Wow...
- Thread starter Simon Sexton
- Start date
You could likely explain the physics behind this to me for hours - and I'm still not sure I'd understand how this works.
(My brain is likely still scarred from a bad experience with vectors in the one - and only - physics course I took in college.)
(My brain is likely still scarred from a bad experience with vectors in the one - and only - physics course I took in college.)
I honestly don't, either. My question is: How does the sail fill? I ask that because they used riding a bike as an example for generating wind, and that works, yes, but it only generates wind coming directly from straight ahead. That prevents the sails from being filled, doesn't it?
The commentator is playing with numbers a bit. The boat doesn't actually travel back up the river at 30 miles per hour. The AC72 boats don't sail directly down wind, they tack at somewhere around 45° true. They do travel faster than the wind, around 2.7 to 3 times the wind speed. This means the apparent wind goes forward and ends up being much higher than true wind so even heading down wind (~45° off true DDW) in 10 MPH wind (if you were travelling with the current in the example above), once you get up to speed you end up with somewhere around 21 MPH wind about ~20° off the bow and you are sailing at about 30 MPH at a ~45° angle INTO the current, making good about 14 MPH over the bottom upstream.
This guy does a good job explaining this if you want more detail http://www.nauticed.org/sailing-blog/americas-cup-apparent-wind/
The thing that is hard to comprehend is how ridiculously efficient these AC72 boats are. They make a J121 feel like a Viking Longship.
This guy does a good job explaining this if you want more detail http://www.nauticed.org/sailing-blog/americas-cup-apparent-wind/
The thing that is hard to comprehend is how ridiculously efficient these AC72 boats are. They make a J121 feel like a Viking Longship.
Ok so I think there wrong. If the current gives you 10 mph down wind and the boat is 3 times as effecent with the wind yes you can go 30 mph. Now turn the boat around
The drag on the hull by the current would propel you backwards at 10 mph. The boat is still 3 times as efficient as the winds so it could in theory do 30 mph down wind (up river) making 20 mph headway once you overcome the current.
Here is the problem though. The reason the boat is so efficient is that it comes out of the water on a foil and reduces drag. Once the drag is reduced there is no longer a 10 mph current counteracting on the boat and the boat will eventually slow to a crawl or stay in one place. I don't see how you can have a drag on the boat of 10 mph that could be overcome to continue progress up river.
So what say you am I not understanding something here or have they solved perpetual motion where you get more work out of something than effort than you put in.
The drag on the hull by the current would propel you backwards at 10 mph. The boat is still 3 times as efficient as the winds so it could in theory do 30 mph down wind (up river) making 20 mph headway once you overcome the current.
Here is the problem though. The reason the boat is so efficient is that it comes out of the water on a foil and reduces drag. Once the drag is reduced there is no longer a 10 mph current counteracting on the boat and the boat will eventually slow to a crawl or stay in one place. I don't see how you can have a drag on the boat of 10 mph that could be overcome to continue progress up river.
So what say you am I not understanding something here or have they solved perpetual motion where you get more work out of something than effort than you put in.
The sails don't fill, because they aren't sails, they are wings. The material is solid, not fabric.
now, if you believe that, i have some land in Louisiana i'd love to sell you. 
One of the hardest parts of this video to wrap my head around was the notion that while there is a 10 kt apparent wind from the stern, the boat is moving backwards relative to the ground and stationary relative to the water, until it accelerates past the current speed. Once the boat starts moving up river at a 10kts over the ground, then it will be stationary relative to the ground with the river flowing past. Since the wind speed is 0 relative to the ground, the boat will become becalmed, the apparent wind drops to 0.
I just don't see how the boat could move upstream under those conditions. I can see that the boat could sail back and forth across the river and not move down stream, but not move faster than still air.
I just don't see how the boat could move upstream under those conditions. I can see that the boat could sail back and forth across the river and not move down stream, but not move faster than still air.
Forget the river I just realised that according to their video they have solved the problem of light speed travel! As the boat gains speed the apparent wind gains speed into infinity! So why can't this boat accelerate to 100+ under normal sailing not in the river! If it's moving forward at 30 kts then the wind is 30 kts so it should do 90 kts this continues to increase. Someone better call NASA!
... forget light speed... they have overcome the first law of thermodynamics.
Schrodinger's cat is dead!!!!
with no true wind the apparent wind will always be dead ahead. end of discussion
That's the thing, there is 10 MPH of true wind. The boat doesn't care what the wind speed is relative to land, it cares what it is relative to the water it's sitting in.
You have to think about apparent wind, not true. Once the boat starts moving, there will be apparent wind. They never head straight up stream and the true wind is directly against the current so apparent wind will never be zero.
Now, this is assuming they can build up enough speed to get foiling in 10 MPH wind, which I'm not sure is possible. Once they lift off, drag drops dramatically and they pick up a lot of speed. They would never be able to get upstream low-riding.
at the thirty second mark of the video he clearly states there is no wind.
i stand by my remarks
i stand by my remarks
you confused me. true wind is the wind over the earth surface. apparent wind is the wind felt on the vessel, in this case created by the 10 mph current. ok so now we can compare apples to apples.
if the vessel is pointed up stream with sails out it would be going through the water but still down stream vmg's. but running is slow. we can agree on that. so what he is suggesting is that with the vessel headed in an upstream diagonal the vessel will then have a wind angle to propel the vessel through the water at reaching speeds of this kind of vessel. however, if the vessel achieves a vmg upstream that is a positive number then the current made apparent wind is gone.
this is my thinking. but hey, i'm wrong allot.
if the vessel is pointed up stream with sails out it would be going through the water but still down stream vmg's. but running is slow. we can agree on that. so what he is suggesting is that with the vessel headed in an upstream diagonal the vessel will then have a wind angle to propel the vessel through the water at reaching speeds of this kind of vessel. however, if the vessel achieves a vmg upstream that is a positive number then the current made apparent wind is gone.
this is my thinking. but hey, i'm wrong allot.
I don't buy it. The energy has to come from somewhere. Otherwise we could do that with airplanes. Just get going and let the apparent wind take over. You could shut off the engine.
We saw this played out in San Francisco. Once the AC72 boats come up on their foils, their wing sails are generating so much power relative to hull drag that the boat will be able to convert downwind true wind to effective lift-effect apparent wind and sail trimmed close-winded. It took the crews some time to figure this out and early Louis Vuitton qualifiers did not get it figured out downwind. The boats eventually were ALWAYS in close-wind trim. Once it became clear how apparent wind force could be converted, boat speeds increased downwind and the challenge became keeping the boat on her feet in that 60mph control range.
But it isn’t really 30kts COG up river it is 30kt-10kts = 20kts SOG up river. - as long as the river current is imparting movement through the air. That may what some of you are not accounting for - the river current motion is a source of wind power.
But it isn’t really 30kts COG up river it is 30kt-10kts = 20kts SOG up river. - as long as the river current is imparting movement through the air. That may what some of you are not accounting for - the river current motion is a source of wind power.
No wind, no move. That defies the laws of physics (I think). A sailboat cannot generate it own wind to sail. Granted, the boat moving because of current will generate apparent wind, but not off the side of the boat. It will be head-on. Sorry, I call BS on this.
