Interesting facts about the AC72...

[flaming]

Right, so for a boat sailing at 45 degrees from a true downwind course.

Let's assume the wind is blowing from the SOuth, so the boat is trying to get North, and that it's sailing at 045 degrees.

With 10 knots TWS, when VMG = 10 knots, boat speed = 14.1 knots (Pythagarus)

So apparent wind is the vector of 10 knots at 180 and 14 knots at 045.

I don't have a scientific calculator to hand but it's pretty obvious that the resultant wind is not 0.

So if the resultant wind still causes your boat to accelerate, then that's what it's going to do.

 

[A.T.]

Remind me what energy source creates that apparent wind?

The energy to propel the boat comes from the true wind, which is constant, regardless how fast the boat goes.

 

[A.T.]

As the grey arrow on the diagram increases the green arrow decreases ....

The green arrow is the free stream true wind, which is constant.

 

[Tempus]

The green arrow is the free stream true wind, which is constant.

You're wasting your time guys..

"There's none so blind as cannot see" - even when it's been on the TV!!

Is this the sailing equivalent of the Flat Earth Society - the "It's impossible to sail faster than the wind, even when I've seen it with my own eyes" society?

 

[JumbleDuck]

You're wasting your time guys..

"There's none so blind as cannot see" - even when it's been on the TV!!

Is this the sailing equivalent of the Flat Earth Society - the "It's impossible to sail faster than the wind, even when I've seen it with my own eyes" society?

There was a user called "Toad" on the uk.rec.sailing newsgroup who for years persisted in claiming that it was theoretically impossible to build a boat which could use coupled wind and water screws to travel directly into the wind. Even when it was pointed out that it had been done by several independent groups he persisted. I believe his final claim was that it might work at 0.0000001 degrees off the wind on either side, but not straight on. Of course it may well be no relation.

 

[A.T.]

There was a user called "Toad" on the uk.rec.sailing newsgroup who for years persisted in claiming that it was theoretically impossible to build a boat which could use coupled wind and water screws to travel directly into the wind. Even when it was pointed out that it had been done by several independent groups he persisted. I believe his final claim was that it might work at 0.0000001 degrees off the wind on either side, but not straight on. Of course it may well be no relation.

Going directly upwind is just the reverse case of DDWFTTW. It is also dealt with here, along with DDWFTTW:
http://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content

And on land, you can even go directly upwind faster than the wind, demonstrated here by the modified Blackbird, 2 years after its DDWFTTW reocrd:

 

[JumbleDuck]

Going directly upwind is just the reverse case of DDWFTTW.

Absolutely. All you have to do is turn the interface upside down and change the axes: it's precisely the same problem.

Or, to put it another way, travelling direct down wind faster than the wind is the same (shift axes to move with the wind) as travelling directly into the tide on a windless day, which is the same (invert) as travelling directly into the wind on a tideless day.

 

[DJE]

The carts work because the propellor blades are not travelling directly downwind (or upwind) they move in a helical path a constant angle to the wind direction.

 

[A.T.]

The carts work because the propellor blades are not travelling directly downwind (or upwind) they move in a helical path a constant angle to the wind direction.

That's true. But as you see here, some people even have problems with downwind VMG > TWS, while tacking across the wind. Once you understood that, the cart trick is quite obvious:

 

[DJE]

It's starting to feel like Groundhog Day.

 

[toad_oftoadhall]

With 10 knots TWS, when VMG = 10 knots, boat speed = 14.1 knots (Pythagarus)

In order to show a boat can sail a net course DDW faster than the wind you've started with an assumption it's *already* doing 10 knots with a CMG DDW in a 10kt wind and shown that under those circumstances the apparent wind looks hunky dory.

Anyway, I've yet to see a fruitful DDWFTTW[1] debate, so I'm out. I doubt anything will come to light that isn't already on talkrational.

[1] Please don't anyone say this is not DDWFTTW. if you beat the balloon to a point downwind with a CMG the same as the balloon then the net speed and course are DDWFTTW.

 

[A.T.]

you've started with an assumption it's *already* doing 10 knots with a CMG DDW in a 10kt wind

You claimed the acceleration would stop at downwind VMG = TWS. Now that the opposite was shown, you complain that we used the conditions that you stated yourself? You can show the same thing, in the same way, for any speed from 0 to way beyond downwind VMG > TWS.

Anyway, I've yet to see a fruitful DDWFTTW debate, so I'm out.

So you were never able to convince anyone in those debates? Maybe it's the others that are right?

 

[flaming]

In order to show a boat can sail a net course DDW faster than the wind you've started with an assumption it's *already* doing 10 knots with a CMG DDW in a 10kt wind and shown that under those circumstances the apparent wind looks hunky dory.

Well, there are plenty of boats that would do that easily.

Anyway, I've yet to see a fruitful DDWFTTW[1] debate, so I'm out. I doubt anything will come to light that isn't already on talkrational.

[1] Please don't anyone say this is not DDWFTTW. if you beat the balloon to a point downwind with a CMG the same as the balloon then the net speed and course are DDWFTTW.

When DDWFTTW was first discussed here I didn't believe it could work.

What proved it to me was the last AC, where someone pointed out that the 90 foot tri had got to the leeward mark about twice as fast as the wind. That gave me the revelation that of course the prop blades are also not going in a straight line.

What amazes me is that you can see footage of the AC races, where speed and angle and wind speed are there for all to see and then confidently state that your maths must be correct, and the evidence of the two boats actually sailing is wrong.

That must take truly epic amounts of self belief.