Wow...

[dlochner]

But your explanation suggests that boats could never sail downwind at VMG above true wind speed. We've seen them do that.

The key is that the boat isn't going purely upstream / downwind - it's also going across the wind/current. That combination of cross wind with downwind sustains an apparent wind, which is sufficient to keep the boat moving forward.

VMG is velocity made good towards a destination, in this case the some point upstream on the river. When VMG equals the current, then there is no wind.

Think of sailing downwind in a dying breeze. Initially the wind is high and you can run straight down wind, as the wind dies it become necessary to head up to maintain speed, but while heading up the boat is adding distance to be sailed, so VMG can go down, even while boat speed through the water is maintained or increased.

Remember the only breeze is that which is generated by the current pushing the boat through still air. If the current stopped, then there would be no breeze. The current can be effectively negated by moving in the exact opposite direction at the same speed as the current. The boat will look like it is moving through the water, but will not be moving and the air around it will not be moving.

Since all the energy driving the boat is coming from the river, not the air, motion will stop. When viewed from the shore, the boat will be standing still, even if the knot meter is registering 10 kts.

The red herring in this discussion is the wind, there isn't any, it is all about the current and how that affects the air moving past the boat.

 

[Cowpokee]

This is the answer. All the rest is ultimately distracting. As the VMG going upstream approaches the current speed, the apparent wind decreases until the VMG and current speed are equal, at which point there is no wind to power the boat and the boat stalls and starts to drift downwind. This causes the VMG to go negative and the apparent wind to increase and we start over again. Practically it would be like sailing downwind in puffy conditions, down in the puffs, up in the lulls, except, that going against the current means it is always a losing game.

Vector diagrams can be helpful but they don't really answer the question, because the solution is not linear. If we were to do the calculations, the answers would likely be hyperbolic. That takes some calculus to solve and that's when the nightmares started.

Dave,
I challenge your explanation. The understanding has to be relative to the boat. Uncle Al got quite famous explaining relativity to the layman, so I will make an attempt here. Davidasailor26 already showed that a sailboat can have a VMG downwind greater than true wind. Let me change a few words (in italics) in your statement to illustrate my point.

"As the VMG going downwind approaches the true wind speed, the apparent wind decreases until the VMG and true wind speed are equal, at which point there is no wind to power the boat and the boat stalls and starts to drift downwind. "

 

[Cowpokee]

Since all the energy driving the boat is coming from the river, not the air, motion will stop. When viewed from the shore, the boat will be standing still, even if the knot meter is registering 10 kts.
.

This can't be true. In order for the boat to appear standing still from an observer on the shore, It would have to be sailing directly up stream (Upwind). Now a helmsman could pick a course upwind where the boat is making zero VMG upstream but it would still be moving laterally across the river in order to keep his sails full.

 

[Scott T-Bird]

The whole thing is completely silly. You can't convert river current to apparent wind. You can only generate apparent wind from true wind and boat velocity. When the boat has its bow pointed downwind, the current is causing a true wind vector pointed straight up current. The vector of true wind never changes, it is always pointed straight up-current. As soon as the boat points off the current, the true wind vector is diminishing. Sure, via some trickery of reducing friction and foil shape, the boat speed can increase, which increases apparent wind. But the more the boat points off the true wind vector, the greater the true wind vector diminishes to the point where it is 0 when your velocity made good up-current reaches the current speed and negative as soon as you go more up stream. As the true wind vector diminishes, so does the apparent wind vector at increasing rate. The only force that assists apparent wind at this point is momentum, which is killed by friction and current. Even if friction were zero, the current is killing momentum and the apparent wind vector. The key is that there is no sustainable apparent wind when the true wind vector disappears. You can't have a true wind vector unless there is actual wind, or the current is pushing your boat in a direction that causes true wind.

When I windsurfed, I knew that apparent wind could keep my board on plane as the wind diminished, but it was always a losing battle. As long as momentum allowed apparent wind to stay high enough for planing, I could stay on plane for just a little longer, but as soon as I altered course downwind, or friction eventually caused momentum to diminish, the board dropped off plane.

The whole mistake is a belief that the true wind vector isn't necessary to sustain apparent wind. True wind disappears, and so does apparent wind soon after - as soon as momentum disappears. That is an unavoidable fact of physics.

 

[Davidasailor26]

Dave,

Let me try to give my perspective on your points to figure out why I don't follow. If my logic fails at some point please help me understand where:

VMG is velocity made good towards a destination, in this case the some point upstream on the river.

Concur.

When VMG equals the current, then there is no wind.

I disagree. To give an example of why - If the boat is pointed at an angle to the river, then when its VMG equals the current, there is no upriver/downriver component to the wind, but there is still the boat's motion perpendicular to the river, so still apparent wind.

Think of sailing downwind in a dying breeze.

But in this case, the breeze isn't dying. The current is constant, so the water is always moving relative to the air, so there is a constant "breeze" of air over water.

If the current stopped, then there would be no breeze.

I agree. But I don't think their video suggested that the current would ever stop - They're in a constant current river.

The current can be effectively negated by moving in the exact opposite direction at the same speed as the current. The boat will look like it is moving through the water, but will not be moving and the air around it will not be moving.

I agree - But the boat will never just sit there moving in the opposite direction and same speed of the current. Yes, if you just point the boat straight upstream, it will move downstream at a speed of (current speed - effect of drag of the sails through the air). The key is to not have the boat point straight upstream - always at an angle, so you have that sideways component of wind which keeps you moving. On these hulls, as the America's Cup itself (not the video) showed, that sideways component of apparent wind is enough to move the boat away from the source of the wind at a VMG higher than the wind speed itself.

 

[Davidasailor26]

the more the boat points off the true wind vector, the greater the true wind vector diminishes to the point where it is 0 when your velocity made good up-current reaches the current speed and negative as soon as you go more up stream.

Yes, absolutely. But the boat is also moving sideways across the river, which generates a sideways vector to the apparent wind. Add the sideways vector to the negative vector you describe, and you have a boat that's sailing on a close reach.

 

[Scott T-Bird]

On these hulls, as the America's Cup itself (not the video) showed, that sideways component of apparent wind is enough to move the boat away from the source of the wind at a VMG higher than the wind speed itself.

The boats are only moving because there is a true wind vector. They can't sail at speed due downwind, when the apparent wind diminishes to the true wind speed. Once momentum is consumed, they drop off their foils and sail no more than but slightly less than true wind speed.

 

[Davidasailor26]

They can't sail at speed due downwind, when the apparent wind diminishes to the true wind speed

I agree. But the idea is not to sail due downwind, but at some angle off the wind, so that there is always that sideways component to the wind speed moving them forward. Same as sailing on a reach at VMG above true wind speed in an AC race.

 

[Scott T-Bird]

@Davidasailor26, I think that you are completely ignoring that true wind vector diminishes as soon as the boat starts moving across the current. Apparent wind is not sustainable without true wind and it diminishes as true wind diminishes. Sure, it is feasible to sail a boat back and forth across a current, but the boat will always have to be moving down current as well. By increasing efficiency, it is even possible to sail across the current faster and with less loss of ground, but as soon as the boat reaches a VMG up-current that matches the current, ALL true wind is gone and apparent wind will soon follow, only surviving to the extent that momentum survives. Momentum will always concede to the current eventually.

 

[Simon Sexton]

got a duck thawing that i will roast tomorrow.

I envy you! Just boiled 12 pounds of crawfish last night for the family, first boil of the season! Having friends over and boiling crab legs and sausage next week!

 

[dlochner]

Part of the confusion is over the differences between VMG, SOG, and speed through the water. They are not interchangeable. A boat can have a very high SOG or speed through water and have 0 VMG or even a negative VMG.

It is not possible to have a VMG unless there is a specific destination. In the video that destination is somewhere directly upstream. So while a boat might have good velocity (speed) reaching across the river, if the goal is some point upstream the VMG will be very low.

Sailing upstream will add a negative component to the VMG calculation. When the VMG upstream equals the current, the net result is 0 and the wind dies. Current speed = -10kts, VMG = 10 kts. Add them together and they equal 0, there is no motion towards the destination and there is no wind because the VMG and Current are equal. Once the boat stalls because there is no wind, it will drift downstream until there is wind again and repeat the process.

 

[Davidasailor26]

I'm going to mull this over and think about if there's a way for me to express myself better later. I'm still quite sure it's possible to sail upstream in these circumstances, but some of you probably feel just as strongly that it isn't. I'm just frustrated that I can't find the words to describe how it makes sense to me.

 

[Will Gilmore]

As soon as the boat points off the current, the true wind vector is diminishing.

This part is not quite true. When the boat is on a heading anywhere between dead down stream to perpendicular to the current, there will be an addative 10 knot component to the wind felt across the deck. Even with a heading perpendicular to the current, the boat is still moving down stream at ten knots. VMG is in reference to true heading not compass heading. So VMG is the actual direction and speed of progress across the ground. When the boat starts pointing upstream, then the 10 knot wind generated by the current will begin falling off. When the angle of the boat's heading gives her a velocity that equals the upstream movement of 10 knots. That component drops to 0. At this point, the wind direction on deck will have moved to a point of sail that the boats can't convert to good apparent wind. Remember that to get to this point, the boat has been falling away from the wind the whole time. Once the boat begins gaining upstream velocity, the apparent wind is entirely self generated. The current generated component is lost. The boat is now running away from that wind. So, which direction is the wind coming from?
Has anyone taken one of these boats out on a windless day and given her a push start of 10 knots and seen her keep going? No!

-Will (Dragonfly)

 

[dlochner]

VMG is in reference to true heading not compass heading

This is incorrect, VMG is Velocity Made Good towards a destination. In racing, it is the velocity made good to the next mark. When cruising it is velocity made good towards the next waypoint or destination. I'd give you a reference, but I can't access a search engine at work.

 

[Cowpokee]

I'm going to mull this over and think about if there's a way for me to express myself better later. I'm still quite sure it's possible to sail upstream in these circumstances, but some of you probably feel just as strongly that it isn't. I'm just frustrated that I can't find the words to describe how it makes sense to me.

I agree with you 100%. If a boat can sail a greater VMG downwind than the wind speed, it can therefore sail upstream against a current with zero true wind.

 

[Davidasailor26]

Has anyone taken one of these boats out on a windless day and given her a push start of 10 knots and seen her keep going? No!

But current isn't a "push start", it's a constant motion of water. In this case, since the air is stationary relative to the ground, current is constant motion of water relative to the air. Constant motion of water relative to air is, well, wind. Boats sail in wind.

 

[Scott T-Bird]

@Cowpokee said it very succinctly above my posts. Without an actual true wind to propel the boat, it is simply impossible to sail a boat in opposition to the current, even if you go sideways enough to generate an apparent wind. It may only be feasible to momentarily sail up-current with momentum that is generated by apparent wind. But it simply can't be sustained. Without actual wind, the true wind velocity drops to zero as soon as the up-current boat speed matches the current. True wind is zero, apparent wind has to disappear no matter how efficient the boat and sails are.

 

[Simon Sexton]

You're on your boat. It's got that big fancy wing sail and carbon foils all over your hull. You're out in the bay. There is no wind. At all. Zero. You run by motor power with your sail up trimmed as close as you can get it, and you get moving at a pretty decent speed with your motor. I don't know who in their right mind would think, "Oh, I feel wind on my face, I think I can probably sail now," and turns off their motor and magically is able to keep sailing with the air that they've been feeling on their face. Sorry, there's just no way. I don't care how many "wind vectors" and "variable forces" there are in the technical equation. When you're sailing, if no wind, no go. Doesn't matter how fancy or light or drag-resistant your rig is. Here's my equation:

sailboat + no wind + no motor = no moving. None. Especially with an opposing current.

 

[Simon Sexton]

it simply can't be sustained.

YESSS, THANK YOU.

 

[dlochner]

Without actual wind, the true wind velocity drops to zero as soon as the up-current boat speed matches the current. True wind is zero, apparent wind has to disappear no matter how efficient the boat and sails are.

@Scott T-Bird You are absolutely correct with one significant word change, it is not boat speed that makes the difference, it is VMG, or the speed towards the upriver destination. A boat may be screaming along at 30 kts, but if it is sailing at 90* to the destination, it will never get there. Its VMG will be 0. It can point down wind a little and not lose much speed, but at some point if it reaches a VMG (not boat speed) equal to the current speed, the wind will die.