Scott T-Bird, In order to understand how a boat can sail upstream, against the current, you will have to understand how it can sail downwind faster than the wind speed. I can't explain completely how that happens but I know it does. The boat uses the small component (vector) of a favorable direction and amplifies it with its apparent wind. Kind of like a heat pump that takes cold air from outside the house, makes it colder, but puts hot air inside the house. Until you agree that a boat can sail faster downwind than the wind is blowing, you won't understand how it can go up stream against the current.
VMG is the speed that you are making up current - that's the goal, right? It doesn't matter if you are reaching across the current at 50 knots boat speed, the vector up-current is the VMG. You're making a big deal about the boat getting on foils and VMG exceeding True Wind Speed (TWS). According to
@Davidasailor26 's polars, the AC boats were achieving VMG at, say, 35 Knots with TWS at 20 Knots. That's impressive, right? VMG/TWS = 1.75.
BTW, the polars show the boat's highest speed direction at 150 to 165 degrees to TWS. I thought ya'll were saying that sailing downwind couldn't be considered because it was slower than a broad reach, yet 165 degrees is pretty close to DDW, but that's really besides the point, agreed?
So, we know that 1.75 is a pretty awesome ratio. What happens when we consider the ratio in this little hypothesis? Wind energy is created by the boat drifting down stream at 10 Knots. When we start sailing the boat, we pick up speed. let's say we get boat speed up to, say 25 Knots when we are traveling across the current and the boat lifts onto it's foils to make it more efficient yet. Now we're cooking with gas! We have 10 Knots TWS on a beam reach, so we start turning up current to make some headway. Good news, the polars (of our magical boat) tell us we will pick up speed! It might be possible for us to get up to say 50 or 60 Knots boat speed. We turn up-current enough to make some headway against the current (and our polar position on earth in an east-west position in this instance). Our boat speed picks up a little bit, but guess what, VMG goes to just 1 Knot relative to the current. Oops, we also lost 1 knot of TWS. VMG/TWS is now 1 Kt/9 Kt or 0.111. No big deal, we can still increase our speed according to the polars. Let's turn more downwind! We're flying now! Boat speed is up to at least 45 Knots! We're still going mostly north, when we need to go east to improve VMG. What happens when our VMG is 5 knots in the current. We're still going backwards, but only half as fast as the current is flowing. Darn! that TWS keeps dropping! It's now only 5 Knots. But guess what! The ratio is now 5Kts / 5kts = 1.0. We're improving efficiency! Because we are improving efficiency our boat speed keeps going up! Let's turn a little more up-current. The polars show us leaping to about 50 knots. We're going north like a bat out of hell, but we make good on the current another 3 knots … we're up to 8 knots VMG! That darn TWS keeps dwindling now … we're down to just 2 Knots TWS. But that ratio is 8 / 2 = 4! Now we are really efficient!
Guess what happens when we make our way up-current at 10 knots - standing still. TWS = 0 knots. Efficiency is 10 / 0 =
INFINITY!!! That's the best we can do if we are able to eliminate all resistance. Our boat speed might be 65 Knots (pick any number) due north. The best we can do is standing still in 0 wind.
@dlochner is right about the transfer of energy. Energy from the current can't overcome the current. The only way that works is to detach all resistance from the current. When that happens, the boat is completely detached from energy generated by the current … the boat may as well be a balloon floating in the still air above the current.