Wow...
- Thread starter Simon Sexton
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OK, ignore the fact, for just a moment, that we are discussing river current. Say we are out in the open ocean on a high performance foiling boat, you are making progress downwind at 14 knots in only a 10 knot wind. (downwind progress, not boat speed) Only when you turn your GPS on, you realize your COG is only 4 knots. You quickly figure out that you have been in a 10 knot current the whole time with zero true wind. Your boat never knew the difference. Do you see any point inherently wrong with this scenario?
Me too.
Basta!
I've run out ideas on how to explain this.
Stepping back. The funny thing is that as sure as I am that I'm right, I'm sure you, @Scott T-Bird, and some of the others feel the same way about your perspective. Someone many years from now can probably do an interesting psychological study from this thread on how debates on the internet work. Happily, I think we've had a good, remarkably civil discussion and everyone got a chance to think through some ideas about how boats move.
I understand this picture … but you haven't described a scenario where the boat actually makes progress against the current.
So to start out, I suppose you are pointed Northeast, drifting with the current and you have a true wind pointed due west at, say, 10 knots because of the current. Your boat starts moving faster down current and you develop apparent wind that moves aft (a lift) as true wind strengthens and causes the boat to accelerate onto its foils. So then you start your move by steering to port. You accelerate and now your bow is pointed due north, your true direction is northeast, but mostly north because you picked up speed. The wind is still on starboard side but as you accelerate northward, the apparent wind is shifting forward because the true wind has steadied at 10 knots while forward motion north increases and you are now also moving east along with the current.
Your apparent wind speed now exceeds true wind speed substantially and you have sails in close-hauled even though you are beam reaching. As you turn the boat westward, you reach equilibrium with the current. The boat is moving quite fast in a northerly direction, but it isn't making any way westward with respect to a global position. Your True wind has dropped off to O because you are now not moving with respect to the air in an east-west direction, but you are moving quite fast in a northerly direction with your bow pointed northwest. You are right, the apparent wind is still on your starboard side. If your boat is quite efficient, I suppose you could go on indefinitely in a northerly direction as long as current is providing resistance on the port side of your keel.
Now, I suppose that since you believe that it is possible to maintain this direction, you are thinking that the boat can also begin to creep forward in a westerly direction with the apparent wind on your starboard side. However, with any advancement west, you are moving the apparent wind forward to your bow. Why? Well the air is still in respect to east-west direction, so any advancement west is causing a wind vector aimed due east. That puts this vector on the port side of the sail. Now you have wind vectors on opposing sides of the sails and the apparent wind moves ever more forward as you attempt to make more progress up stream to the west. Now, I suppose that you are suggesting that the wind pressure on the starboard side is far greater than the pressure on the port side, so progress can be made upstream. I think it is a zero sum game and can't be done because any progress attempted to the west turns apparent wind into drag. In the meantime, the best you can do is slip sideways at high speed. You can't advance to the west because that moves apparent wind forward, increasing drag.
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Same way a sailboat sails without wind... it doesn't.
A heat pump does not put hot air inside the house. it removes the cold air.
No, not in that video. I'm trying to find where I read that. The concept is based on the same idea as the Amazon demo video, which is based on a comment one of Artemis racing captains made about the efficiency of their designs.
-Will (Dragonfly)
Damnit ;-) I said I was done but I feel like you're so close...
Yes, everything you said above this is along the lines of what I'm asserting.
So this is where the polar diagrams and the fact that foiling cats can go away from the wind at a rate faster than the true wind speed comes in.
If you're on a conventional boat, then you're exactly right, it can't be done. There are three forces at play at this point - Lift from converting the sideways component of the apparent wind to forward thrust, drag from the forward component of the apparent wind, and drag from the hull in the water. Still good on those 3 forces?
On a displacement hull the sum of those two drag components exceeds the lift.
Here's the key though - lifting the boat onto foils reduces its friction through the water so much that it takes very little apparent wind to keep it there. The drag through the water is tiny. That's why the AC boat polar shows a VMG downwind of about 35 knots when the wind is only blowing 20 - The lift that the sails generate harnessing the sideways component of the apparent wind exceeds the sum of drag caused by the forward component plus the water drag. As the boat speeds up there is more drag through the water, so eventually you reach an equilibrium point.
The efficiency and boat speed does nothing to counter-act the pressure that will be induced on the port side. Now that the boat is moving up-current wind vectors are on both sides of the sails. The more VMG increases, the more forward apparent wind goes. Apparent wind moving forward on the sails is drag, not lift. The boat stops and gets carried downwind by current. There is no equilibrium until the boat settles into the water and drifts.
Actually Will, that's a good point that gave me another idea on this.
In chemistry terms there are exothermic reactions that have a net release of energy (eg burning wood), and endothermic reactions that require energy to continue (eg boiling water). For exothermic reactions, once you give them activation energy (eg a spark), they will release enough energy to sustain themselves. Endothermic reactions need a continued source of energy to continue. If anyone insists they can run an endothermic reaction with just an initial charge of energy, it's "perpetual motion" (eg false). Moving a boat through water is inherently endothermic due to friction in the water, so it needs a sustained source of energy. If someone tells you they can keep it going just by giving the boat a push, they're wrong. In this case, the motion of the air relative to the water provides the sustained source of energy, just as it does when there is "regular wind".
Actually, I am a psychologist and I'd be happy to explain what happens in these discussions and why it happens. However, we would have to swear to not mention anything political, or @Brian D will show up with his drill sergeant expression on his face.
Yes, this is a good analogy. And it mostly works, except for this: the motion of the air relative to the water provides the sustained source of energy, just as it does when there is "regular wind" In the Artemis scenario, the air is not moving across the boat's sail/wing, the wing/sail is moving through the air. Since sailing is endothermic, that is it takes engergy to get the boat moving, the energy has to come from somewhere. When the boat is sailing "up wind" which is downstream the river current is supplying the energy and the boat could sail using this energy and accelerate to a velocity above the river current's velocity.
However, when the boat turns around heads "down wind" which is upstream, the river current is no longer supplying energy, in order for the boat to sail faster against the current it must get energy from somewhere. And there is no somewhere to get it, especially when the boat velocity upstream equals the current velocity down stream.
If we change the scenario ever so slightly by adding a low velocity upstream wind, a real wind not a current developed wind, your position becomes more tenable. In this case, the upstream wind would become 10 knots + the actual windspeed. If the wind is strong enough to provide the necessary energy to overcome the energy consumption required to exceed the current velocity, the boat will indeed move upstream to the source.
This all works better of course on a foiling cat with a wing than a monohull dragging a lump of lead because the wing is much more efficient at extracting energy from the wind and the foils offer much less resistance than a big chunk of lead and fiberglass.
Yeah, who would've thought the anchor selection thread would be the quiet one.Well, glad to know I could offer a conversation topic of interest! Good talk, guys
Are you kidding? Who wants to get weighed down by a boring topic. Cold fusion is much more interesting.Yeah, who would've thought the anchor selection thread would be the quiet one.
The head-banging smilie has never been more appropriately used than in this thread. I'm done trying.
Basta!
I've run out ideas on how to explain this.
And by the way ... if a tree falls in the forest and no one hears it, YES IT DOES make a noise, dammit!
But this explanation still suggests it's not possible to sail downwind with a VMG faster than true wind speed. That is possible. The energy comes from the energy of the moving fluids. That energy is still available, you just need to vector it with the sails.
@dlochner , that is Drill Instructor, not drill sergeant. We Marines be very sensitive to what we are/were called. And... that would be the SBO Police Bot that would get upset, more than me. And he/she (I still don't know who it is) will be all over this like white on rice! LOL
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.
How about this - take your normal day where there is no current and a 10 mph wind from the east. Go into space and sit above the earth, so you see the earth spinning below you. At the equator, the earth's surface is spinning at 1000 mph. The air, if you could see it, would look like it was spinning 9990 mph right?
Now, put an AC boat on that water and have it sail on a broad reach. We know the AC boat can have a downwind speed greater than the wind speed, so it's going, say 20 mph over land/water from East to west (plus north at some speed, since it's reaching). From your perspective in space, that 20 mph over the water looks like 980 mph. Still with me?
Now, for demonstration purposes, slow down the land by 10 mph, but not the water or the air. Now it looks to you as if the land is moving 9990 mph. You haven't touched the boat or the wind, so the boat is still going 9980 mph as far as you can tell, the air is still going 9990, and the water still 1000. Still good?
Now, accellerate your chair in space so it's moving 990 mph. Now, the land looks to you like it's not moving. The air looks like it's not moving. The water looks like it's moving 10 mph from West to east, and the boat looks like it's moving 20 mph from East to west. Sound familair?
Now, put an AC boat on that water and have it sail on a broad reach. We know the AC boat can have a downwind speed greater than the wind speed, so it's going, say 20 mph over land/water from East to west (plus north at some speed, since it's reaching). From your perspective in space, that 20 mph over the water looks like 980 mph. Still with me?
Now, for demonstration purposes, slow down the land by 10 mph, but not the water or the air. Now it looks to you as if the land is moving 9990 mph. You haven't touched the boat or the wind, so the boat is still going 9980 mph as far as you can tell, the air is still going 9990, and the water still 1000. Still good?
Now, accellerate your chair in space so it's moving 990 mph. Now, the land looks to you like it's not moving. The air looks like it's not moving. The water looks like it's moving 10 mph from West to east, and the boat looks like it's moving 20 mph from East to west. Sound familair?