Wow...

[Davidasailor26]

The wind is 0

No, the boat is sitting in the water. Don't think about the land. The wind is blowing 10 kts relative to the water, so there is 10 knots of true wind for you to sail in.

when a boat is sailing in 10 knots and then the wind drops to nothing. We've all experienced

Sure, but not in 10 knots of current.

If you say that the boat "feels" 10 Knots of wind when it is drifting downwind, you can't say that the boat "feels" 10 knots

Sure you can. The boat still "feels" the 10 knots of wind when it's sailing upstream, it just also feels 10 knots of apparent wind offsetting that 10 knots of true wind.

The current doesn't cause wind

Yes it does. Wind, as far as the boat floating in water cares, is the motion of air relative to the water - 10 knots.

True wind is eliminated incrementally with any progress up current.

True wind can't be eliminated, it doesn't ever vary with boatspeed.

 

[Scott T-Bird]

Think of an airplane. How could it ever fly up? The air isn't going up.

It only flies up because an engine creates air speed. It can't do it on its own. It never will, even when you give it a push with an engine. When the engine shuts off, the plane must fall.

 

[Davidasailor26]

When the boat is still, in relation to land, the wind is zero.

Yes, and in relation to water, the wind is 10 knots, right?

If we are motoring at 10 knots, do we say the wind is 10 knots? no, we say the air is still (TWS = 0)

Right, because the air isn't moving relative to the water. In the river the air is moving relative to the water, thust 10 kts of true wind.

 

[Davidasailor26]

Yes, and in relation to water, the wind is 10 knots, right?

Right, because the air isn't moving relative to the water. In the river the air is moving relative to the water, thust 10 kts of true wind.

It only flies up because an engine creates air speed. It can't do it on its own. It never will, even when you give it a push with an engine. When the engine shuts off, the plane must fall.

I'm glad you said that; I knew that question would come up. Honestly I had to think that through myself after I posted the analogy.

Here's the thing - you have the glide plane in the air, moving at say 100 kts. That plane has a certain amount of energy. The energy comes in 2 forms - kinetic energy from the 100 kts of airspeed, plus potential energy from your altitude. Now, a pilot can tell you that you can "trade" the potential energy for kinetic, flying upward, or vice versa flying down. Put the drag of the plane slowly eats the energy away, so you can't go up indefinitely. Still good?

Here's the thing though, the plane is in one medium - air, with no other source of energy beyond the potential and kinetic energy we started with. The boat in the flowing river is in 2 mediums, air and water. Those two mediums have motion relative to each other. That relative motion, of the air moving relative to the water, is energy that is available for the boat to harness and turn into its own motion. See that last sentence - motion of air relative to the water being harnessed - that is exactly the same regardless of the relative motion of some body of land far away.

 

[Dr. D]

OK, I haven't read all the replies because the video is BS. "Imagine the boat on the Amazon river," so the boat was never actually on the Amazon river. Next, "with 10 mph current." Please find me the source that has 10 mph current in the Amazon river; I can't. Finally, as a few replies touched upon, it all works while the boat is being moved by the current through the still air. When it turns 180 degrees there is no other source of energy other than boat's own motion. As there must be some resistance, the boat will slow down until stopped (then drift backwards).

Oh, if the boat will generate forward motion while drifting backwards in the current -- ending up sailing up the river -- then why start with sailing down the river? Have the boat on a mooring. Release the mooring line and start sailing up the river in still air all due to the current pushing the boat backwards.

 

[Scott T-Bird]

Not overcome - just be redirected to move a piece of material in the other direction. Saying "the energy from the current can't overcome the current" is like saying "you can't sail up-wind, the energy from the wind can't overcome the wind".

That's an interesting analogy. Why are we able to sail upwind? I think that the answer is because there is such a great difference in the resistance vector between air and water. We can't sail a plane against the wind, unless we have a force from an engine or gravity or an updraft. There has to be some form of differential resistance for movement to occur. Air has less resistance than water, so you can't do it in reverse. You can't sail up current without an independent source of air current to overcome the water current. Apparent wind can't be sustained when there is no actual wind. Resistance overcomes.

 

[Davidasailor26]

OK, I haven't read all the replies because the video is BS. "Imagine the boat on the Amazon river," so the boat was never actually on the Amazon river. Next, "with 10 mph current." Please find me the source that has 10 mph current in the Amazon river; I can't.

Yes, for all of those reasons I agree the video is nonsense. But that doesn't change my assertion that the physics supports sailing a boat upstream when the air isn't moving relative to the land.

When it turns 180 degrees there is no other source of energy other than boat's own motion. As there must be some resistance, the boat will slow down until stopped (then drift backwards).

Yes, if you sail 180 degrees upstream you stop, then drift down again. But broad reaching upstream it works.

Oh, if the boat will generate forward motion while drifting backwards in the current -- ending up sailing up the river -- then why start with sailing down the river? Have the boat on a mooring. Release the mooring line and start sailing up the river in still air all due to the current pushing the boat backwards.

Well, to make the sailing upstream possible you need to be going fast enough to be on your foils. To do that you need to sail closer than a broad reach for a bit.

 

[Davidasailor26]

That's an interesting analogy. Why are we able to sail upwind? I think that the answer is because there is such a great difference in the resistance vector between air and water. We can't sail a plane against the wind, unless we have a force from an engine or gravity or an updraft. There has to be some form of differential resistance for movement to occur.

Exactly.

Air has less resistance than water, so you can't do it in reverse.

True, if you have the same amount of surface resisting against the air as the water, like most boats. But once you have a foiling cat on its foils it has very small resistance to the water, and significant lift from any sideways component of apparent wind that you've managed to maintain as you bear off. That difference between the huge lift and tiny drag is how the AC boats sail downwind faster than the wind, and thus how the river thing is possible.

 

[Davidasailor26]

How about this analogy -

You take your sand yacht out on a day when the wind is blowing 10 knots over the land, from east to west. Since your sand yacht can go downwind with a VMG greater than the wind speed (see https://en.wikipedia.org/wiki/High-performance_sailing), you can sail yourself west with a VMG of, say, 20 knots. So you're going along in a north-westerly direction, moving west at 20 knots and north at, say, 10 knots. Still good?

Now, you look out toward the horizon and you see a body of water. The water is moving relative to the land from east to west at 10 knots. Therefore, the wind is not moving over the water. You're going west at 20 knots, so relative to some point on the water you're going west at 10 knots. Still good?

Ok, now, in the paragraphs above, do a copy/replace. Replace every instance of "sand yacht" with "foiling catamaran", every instance of "water" with "ground", and every instance of "land" with water. Those find/replaces don't change the physics of the situation in any meaningful way. What do those paragraphs say now?

Here's what it says. I promise, all I did were those 3 find/replaces.

You take your foiling catamaran out on a day when the wind is blowing 10 knots over the water, from east to west. Since your foiling catamaran can go downwind with a VMG greater than the wind speed (see https://en.wikipedia.org/wiki/High-performance_sailing), you can sail yourself west with a VMG of, say, 20 knots. So you're going along in a north-westerly direction, moving west at 20 knots and north at, say, 10 knots. Still good?

Now, you look out toward the horizon and you see a body of ground. The ground is moving relative to the water from east to west at 10 knots. Therefore, the wind is not moving over the ground. You're going west at 20 knots, so relative to some point on the ground you're going west at 10 knots.

Those last 2 paragraphs are you, sailing upstream through a current, with no air blowing over the land.

 

[jon hansen]

Well, that's a fine kettle of fish
The END

 

[walt]

Some of the arguments here say an ice boat wont exceed wind speed or a VMG greater than wind speed going down wind. They do.. Try and understand that first.

Also, you really dont need to speculate on this as there is data showing that what is presented in that video could be true.

You can find actual data on AC boats for ground speed and VMG here http://www.cupinfo.com/cupstats/index-ac34-statistics-polar-plots-02.php#SOG These polar plots show actual speeds for a bunch of races. In this case, its the "VMG" plots that are most interesting regarding the discussion.

For each of those races, you need to know the wind speed and that is here http://www.cupinfo.com/cupstats/americas-cup-2013-stats-wind-distribution-01.php#race-6

I will just sumarize a couple points of the data
Race 6 Wind speed average of 11 knots. Downwind VMG looks like 25 knots at around 165 degrees
Race 7 Wind speed average of 16 knots. Downwind VMG looks like 32 knots at around 160 to 165 degrees
Race 10. Wind speed average of 18 knots. Downwind VMG looks like 35 knts at 165 degrees.

FYI, In a sailboat – VMG is the velocity component of the boat in an upwind (or downwind) direction

That data clearly shows that an AC boat can greatly exceed the true wind going deep down wind. The case most interesting for that video is the 11 knot data point which shows a down wind VMG capability of 25 knots at 165 degrees (downwind). The video said the current (and generated wind) is 10 mph which would only be 8.7 knots but there is a clear data point in that link that in 11 knots of wind, the AC boat going deep down wind could exceed the true wind speed by 14 knots. At some speed the boat wont foil but I choose to believe Artimus that they would get somewhat similar performance at 8.7 knots wind speed.

Easy for me to see that a boat drifting in 10 mph of current would see a 10 mph wind coming from the direction you are drifting. From the boats perspective, its no different than a 10 mph wind if the body of water is not moving.

All the boat has to do is then sail downwind with a VMG greater than 10 mph. The actual data says this is completely possible if the current was 11 knots.

Why in the world someone mentioned climate change in that video is beyond me (and maybe that is throwing some people off of what is actually interesting here).

 

[Davidasailor26]

Why in the world someone mentioned climate change in that video is beyond me (and maybe that is throwing some people off of what is actually interesting here).

Yeah, that, and also the fact that the video makes it sound like "sailing in no wind!!!". They say that so enthusiastically to make it sound like some revolutionary thing that they've done to make energy. Unfortunately it just confuses the topic and obfuscates the fact that there is wind relative to the water that the boat is sailing on. I think that obfuscation throws off some people, making them get in the mindset that since you can't sail without wind, the whole thing must be a lie.

 

[Davidasailor26]

Why in the world someone mentioned climate change in that video is beyond me (and maybe that is throwing some people off of what is actually interesting here).

Just one more thing - the potential to sail upstream is the interesting part for us on SBO, but the (false) promise of a climate change solution is what makes it interesting to environmentalists, consumers, and investors, which is a much larger and more important audience to them than sailors.

 

[Will Gilmore]

https://www.wired.com/2011/02/ff_fasterthanwind/

Imagine the sail is one of those propeller blades. 0 wind, ground/water is moving.

-Will (Dragonfly)

 

[dlochner]

Imagine the sail is one of those propeller blades. 0 wind, ground/water is moving.

A couple of observations.

The energy required to propel a light weight dirt boat is much less than it takes to propel a boat in the water due to the resistance from the water and the energy required to lift the boat up on its foils.

The treadmill model is interesting, however, the propeller is not going dead down wind. Look closely, it is angled. While we tend to think of apparent wind and tacking angles in the lateral plane, i.e., left to right, it is not inconceivable to think of them in a vertical plane, i.e, tipped forward or aft. We do that on boats by raking the masts aft.

The article mentions that the propeller torque caused the dirt boat to twist such that one wheel was no longer in contact with the ground. That torque may also have changed the angle of attack for the propellers, which would in effect cause an increase in the apparent wind at the propellors. Also the photos were not clear about the structure of the propellor support, were they perpendicular to the ground or at an angle?

 

[jon hansen]

Will, your examples don't apply to the op video. it is stated that there is no true wind. the apparent wind from the current will drop to zero as the vessel attempts to crawl upstream. the only apparent left is dead on the nose. sailboats cannot sail straight into the wind.
easy peezee

 

[Davidasailor26]

The energy required to propel a light weight dirt boat is much less than it takes to propel a boat in the water due to the resistance from the water and the energy required to lift the boat up on its foils.

Yes, it takes less energy for the dirt boat than the catamaran. But not so much less as to make the analogy fail. A displacement boat has more resistance still, to the point where it doesn't work anymore. That added resistance is why the dirt boat and cat can sail downwind with a faster VMG than the wind, and the displacement hull can't.

 

[Davidasailor26]

the op video. it is stated that there is no true wind.

And that is the misrepresentation in the video that is throwing you off. The wind over the water is 10 knots. That's true wind as far as things on the water are concerned, in the same way that 10 knots of airflow over the land is true wind as far as a boat on land is concerned.

 

[Davidasailor26]

The treadmill model is interesting, however, the propeller is not going dead down wind. Look closely, it is angled.

I think it's just mounted that way to make the structure simpler and need less gears for the drive shaft. The propeller would probably be even more efficient if it were horizontal, but there would be more nergy losses to the friction of the extra gear.

 

[jon hansen]

at 28 seconds into the video he states there is no wind at all. that means no true wind. all other winds are apparent. i stand by my posts