Wow...

[Davidasailor26]

I'm not sure I understand your argument correctly.

@Scott T-Bird - I think the key is that you're thinking purely about sailing straight upstream, but that's not how to do this. Let me go through your points and show how I see them...

That would mean that True wind is whatever the current is.

Yes, exactly. Current is 10 knots from the west, so True wind is 10 knots from the east.

I agree that if you pointed the bow of the boat down current at a 45 degree angle, you would have 10 knot True Wind (in their case) and an apparent wind with vector that is a little bit more aft that would actually give you lift.

Well, you wouldn't have the apparent wind until you start moving, but yes, let's say you start sailing 5 kts through the water, 45 degrees down stream (north east). Now you have the 10 knots of true wind from the east from before, plus 5 knots of boat-generated wind from the north east. The total apparent wind is now 14 kts from the east north east.

It is an interesting question if you can actually "sail" downwind at higher SOG than the current when the actual wind is 0

Don't think of tje ground yet, or the actual wind relative to the ground. Think of the boat's motion through the water, in the 10 knots of true wind - like you said above. Since the true wind is 10 knots, you can sail away from that wind with a VMG greater than 10 if you have a foiling catamaran. This means that relative to the water's surface, you're sailing west with a VMG more than 10. Since the water is moving east relative to the land at 10 knots, and you're moving west relative to the water faster than that, you're sailing west relative to the land.

#1: True wind goes to zero when your boat speed (in the water) equals the current (when SOG = 0).

But the idea is to never just sit in the water with boat speed equal to current speed. Again, you're not going straight upstream or downstream - you're always reaching. For this next part you might want to sketch out the vectors on some paper... I had to... Yes, say at some point you will have a VMG upstream that is 10 knots. But at that time you'll be broad reaching, so you're also moving north across the river. That motion across the river generates an apparent wind from the north. So your boat is pointing West North West (35 degrees north of the oncoming current). Through the water, you're going 12.2 knots. The component of your speed up-river is 10 knots, which cancels the true wind. But you also have a component of your speed, 7 knots, directly north. That northward movement, at 7 knots, generates a 7 knot northerly apparent wind. That apparent wind hits your sails at a 55 degrees off the bow (a close reach). The key is to have a boat that's fast enough that it can still accelerate to a higher speed when it is going 12.2 knots through the water, with an apparent wind of 7 knots from 55 degrees. The AC boats are efficient enough to do that. Therefore their speed through the water increases. The increase in STW changes means you have VMG upstream.

Think about what happens when you sail DDW

No, definitely not DDW. I agree that you can't sail DDW at the wind speed, which is why you can't go straight up the river. You (well, not you, but a foiling cat) can, however, sail on a broad reach at a greater that VMG than the TWS (see their polars and the data from the AC races). That's all that's happening here - Sailing with a greater downwind VMG than the 10 knots of True Wind.

Edit - Sorry, I adjusted the numbers in my example because I realized the triangle calculator I was using was changing the triangle legs around.... The concepts hold and now the numbers should work.

 

[Davidasailor26]

TRUE WIND: the natural wind over the surface of the earth. (land or water)
there is no other definition

So, land or water then? Since the water is moving relative to the land, they won't be the same number, so you have to pick one for the system you're interested in. Since we're talking about sailing a boat over the surface of the water, where its speed is dictated by its friction in the water, I suggest the true wind, as far as the boat is concerned, is the natural wind over the surface of the water.

How about this - Tell me if any of the below are not right... Put a sailboat in a lake, when there is no wind (relative to land or water, because there's no current). The sailboat sits there, sails empty. Now, put that same sailboat in a 10 kt river current, still with no air motion relative to land. The sailboat starts to drift down-stream, right? When it does that, it's being pulled against the stationary air, so its sails fill, right? Those sail present drag, right? That drag will reduce the speed at which the boat drifts, right? Since the boat isn't drifting at 10 knots, it is moving relative to the water under it, right? That motion through the water - that's sailing. It enables you to steer, have a wake, etc.

 

[jon hansen]

RELATIVE WIND: wind felt on the boat created by the combination of the 'true wind' and the vessels movement over the 'surface of the earth'.(land or water)

there is no other definition

 

[dlochner]

But the idea is to never just sit in the water with boat speed equal to current speed. Again, you're not going straight upstream or downstream - you're always reaching. For this next part you might want to sketch out the vectors on some paper... I had to... Yes, say at some point you will have a VMG upstream that is 10 knots. But at that time you'll be broad reaching, so you're also moving north across the river. That motion across the river generates an apparent wind from the north. So your boat is pointing West North West (20 degrees north of the oncoming current). Through the water, you're going 22 knots. The component of your speed up-river is 10 knots, which cancels the true wind. But you also have a component of your speed, 20 knots, directly north. That northward movement, at 20 knots, generates a 20 knot northerly apparent wind. That apparent wind hits your sails at a 70 degree angle (just closer than beam reach). The key is to have a boat that's fast enough that it can still accelerate to a higher speed when it is going 22 knots through the water, with an apparent wind of 20 knots from 70 degrees. The AC boats are efficient enough to do that. Therefore their speed through the water increases. The increase in STW changes means you have VMG upstream.

This is just simply inaccurate. VMG is not boat speed. It is velocity towards a destination, which is directly upstream. When VMG = Current velocity, there is no airflow, any movement the boat has is simply momentum that will fade due to friction with the water and still air.

If there is no airflow there is no energy to drive the boat to overcome the inherent friction from the water. The boat stops moving relative to the water and drifts with the current. At some point the air flow returns due to the boat's drifting downstream and the boat may sail again, until VMG = Current Velocity. And then it stalls due to no air flow.

The boat's velocity through the water may at time exceed the current velocity but that is not VMG. As the boat's velocity increases, the VMG increases, and the primary air flow, decreases.

 

[Davidasailor26]

VMG is not boat speed.

Concur.

It is velocity towards a destination, which is directly upstream.

Concur.

When VMG = Current velocity, there is no airflow

Disagree. VMG does not equal the motion of the boat. The boat can move North or South all it wants, keeping VMG=current velocity. This north or south movement generates apparent airflow.

 

[dlochner]

Disagree. VMG does not equal the motion of the boat. The boat can move North or South all it wants, keeping VMG=current velocity. This north or south movement generates apparent airflow.

No. When the boat sails due N or S, the VMG towards the upstream goal is at best 0 or perhaps negative.

If the destination is 10 miles downstream, and the boat sits motionless relative to the water, i.e., drifting with the current, the boats velocity through the water is 0 but the VMG is 10 knts. because the current is carrying the boat at 10 knts towards the destination.

When racers talk about VMG they are always talking about the velocity towards the next mark, not boat speed.

 

[Davidasailor26]

When racers talk about VMG they are always talking about the velocity towards the next mark, not boat speed.

Ok, we'll go with that (even though I disagree with it, but we'll save it for another thread )... So if VMG is velocity toward a point upstream, then how could it also be true that -

When VMG = Current velocity, there is no airflow

If VMG were defined as speed toward a mark upstream, then when VMG = current velocity you're moving 10 knots relative to land. If you're moving 10 knots relative to land, you have airflow, because the air isn't moving relative to land.

 

[dlochner]

If VMG were defined as speed toward a mark upstream, then when VMG = current velocity you're moving 10 knots relative to land. If you're moving 10 knots relative to land, you have airflow, because the air isn't moving relative to land.

I took an arithmetic short cut.

(Current velocity = CV, Boat speed through water = VB)

When CV - VMG = 0 then there is no air flow.

When CV > VMG there is airflow

 

[dlochner]

I wrote:

When racers talk about VMG they are always talking about the velocity towards the next mark, not boat speed.

Ok, we'll go with that (even though I disagree with it, but we'll save it for another thread )

Racers will talk about meeting their polars, but polars are about optimal boat speed under different wind conditions, they are useful when trying to calculate the optimal VMG on the course.

Optimal VMG is a function of the bearing to the mark, boat heading, boat speed, and apparent wind angle. There is a trade off between increased boat speed and distance sailed.

 

[Will Gilmore]

When racers talk about VMG they are always talking about the velocity towards the next mark, not boat speed.

As a new convert to this concept and having resisted it as best I could, here is what I have come to understand.
1st, the strict definition of VMG has been determined to mean, directly with or against the wind. In racing, there is an upwind mark so the upwind VMG is progress towards that mark, but the strick definition of VMG is in relation to the wind. There is also a downwind VMG.
2nd. In terms of this discussion, VMG is used to talk about progress "downwind", but not about DDW sailing.
3rd. It is true and demonstrable, that high performance sailing can make a VMG that is faster than true wind speed. This is not achieved by pointing DDW. It is achieved by sailing on a broad reach (somewhere near 40 degrees off of the wind direction or 140 degrees on a polar plot). In fact, this is the most efficient point of sail. This is where records are broken. The downwind component of the greater than 2 times windspeed is IS still greater than windspeed.
The hard part to understand is, how do you get there? The Artimus people are taking about using a small electric motor to give them a push to get them going. What they will need to do is convert head on wind to reaching wind. I don't see that happen very easily since they need to establish lateral resistance to the wind direction. In a river, they can do that by giving one end or another a push to rotate them in the stream. What they will need to do is pull/push their boat sideways to get the wind on the right quarter, but then they have no lateral resistance. So, I feel they are stuck.

-Will (Dragonfly)

 

[Scott T-Bird]

@Davidasailor26 it doesn't matter that you are attempting to sail across the current, if you are going to make progress against the current, no matter how far off you are reaching, you will eventually negate all of the "true wind" when your progress against the current matches the current velocity. When that happens, there is no vector component except your supposed "apparent wind" (if momentum is still moving the boat relative to land). In that case, the apparent wind is on the nose, no matter which direction the boat is moving, and the wind is drag. It won't be creating lift. Simply put, it is impossible to sail up current on ANY vector when there is no wind. It doesn't matter if you were able to sail across the current at 100 mph. When the up-current vector reaches 10 knots, it's game over, there is NO true wind anymore, it's completely still.

You can't sustain apparent wind when you eliminate True wind. When True wind goes to 0, any remnant of apparent wind that is caused by momentum becomes drag in the face of the boat no matter what reaching angle you attempt to maintain. The boat will stop and drift downstream.

 

[dlochner]

he strick definition of VMG is in relation to the wind. There is also a downwind VMG.

VMG is velocity made good towards a destination regardless of whether is upwind or down wind.

In terms of this discussion, VMG is used to talk about progress "downwind", but not about DDW sailing.

Yes. The destination is the headwaters of the Amazon, which is upstream. It is downwind only because of the conditions placed on it, there is no air movement only current.

The hard part to understand is, how do you get there? The Artimus people are taking about using a small electric motor to give them a push to get them going. What they will need to do is convert head on wind to reaching wind. I don't see that happen very easily since they need to establish lateral resistance to the wind direction. In a river, they can do that by giving one end or another a push to rotate them in the stream. What they will need to do is pull/push their boat sideways to get the wind on the right quarter, but then they have no lateral resistance. So, I feel they are stuck.

This may be what they are saying, but, it is nonsense. It takes energy to move a boat. Sailboats get energy from the wind, if there is no wind, there is no energy to be had. The boat doesn't move. This is akin to cold fusion and perpetual motion machines.

 

[Brian D]

Remember Shoot the Moon?

-Will (Dragonfly)

He missed three times. So he only got a 7. Shoot the Moon is like hand granades. Near misses don't count.

 

[Scott T-Bird]

You guys seem to think that apparent wind is the holy grail. You seem to think that once it is attached to a point of sail, that it is static - never changing. That's where it is all wrong. Anytime True wind is perfectly still, apparent wind is directly on the nose, no matter which direction that you point. When apparent wind is on the nose, it is drag and brings you to a screeching halt. There is only a True wind vector when the boat is drifting down stream. As soon as you turn the boat on any vector that is pointed up stream, you are eliminating the true wind, and apparent wind moves forward towards the nose, slowing your boat. It doesn't matter if it is on a foil or not. It doesn't matter if you achieve a speed that is greater than any true wind vector. When the true wind vector is eliminated, apparent wind shifts to the nose and becomes drag.

 

[Cowpokee]

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.

 

[Brian D]

@Scott T-Bird I would agree almost 100%. The disagreement is that only when the boat is moving in the direction of the bow will the apparent wind be on the bow. If the boat is pushed from the side then the apparent wind will be from the opposite side. Thus the sails can be trimmed and the boat will move both in the direction of the bow AND the direction of the to beam (but less so because of the keel). However, this will not last long as the apparent wind will shift forward. All for naught.

 

[Davidasailor26]

The hard part to understand is, how do you get there? The Artimus people are taking about using a small electric motor to give them a push to get them going. What they will need to do is convert head on wind to reaching wind. I don't see that happen very easily since they need to establish lateral resistance to the wind direction. In a river, they can do that by giving one end or another a push to rotate them in the stream. What they will need to do is pull/push their boat sideways to get the wind on the right quarter, but then they have no lateral resistance. So, I feel they are stuck.

I just listened again and didn't hear anything about a small motor. What they said was "The boat starts floating along with the flow". As long as they have the boat turned at some angle to the wind when it starts drifting, that's what starts the apparent wind cycle. From there on, I think you've got it.

 

[dlochner]

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.

A boat can sail as fast or faster than the wind. My 15,000# boat can make 6 knots in 7 knots of breeze, reducing drag would let it sail faster.

Assuming the destination is directly upstream, yes a boat can sail against the current and reach that destination if there is wind.

In the Artemis scenario, there is no wind, only current, when the upstream VMG = the current's velocity the boat's motion relative to the wind stops because there is no wind.

Sailors sometimes talk about a boat making its own wind when they are referring to apparent wind velocity increases. That isn't really accurate, they don't make wind, it just feels that way.

In order to make wind there needs a thermal difference between 2 areas, then air moves, and we have wind. If the air isn't moving there is no energy to move the boat. The current gives the illusion of wind while the boat is in the current, but there is no wind.

 

[Scott T-Bird]

No Mas!

 

[Davidasailor26]

In that case, the apparent wind is on the nose, no matter which direction the boat is moving

Not quite. The boat is moving North relative to land. It's moving northwest relative to the water, but the western component is cancelled out by the water flowing east, leaving only the north component, on a boat pointed at some angle between north and west. Thus, apparent wind not on the nose.