Wow...

[Will Gilmore]

OMG!
As long as the water is pushing the boat relative to the air, there is wind across the deck (apparent wind), filling the sail, when at the right angle. Not head on!
When the water is not a factor, there is nothing opposing the wind across the deck. A sailboat can't sail in any direction counter to the apparent wind direction if there is not keel in the water giving her leeward resistance. Even if you can sail faster than the wind and generate a high apparent wind, the boat would begin to change direction and fall down wind until the was no apparent wind left.

This is BASIC sailing 101.

-Will (Dragonfly)

 

[Davidasailor26]

Will, I'm not sure I see your point. Help me understand...

As long as the water is pushing the boat relative to the air, there is wind across the deck (apparent wind), filling the sail, when at the right angle.

Yes, I agree with this - Put Artemis in the water and the current pushes it against the air, generating what it perceived as "wind".

Not head on!

Correct, you need to but Artemis in the water pointing, say, 45 degrees off the direction of the current.

When the water is not a factor, there is nothing opposing the wind across the deck.

Ok, but when is water not a factor? The boat, sitting 45 degrees relative to the current, will have some amount of keel exposed to generate resistance to leeway.

A sailboat can't sail in any direction counter to the apparent wind direction if there is not keel in the water giving her leeward resistance.

I guess I agree with that, but the video doesn't show that happening. The boat starts moving forward diagonally across the river, with its foils in the water pulling it downstream while its sails in the wind pull it upstream and cross-stream.

Even if you can sail faster than the wind and generate a high apparent wind, the boat would begin to change direction and fall down wind until the was no apparent wind left.

Why? The boat is sailing through the water, with air piling up on one side of the sail, and water piling up on the opposite side of the other. The boat is going forward through the water, so it has steerage.

Don't get me wrong, I'm not buying the whole sustainable energy / fix climate change angle of this. If there is current and no wind then just put a turbine in the water and get all the energy that you need without involving a boat, but still, the physics of sailing upstream are sound.

 

[Will Gilmore]

the physics of sailing upstream are sound.

When you are not reliant upon the water movement generating your wind, you have not got enough lateral resistance to the wind you are generating to keep it going. If the air isn't moving except in relation to your motion, it is always moving on the head. No lateral resistance, no lifting force. You're just a dandelion seed floating in the wind until you settle back down in a current going the wrong way.

-Will (Dragonfly)

 

[jon hansen]

10th grade geometry. chapter on vectors. simple really. once the vessel starts making positive vmg's upstream the wind from the current vector goes to zero. all apparent wind goes dead ahead. the vessel cannot go upstream.
this is what i think, what say you

 

[Simon Sexton]

Man, it seems I've started another one of those soon-to-be novels!

 

[Davidasailor26]

Let me try to explain myself this way - look at it in phases. In the pictures the blue arrows are the boat's motion relative to land, and yellow is boat's apparent wind.

Step 1, put boat in river, stationary:

Step 2: Current starts to carry boat downstream at 10 kts. Boat feels 10 kts of apparent wind.

Step 3: Due to apparent wind over its sails, boat begins to move forward. Still being carried downstream. Apparent wind is now a combination of boat motion forward and being carried downstream.

Step 4: Boat bears off to a reach. Still going downstream at 10 knots from current, but also going across the river at 30 kts. Apparent wind is the sum of the two vectors.

Step 5: Boat bears off to broad reach. Still getting carried downstream by current, but its own motion is still forward at as much as 3x that speed. Total boat motion is the sum of those two vectors. Apparent wind is the 10 knots from being pushed by the current plus the opposite vector of the boat's forward motion, which still "feels" like a close reach.

This is exactly how the AC boats sail downwind at faster than wind speed, regardless of current.

 

[jon hansen]

simon, i'm retired. the kids have all left the nest. it's minus 11 out. it's fun having this discussion and see if my concepts are correct.
i think i'm right
i've got a duck thawing that i will roast tomorrow.

 

[Davidasailor26]

For those that don't like vector math, how about this example -

Get a tractor trailer. Put a kiddie pool in the trailer, with a fan at the front of it. Set fan to blow at 10 kts. Put model AC boat in pool, and it will sail up the pool, turn downwind, and go at 30 kts. That models the sailing world as we see it all the time.

Now, cut the roof off the trailer and remove the fan. Drive the trailer down the road at 10 kts. All you've done is replace the fan with the stationary air that the pool is moving through. The boat will still go downwind at 30 kts relative to the pool, just like before, because all you've done is replace the fan with another source of wind. Relative to the land around the truck your sailboat is going about 20 kts, but it's still going backwards.

 

[Davidasailor26]

i'm retired. the kids have all left the nest. it's minus 11 out. it's fun having this discussion

Hmmm, I'm also enjoying the discussion, but I'm not retired, and I have two kids that are going to get up at 5:30 like it or not. How's this going to work out for me?

 

[Michael Davis]

Hmmm, I'm also enjoying the discussion, but I'm not retired, and I have two kids that are going to get up at 5:30 like it or not. How's this going to work out for me?

You should go to bed!

 

[Will Gilmore]

10th grade geometry. chapter on vectors.

Yes. Do the math. It's not supportable to sail anywhere, much less, upstream on your own wind alone.
However, here's a video of a wind turbine powered electric car that can move into the wind.

As the car moves into the wind, the apparent wind increases and the turbine spins faster and more electricity gets created, but so does resistance and eventually, the energy to velocity curve will level off because the increase in power isn't enough to continue acceleration against energy lost due to heat and friction.

-Will (Dragonfly)

 

[Davidasailor26]

Ok, my last thought for the night. The more I think about the video, the less I like it. I still say the physics is sound, but I don't like how it's being marketed at all. They're pretty clearly trying to sell this efficient boat design as a way of moving something backwards against a current, with an implied undertone of "OMG, this is so great it's like free energy!". Some viewers will be mislead by that and think "OMG, free energy, they're awesome!", which is obviously not really the case. A second set of viewers (and maybe some respondants to this thread) will see the free energy implication, and think "huh, they say they have free energy; that's not a thing; it must all be a scam", and throw out the whole thing. Either way, what's really their point, and what worthwhile have they gained? Yes, I think it's possible to sail a boat that way, but it's no more valuable as an energy source than using wind or water motion anywhere else.

 

[Brian D]

I went to Windy.com and looked at the part of the Amazon River where they claim the boat was sailing. Nominally the wind is 3 to 9 knots and direction is west to south west. I still call BS!

 

[Davidasailor26]

I went to Windy.com and looked at the part of the Amazon River where they claim the boat was sailing. Nominally the wind is 3 to 9 knots and direction is west to south west. I still call BS!

Well yes, that's another part of why it's a bit of a silly idea. But the fact that it's usually a little breezy there doesn't really take away from the physics that would let it work if you went out on a day when there was no wind.

 

[Will Gilmore]

Let me try to explain myself this way

Sorry David, I didn't see the above post when I responded to Jon's post. I'm not trying to belittle or trivialize your points.
I'll try to address these points, because I too am having fun.
There is no problem with sailing across the river in 10 knots of current generating 10 knots apparent wind. When the boat is being controlled by the water movement, it can use the wind to sail at a right angle to the apparent wind. Let's say the river is very wide so we can set the boat on a broad reach across the river and agree that the boat is efficient enough to achieve 3 times the wind speed in VMG. So now, the boat is sailing perpendicular to the current at about 28.3 knots while also moving down stream at 10 knots so that the vector sum across the river bottom equal 30 knots. We can, at this point, agree to this dynamic. I don't think anyone has argued against this part.
To break out the components of the wind we can take the overall velocity as the hypotenuse of a right triangle with heading (the direction the bow is pointing) as one leg, and river generated wind as the other leg.
900 (30kt^2)-100(10kt^2)=800(28.3kt^2).
This moves the angle of the apparent wind ahead of the beam and apparent wind speed is (800+100)^(1/2)=30 knots. Exactly the same as the velocity because there is 0 true wind. (Note: we are also treating this problem as if there is 0 leeward movement from the wind of 10 knots with perfect lateral resistance).
Now, if the boat changes course to fall off the wind (begin a turn up stream), the 10 knot vector moves aft of the beam, assuming the river movement still has influence on the heading and movement of the boat that has come up onto her foils.
This has the effect of reducing the apparent wind because some component of it now gets subtracted from the apparent wind total. This means that our boat can't sail 30 knots any more. She loses some headway. Her new heading vector constantly gets reduced and apparent wind falls with it.
Now look at the vector components of the wind. The more you turn away from the apparent wind to turn upstream, the farther the component vectors diverge from each other and there is no longer a lateral component to resist. Forward movement stops. Even if the boat were to remain efficient enough to travel at 3 times the wind speed for most of her points of sail, she can't do it going with the wind, which is what sailing upstream is effectively doing. Her only wind component becomes the 10 knot current driven breeze that will fill her spinnaker, but not move her upstream.

-Will (Dragonfly)

 

[Davidasailor26]

Thanks Will. That explanation helps me understand where we disagree.

This has the effect of reducing the apparent wind because some component of it now gets subtracted from the apparent wind total. This means that our boat can't sail 30 knots any more. She loses some headway. Her new hearing vector consistently gets reduced and apparent wind falls with it.

I think this is the key point. I totally agree with what you say in it. But my assertion is that the boat doesn't just keep turning upstream. It turns to some broad reach angle where it has optimal downwind VMG. At that point the river is still pulling the boat downstream at 10 kts, so there's apparent wind from that, but the boat is going up-wind and across the stream, generating an opposing apparent wind. The result of adding those two vectors is some kind of closer reach. True, the boat still isn't going the full 30 kts, but it is still going fast enough that its VMG is greater than 10, so it goes upstream.

Her only wind component becomes the 10 knot current driven breeze that will fill her spinnaker, but not move her upstream.

But the boat never does turn straight upstream to fly the Spinnaker, because then her speed would be limited to 0 as you say. The AC boats never flew a Spinnaker for just that reason - no matter how efficient it would limit their VMG to wind speed, whereas broad reaching they could get much higher VMG using their apparent wind.

This whole cycle stops if the boat falls off its foils when it's reaching. At that point the resistance is too great and it loses that apparent wind. This happened to the US boat in one of the last AC races - they were reaching along flying at like 30 kts, but did a maneuver that brought them off the foils. They quickly lost their boatspeed, and with it the apparent wind. They had to turn up-wind again, build boatspeed, rise up on the foils, then turn down and use that apparent wind to go above true wind speed again.

 

[Will Gilmore]

David, that sounds very interesting. I am unfamiliar with the AC sailing like that. I can see that there may by the possibility of gaining some upstream movement at such an acute angle, but I can't see that as sustainable. This is also ignoring the need to tack and the loss of headway like that. I feel it would mostly be a matter of momentum that slowly gets lost until downstream movement returns. I'll mull this over for a while.

-Will (Dragonfly)

 

[Davidasailor26]

David, that sounds very interesting. I am unfamiliar with the AC sailing like that. I can see that there may by the possibility of gaining some upstream movement at such an acute angle, but I can't see that as sustainable. This is also ignoring the need to tack and the loss of headway like that. I feel it would mostly be a matter of momentum that slowly gets lost until downstream movement returns. I'll mull this over for a while.

-Will (Dragonfly)

Will,

Yes, it's quite mind-bending to think about boats sustaining downwind VMG above the true wind speed, but thanks to apparent wind that's exactly what they could do. For example, check out the graphic at 43:43 in the video at

. An earlier graphic in that race showed true wind speed around 11 kts, but both boats went through their whole downwind leg at average VMG over 20 kts.

Edit: sorry, the VMG graphic is at 43:43, not 40:43.

 

[dlochner]

10th grade geometry. chapter on vectors. simple really. once the vessel starts making positive vmg's upstream the wind from the current vector goes to zero. all apparent wind goes dead ahead. the vessel cannot go upstream.
this is what i think, what say you

This is the answer. All the rest is ultimately distracting. As the VMG going upstream approaches the current speed, the apparent wind decreases until the VMG and current speed are equal, at which point there is no wind to power the boat and the boat stalls and starts to drift downwind. This causes the VMG to go negative and the apparent wind to increase and we start over again. Practically it would be like sailing downwind in puffy conditions, down in the puffs, up in the lulls, except, that going against the current means it is always a losing game.

Vector diagrams can be helpful but they don't really answer the question, because the solution is not linear. If we were to do the calculations, the answers would likely be hyperbolic. That takes some calculus to solve and that's when the nightmares started.

 

[Davidasailor26]

This is the answer. All the rest is ultimately distracting. As the VMG going upstream approaches the current speed, the apparent wind decreases until the VMG and current speed are equal, at which point there is no wind to power the boat and the boat stalls and starts to drift downwind. This causes the VMG to go negative and the apparent wind to increase and we start over again. Practically it would be like sailing downwind in puffy conditions, down in the puffs, up in the lulls, except, that going against the current means it is always a losing game.

Vector diagrams can be helpful but they don't really answer the question, because the solution is not linear. If we were to do the calculations, the answers would likely be hyperbolic. That takes some calculus to solve and that's when the nightmares started.

But your explanation suggests that boats could never sail downwind at VMG above true wind speed. We've seen them do that.

The key is that the boat isn't going purely upstream / downwind - it's also going across the wind/current. That combination of cross wind with downwind sustains an apparent wind, which is sufficient to keep the boat moving forward.