Foil for current-driven propulsion?

Sorry to contradict you, but ARTHEMIS is absolutely right. From the point of view of the most elementary physics!
An observer bound to the Amazonian water feels a relative (but very real!) wind of 10 kt blowing downstream. the foiler therefore feels a 10 kt wind coming from downstream, and able to sail at 3 times the wind speed can therefore go up the current of the Amazon by pulling in a wide reach at 30 kt relative to the mass of water! and therefore 20 kt relative to the shore.

 

Sorry , wind of 10 kt blowing upstream / amazonian water ! wide reach = broad reach

 

If Charlie is the driving force, Where is Charlie ?

 

Let's forget about this confusing water-air and lift thing for a moment.

We tie Charlie's feet to a truck.
And his hands to another truck.
We let the trucks move away from each other at a speed of 10 mph.

Will Charlie notice a force being exerted?

 

To get back on topic, for a thought experiment let's take the Strait of Gibraltar:

"Water flows through the Strait more or less continuously eastwards and westwards. A smaller amount of deeper saltier and therefore denser waters continually work their way westwards the Mediterranean outflow, while a larger amount of surface waters with lower salinity and density continually work their way eastwards the Mediterranean inflow."

If you manage to place a foil of which you can control the attitude in the upper inflow. And another one in the lower outflow. And connect the two by a line you have something that should be able to move in a controlled way.

 

That's two masses with different velocities. Foils could turn into force then all you need is righting moment equal to the distance between times the force.

 

We have seen the AC boats doing 30 kts downwind in 10 kts of true wind. Change your Frame of Reference from the water to the air, that's the same as sailing in still air in a water current of 10 kts.
My only gripe with the video is that whilst Artemis might be doing 30 kts through the water, it's not sailing DIRECTLY upstream - they have fudged the apparent wind angle. They would have to put in a few jibes to make any progress up the river.
If their AWA was say 30° (I can't find the polars for an AC75 downwind in 10 kts) then their progress towards the next village would be 30 x cos(30°) - 10 knots = 16 kts. So they would "fly back to the source of the Amazon" at 16 kts.
The AC boat doing 30 kts downwind has to jibe a few times to lay the leeward gate.
But it's a real mind bender all the same.

 

The AC boat doing 30 kts downwind has to jibe a few times to lay the leeward (sorry for the edit). Is it not a sufficient proof that there is something fishy with this video ?
@Flotation and @Skyak , I really appreciate your patience with me. I'm pushing perhaps a little too much this old debate, This is my way to try to understand things better. I also deeply hope that the author of this thread will find interessant content. I'm also a bit sad to let Charlie aside.... But that's ok... I'm a full grown up now (sob...). So.... No Charlie, No Amazon. We travel to Gibraltar, and his (not so) funny current effects. Is this the kind of foil configuration that could take advantage of the underwater current ? How will the boat make his way ?

 

But.....Wait....... In which direction is the AWA if the TWS is zero ???? Charlie !!!!! Chaaaaaarlie !!!!!!!!!

 

I'm guessing an AWA of 30° for the AC 75 doing 30 kts downwind in 10 kts TWS. What's the problem?

 

Do you mean this velocity gradient ?


Have you seen the youtube link that I've posted, about dynamic soaring and water pumping ? Can you use these drawings to show me how Artemis, @tws = 0kts, could exploit these gradients ? Did you see any device on Artemis comparable to what was installed on the Suntory Mermaid ? suntory, wave power, mermaid, suntory mermaid II, kenichi horie, yutaka terao, tokai university, transportation, sailing, wave sailing, sailor https://inhabitat.com/transportation-tuesday-the-wave-powered-boat/suntory-wave-power-mermaid-suntory-mermaid-ii-kenichi-horie-yutaka-terao-tokai-university-transportation-sailing-wave-sailing-sailor-2/
Do the Suntory Mermaid sails 30kts ???

How can you create a driving force allowing for the ACC to got 30kts in the conditions given in the video ?

 

Assume you are left on the amazon river in the complete dark on a foiler drifting, with no wind experimented by an observer on the shore, and you are ignoring were you are. You can feel a wind due to the drifting of the current. You are completely unable to say if you are on the amazon or on a lake with a wind. However you can use this wind by pulling the sails to get a relative motion relative to the mass of water, at 3 time this wind (which applies the same thrust on the sail as if you are on the lake with wind). Which count on the physical point of view is the relative motion of air / sails and the relative motion of water with foils. This is the Galiléan principle of relativity. Watever inertial reference frame (A) you use, you results in correct physical results, you can get the resulting velocities in another reference frame (B) using the Galilean transrorm of coordinate : x' = x + V.t , v' = v+V , were x and x' are on axe parallel to the relative velocity V of A relative to B. Obviously, as Arthemis is broad reaching relative to the mass of water there is an AoA of for the foils as well as for the sails due to the relative motion with both fluids !
This is my last tentative to try to learn you !

 

I think it is a lost cause!

 

Can you picture it ? Where is the driving force ? Where is Charlie ?

 

Thank you Pantzefran, and you all, to have put all these efforts to "demonstrate" the existence of an apparent wind angle in the given configuration. I guess we're bound to let this subject in the state were it has been left on the same forum, some years ago. Still, I recommend you all to take a quick look at Will's proposition on this thread Sailing Without Wind. https://www.boatdesign.net/threads/sailing-without-wind.67181/

Your science may be a great help to answer his question.