Foil for current-driven propulsion?

• The Artemis video was posted as demonstration of how foils can be used for current-driven propulsion, which very relevant to the original question
• Nobody in this thread suggested Artemis could do so at an apparent wind angle of zero
  
• We discussed already how that would be impossible
• For example when Sailor AI explained "My only gripe with the video is that whilst Artemis might be doing 30 kts through the water, it's not sailing DIRECTLY upstream"
• Which i quoted and added "To be able to make way exactly upwind or in the direction of the current something like a propeller/propeller driven craft is needed"
• We even discussed whether or not or not Artemis would technically be gibing or tacking when sailing up the current

Edit, i deleted my complaints about an offensive post as Alan deleted the post in question.

 

OK. Post is deleted. I hope that someone will try the experience ! Discussing seems not to be sufficient, to me. Cheers, Flotation.

 

Can you please clarify what you mean by this remark?

 

"We discussed already..."
"We even discussed..."

I need proofs to believe, although I'm not St Thomas. And I don't consider your arguments as proof.

N.B. I see no offense in asking to reveal his identity in the discussion, but I don't want to offense anyone, so I deleted my post, without trying to think more on, how on earth, making a gentleman's agreement before giving an answer could be interpreted as an offense.

 

There is never a "proof", only evidence.
You want to see the Artemis sailing upstream? Watch a replay of the AC boats on a leeward leg and move your observation point with the wind instead of the water.
Change your Frame of Reference.
In the AC36 we saw boats doing 36 kts downwind in 14 kts true wind which , when you change your FoR shows them effectively sailing upriver in a 14 kt current:

 

My science professors always said that : "One can not make additions between cabbages and carrots.

If the first diagram is true, then the apparent wind in the second diagram is wrong. The apparent wind is in opposite direction to the red arrow.

 

Because sails don't make effective keels?

Is it that you're expecting the hydrofoils to live in a consistent water current that the hull/keel doesn't see? (i.e. a river of current that only occurs at depth?)

Only then, would there be a relative velocity of water over the hydrofoil at all. Otherwise the keel, hull and hydrofoil would all catch up to the water, and reduce the relative velocity between water and foil to zero.

(Only the aerodynamic friction of the boat above the waterline would help.... I suppose optimizing that looks like a parachute hanging from a mast, and that would just slow you down whichever direction you're heading in.)

(Or maybe you've got wing-paddles dipping way down on long narrow rods that feel the difference in water movement direction compared to the surface current?)

 

On that same token you could use a small motor to get going in a no wind scenario and do the same thing. The problem with their claim is that you will always be heading directly into the wind you have created; without any tangential component how do you create lift?

I call BS. This wreaks of a perpetual motion machine.

 

Are you saying they go straight upwind on an apparent wind of 20 kt?

 

In this case, since the wind velocity is zero, any movement would make it faster than the wind. However, since the velocity is zero, it has no direction. Ergo, it is impossible to sail downwind.

 

Are you saying that a boat drifting with the current feels zero wind ?

 

No. Firstly, if your frame of reference is the water, the boat won't drift in the same direction the water is moving with respect to the ground. It will drift in the opposite direction due to the resistance of the air. However, since you introduced a second frame of reference, the air, all movement of the boat would make it faster. You need to pick one frame of reference, not mix and match.

 

Good point, we progress !
Using a frame fixed to the water, the boat (foiler) will experiment a wind ( real wind, for him) of velocity equal and opposite to the current velocity.
from the physic's point of view relating to the air, the water and the boat, this is exactly the same as a boat sailing in a true wind (relative to the land !) on a sea without current ( this is the Galilean relativity principle and, if it was false, all the classical Physics established since Galilée would be false). It states that the law of physics are the same in non accelerated reference frames in uniform relative motion.
So we can use the theoretical or experimental results established in the last conditions, namely the velocity polars relative to the true wind and apply the results to the river case. If it happens the established performances are such that VMG downwind is higher than the true wind velocity (broad reaching), Ergo the foiler is able to pogress upstream faster than the current velocity relating to the land. Obviously, as you said , we need some angle to the apparent wind to get a thrust component, and the course dead downwind is not practicable , only tacking dead downwind is possible ( I say tacking, and not jibing as we can observe than the F50 foilers sails always upwind against their apparent wind !). About the perpetual motion objection, this is not pertinent, as perpetual motion needs no energy, here the thrust energy is extracted from the relative motion of the air with the sails.

 

The true wind velocity is zero with respect to ground. If water is the frame of reference, then you can't use that value. The confusion in your arguments comes from using more than one frame of reference at the same time.

 

So in this river scenario you would need to take the energy from the current and transform it into a propulsive force upstream somehow using the air, which is in a zero energy state relative to a given position. You initially have the energy of the river flowing downstream, build up kinetic energy, then start your turn, no matter which way you turn you will be moving directly into the apparent wind so you would have to cock the sail to one side or the other to introduce a tangential component to make the sail/keel system work, the problem being that the net force will be towards the back of the boat, to say nothing of radically increased drag from moving upstream against the current, all serving to decelerate rather than accelerate the boat.

Am I missing something?