Wow...

[Cowpokee]

The head-banging smilie has never been more appropriately used than in this thread. I'm done trying.

And by the way ... if a tree falls in the forest and no one hears it, YES IT DOES make a noise, dammit!

Gene, I don't know about trees in the forest, but if a photon travels through a double slit, It does matter if you are watching or not as to how it behaves.

 

[Gene Neill]

Says YOU!

Aw, shoot, Cowpokee ... who am I kidding. I don't know what a photon IS, let alone how well-behaved they are.

 

[RussC]

Since I didn't see a little emoticon by your post I won't take this as an attempt at being facetious so let me explain. A heat pump takes the energy of the moving molecules (that is what temperature is - a measure of the amount of movement of molecules) in cold air, and transfers that energy to the air molecules inside the house. Outside air molecules move slower as a result and the inside air molecules are now moving faster. Thus energy (in this case heat) is transferred from outside the house to inside air.

My oversight. sorry. I'll use two next time

 

[Will Gilmore]

that is Drill Instructor, not drill sergeant. We Marines be very sensitive to what we are/were called.

Sargent, because you WORK for a living, right?
in place of the solute emoji.

This problem is somewhat similar to driven harmonic systems.

-Will (Dragonfly)

 

[DArcy]

Wow was a very appropriate title for this thread. I am impressed by the level of interest and discussion while remaining so civil. I am beginning to rebuild my faith in humanity

It seems like people are overthinking some aspects. There is no free energy, no perpetual motion. The energy imparted by a river moving at 10 knots is massive. Just look at a power dam. So from that perspective, moving a boat upstream using this energy is incredibly inefficient and wasteful.
And it really doesn't matter if we call any of these things VMG, true wind or apparent wind. The polar plot shows the AC72 can sail at double the wind speed on a broad reach. So that means it can sail away from the wind faster than the wind. Throw it in the river situation and it can sail upstream. The concept of pressure on the downwind side of the sail is just a discussion about apparent vs. true wind and is applicable regardless if the wind comes from drifting with the current or air blowing across still water. Same thing. So it comes back to the polar plot showing down wind performance being faster than the wind.

 

[Will Gilmore]

Just had a new thought. What if you thought of the boat as upside down. The water is the wind flowing down river, the air is the steaming body of fluid. The sail acts more like a keel while the fin keel becomes the wing-like sail. In that scenario, there is bo problem sailing upwind, as it were.

-Will (Dragonfly)

 

[Cowpokee]

Just had a new thought. What if you thought of the boat as upside down. The water is the wind flowing down river, the air is the steaming body of fluid. The sail acts more like a keel while the fin keel becomes the wing-like sail. In that scenario, there is bo problem sailing upwind, as it were.

-Will (Dragonfly)

Will,
Have you been drinking something besides coffee this morning? I can't understand at all what you are getting at.

 

[dlochner]

Just had a new thought. What if you thought of the boat as upside down. The water is the wind flowing down river, the air is the steaming body of fluid. The sail acts more like a keel while the fin keel becomes the wing-like sail. In that scenario, there is bo problem sailing upwind, as it were.

-Will (Dragonfly)

Sure that would work, once the mast came out of the mud.

Have you been making "produce" purchases in VT or MA?

 

[Davidasailor26]

Will,
Have you been drinking something besides coffee this morning? I can't understand at all what you are getting at.

I think what he's trying to say, or at least the point that it makes to me, is this... It's hard to imagine how a boat could sail downwind faster than the wind itself. It seems like that's breaking some law, because there's no wind pushing you that fast in that direction. But really, if it were just about being pushed by the wind, you could never sail up-wind, because there isn't any component of the wind pushing you that way. In Will's upside down example, it's easy to see how you could sail upstream - the water is just the fluid that you're sailing into, instead of the air.

 

[Will Gilmore]

Exactly!

-Will (Dragonfly)

 

[Cowpokee]

Exactly!

-Will (Dragonfly)

OK, I guess I just hadn't enough coffee myself yet. Now I get it after my third cup is finished. But why can't everyone else understand this?

 

[Will Gilmore]

But why can't everyone else understand this?

We aren't taught lateral thinking.

-Will (Dragonfly)

 

[sailnoproblem]

Oh the humanity, this thread has fried my brain what was left of it. (I am a product of the 60's. minus the military part at the end of the decade)

 

[Scott T-Bird]

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.

 

[Scott T-Bird]

BTW, let's use the polars of the AC boat to pretend what we can accomplish in a 10 knot current on the Amazon.
We might be able to sail 6.5 Kts upstream in 3.5 Kts TWS if we sail at about 15 to 30 degrees off the current. So we would be sailing backwards at 3.5 Knots if we could achieve the efficiency of the AC boat in wind less than 10 knots. 6.5 / 3.5 = 1.86. We can pretend!

 

[Davidasailor26]

165 degrees is pretty close to DDW

Pretty close, but critically different. At 180 degrees, DDW, you have no component of the wind from your side, so you have no lift. All you have is pure drag. With pure drag, you can't exceed the speed of the wind, but with lift you can. Think of an airplane. How could it ever fly up? The air isn't going up. If the wings just generated drag (plane falling straight down), you couldn't go up. But as soon as the plane is going forward, you have lift, pulling up. Same with the boat. Straight down wind, no lift. Broad reach, lift.

we also lost 1 knot of TWS

True wind never changes. You feel less apparent wind from the current, but more from your boat speed. This is the critical thing in the rest of your ratios - True Wind Speed is the speed of the air relative to the surface of the water you're sailing on. It never changes. Always 10.

 

[Davidasailor26]

@dlochner is right about the transfer of energy. Energy from the current can't overcome the current.

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".

 

[Davidasailor26]

a 10 knot current on the Amazon.
We might be able to sail 6.5 Kts upstream in 3.5 Kts TWS if we sail at about 15 to 30 degrees off the current.

No, the current is still 10 knots, so the TWS is still 10 knots. When you're on the bay and it's blowing 15 from the south, say you go running north at 5 knots. Do you say the TWS is now 10 knots? No, the true wind speed isn't affected by your boar speed.

 

[Scott T-Bird]

No, the current is still 10 knots, so the TWS is still 10 knots. When you're on the bay and it's blowing 15 from the south, say you go running north at 5 knots. Do you say the TWS is now 10 knots? No, the true wind speed isn't affected by your boar speed.

No, of course not. The wind is 0. That's the point … you can't say wind is a steady 10 knots when it is actually 0. It can only effectively be 10 knots when the boat is drifting downstream. When the boat stops drifting and is making progress against the current the "wind" starts to become eliminated. It's no different than when a boat is sailing in 10 knots and then the wind drops to nothing. We've all experienced that!

Wind is zero when you have no current. You can't have it both ways. 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 when it's opposing the current. The current doesn't cause wind. It is only the resistance of the boat against the air that causes the boat to "feel" wind. True wind is eliminated incrementally with any progress up current.

 

[Scott T-Bird]

True wind never changes. You feel less apparent wind from the current, but more from your boat speed. This is the critical thing in the rest of your ratios - True Wind Speed is the speed of the air relative to the surface of the water you're sailing on. It never changes. Always 10.

But that is not true because the current has nothing to do with the wind. The wind is still, the current is 10 Knots. We only say the wind is 10 Knots because that is the reaction from the resistance of the boat against the air when the boat is drifting with the current. When the boat is still, in relation to land, the wind is zero. If there is no boat, the wind is still regardless of what the current is doing.

If we are motoring at 10 knots, do we say the wind is 10 knots? no, we say the air is still (TWS = 0) and the apparent wind is 10 Knots. In this instance, we are creating apparent wind at 10 Knots and pretending that it is True wind. Now you are divorcing the apparent wind from the true wind vector and pretending that true wind still exists. It doesn't.