Foil for current-driven propulsion?

Potential energy is relative.

 

OK, it is a way to present it. But I doubt Meteorologic codes use a potential function for the wind for their prevision.
What I mean is this unnecessary complicated to adress the present question.
Please, If you have any argument agaist the equivalence of any inertial frames, I would be curious to liten to you

 

So what ?

 

Position from where, in which reference frame ?
I just wanted to say it was a unecessary complication there.
The question is my reasoning using open sea, no current, velocity polars results about F50, and inertial frames equivalence principle to derive the performance in the river case is right or wrong.
If you say it is wrong, so you say the equivalence principle is wrong.

 

Obviously, in this frame, water has zero motion, so no kinetic energy !

 

How does your method of taking energy from the water works if there is no kinetic energy?

 

I don't understand the sentence. If there is no current, then the open sea or ground are equivalent frames of reference. I suppose they could be inertial with respect of the solar system or the galaxy. The river has current, so how do you make that frame of reference equivalent to the open sea without current?

 

You should check any weather prediction program. It is all about potential differences. They may call them fronts and pressure systems though.

 

In the inertial frame fixed to water (supposed flowing at constant velocity), only air has kinetic energy, corresponding to a wind equal and opposite to the velocity of current relative to the land.
Assuming the foiler is initially at rest in this frame, it will use his sails and foils to accelerate and get a static maximum equilibrium velocity on the chosen course. As the forces depends only on relative velocities with water and air, accelerations are independant of the particular choice of the inertial frame, but this one corresponds to the sailboat in still water with the same wind (same airflow refative to water).
So the velocity polars derived in these normal conditions can be used to derive the equilibrium velocities relative to the water of the river.
It is not an assumption of my part, it is a fact, as considering only the relative motion of the air/water, and saiboat relative to the water and to the air everything is equal, the land has no place here.
If the polars are such that the best course downwind has a VMG downwind superior to the velocity of current, she can progress upstream faster than the current.
Obviously, the velocity upstream for an observer on the land will be less than relative to the water , equal to VMG-ABS(Vcurrent)

 

OK for wind predictions , we learn everyday !
but I doubt anybody has used the potential function of a sailboat relating to wind and or water to derive its performance.

 

Sorry, I mean river with current and no wind (for an observer on the land) is equivalent to water at rest with a wind equal and opposite to the current (observer fixed with water). The last being equivalent to open sea , with the same wind.
I was refering to no tidal current for open sea, but this was not necessary except if we are in the gulf stream

Obviously pure inertial frame on earth doesn't exists, as we are subject to Coriolis acceleration, acceleration of the earth CG around Sun and so on but (except for meteorological applications or spatial trajectories computations), you will agree we can use this hypothesis for most applications.

 

I think that for the sailing example minor forces like Coriolis can be ignored.

 

That is what a polar diagram for boat speeds is.

 

Are you still arguing about whether a sailboat can make progress up a flowing river on a windless day?

 

So, as, a this "potential function" depends only on the relative velocities of ship / air and of ship / water, it is independant of choice of inertial reference frame ( and the change of frame conserve the exchange of energy between the boat with water and the boat with air),
Which confirm the reasoning I and Doug Halsey developed !