...

Interesting one, tacking or gybing. When on starboard, they turn to port to come onto port, as if gybing, yet the sail tacks through the wind but in the opposite way to a conventional tack, i.e. turn to starboard to come onto port.

 

[Mark-1]

I really don't know how to lay it out any more simply than that.

Polars are simpler that videos, if this was any other situation and someone said "How fast do that boat go on a specific course" you'd answer with a link to a Polar diagram not with a video. However, if we're going to to try to work it out from a video then I stand by post 63. You can see what's happening from the sails on the downwind leg and the commentators describe the gybes as gybes and the tacks as tacks.

I can't reconcile that with the boats seemingly covering the downwind leg three times faster than the wind speed but sails don't lie, maybe instruments do.

 

[flaming]

Polars are simpler that videos, if this was any other situation and someone said "How fast do that boat go on a specific course" you'd answer with a link to a Polar diagram not with a video. However, if we're going to to try to work it out from a video then I stand by post 63. You can see what's happening from the sails on the downwind leg and the commentators describe the gybes as gybes and the tacks as tacks.

[/QUOTE]
Gybes are still called gybes, because the stern turns through the true wind. If this wasn't the case we'd have to think of a whole new language.

If you think the sails are not behaving as if the apparent wind is going round the front, then can I suggest looking again, they quite clearly are.....

Polars are pretty difficult to get for AC75s, for obvious reasons of teams not releasing them, but here's a reverse challenge. If the AC75 is doing 40 knots in 12 knots of wind as we see on that video, what angle must it be doing in order NOT to be VMGing downwind faster than 12 knots? Bonus question - does the angle you see on virtual eye look like your calculated figure.... Double bonus question, how long would it take them to get to the bottom mark sailing that angle?

I can't reconcile that with the boats seemingly covering the downwind leg three times faster than the wind speed but sails don't lie, maybe instruments do.

Or, with the obvious display of doing 3x the wind speed down wind for all the world to see.... Perhaps you should reconsider your post 63....

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.

The main is on the same side of the jib, what you're seeing there is the return from the hinged boom, the haul the boom over the centreline but twist the top. Again, all this is very well understood. On any skiff type boat this is done to a degree, trav up to bring boom close to centreline, sheet off. Faster you go, more up to the centreline your boom becomes sailing downwind. The apparent wind angle for an AC75 when sailing downwind is thought to be about 20 degrees.

Here's a video from the AC72 era.

Note Dean Barker, helm of ETNZ in that cup, telling you that they "effectively tack their way downwind".

AC75s are faster still....

 

[Mark-1]

....

Save words: Post a time offset in the video for a gybe on a downwind leg you think is obviously a tack on the downwind leg.

Save more words: Polars for a few suitable boats or ice yachts would be much simpler than us all squinting at sails on videos.

 

[flaming]

Save words: Post a time offset in the video for a gybe on a downwind leg you think is obviously a tack on the downwind leg.

They all are. That's the point.... At this point in the history of the cup, it's not even remarkable, they've been doing that since the AC72s in 2013. I cannot for the life of me understand why you seem unable to accept that fact.

Save more words: Polars for a few suitable boats or ice yachts would be much simpler than us all squinting at sails on videos.

That is what the virtual people are using. Best that seems to be available.

It is absolutely staggering that you can listen to words of Ben Ainslie and Dean Barker describing sailing the boats, and still think that the apparent is not ahead in a gybe.

Or you could just explain how you can watch a video of a gybe, where the wind speed was 12 knots, and the boatspeed never drops below 30, and still think the apparent was aft....
Or you could explain how a boat gets to the bottom mark 3 times faster than the windspeed but apparently hasn't VMG'd faster than the wind.

 

[JumbleDuck]

Or you could explain how a boat gets to the bottom mark 3 times faster than the windspeed but apparently hasn't VMG'd faster than the wind.

It's amazing how defensive people can get when their preconceptions are challenged. In the big DDWFTTW thread we had people claiming that yes, it's possible exactly downwind but the slightest deviation, even 1/100 degree, makes it impossible.

 

[Mark-1]

They all are.

As a favour, just humour me and post the time stamp of one.

Thanks that looks conclusive, where's it from?

 

[pyrojames]

Flaming, the VMG is at 153 off the wind and 44.2 knots but the actual VMG DDW is 39.4. Still somewhat in excess of of the wind speed!

 

[Mark-1]

Flaming, the VMG is at 153 off the wind and 44.2 knots but the actual VMG DDW is 39.4. Still somewhat in excess of of the wind speed!

That's fair enough, it's giving velocity at that angle and noting that that is the course for max vmg. Perfectly reasonable to leave it to the reader to work out the actual VMG DDW.

 

[DJE]

If we're agreed that a boat can sail at an angle to the wind and achieve a downwind VMG greater than windspeed, I think all that remains is to note that the propeller tips on the cart are not travelling dead downwind but along helical paths at a constant angle to the wind. So the cart is doing nothing that the high performance sailing boats don't do. That's the way that I look at it anyway.

 

[RichardS]

If we're agreed that a boat can sail at an angle to the wind and achieve a downwind VMG greater than windspeed, I think all that remains is to note that the propeller tips on the cart are not travelling dead downwind but along helical paths at a constant angle to the wind. So the cart is doing nothing that the high performance sailing boats don't do. That's the way that I look at it anyway.

Except that it's the wheels that are turning and providing motive power to the fan rather than the other way around.

Richard

 

[DJE]

Except that it's the wheels that are turning and providing motive power to the fan rather than the other way around.

Richard

The wheels and the gearing force the propeller tips to move at and angle to the wind. Just like the dagger boards on the sailing boats.

 

[Mark-1]

Or you could just explain how you can watch a video of a gybe, where the wind speed was 12 knots, and the boatspeed never drops below 30, and still think the apparent was aft....

Well you or (someone else) post a timestamp for a gybe you regard as a good example and I will, if I think it was.

EDIT: Ok, I'll pick one.

19:26. Gybe not tack.
Starboard foil goes down so Starboard is the leeward side on the new gybe - it is also the side nearest the leeward mark.

Yes, I can't reconcile that with the Polars you found, but it's not really something that can be mistaken. The weight of the wind is on the port side. Bearing away would take it further downwind, not upwind.

QED.

How we reconcile that with the Polars and the distance time calculation I don't know, but it's unmistakable.

EDIT:
FWIW the polar I posted for AC75 yesterday from SA:



Maybe Pyro can explain his as well, I'm still not clear on that.

 

[pyrojames]

I dont really see what there is to explain. The polars say it all, as you mentioned earlier. Your own polar shows a VMG downwind of around 36 knots in 20 knots of wind.

You seems to be forgetting that once they are doing that speed and gybe, they will be exceeding the true wind speed as they turn and the apparent wind is dead ahead during the gybe. The polars and the footage all agree with each other. That's it.