Artemis Technology--Sustainable Future

Discussion in 'Boat Design' started by Doug Lord, Jan 29, 2019.

  1. Doug Halsey
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    Doug Halsey Senior Member

    Except that you are discussing VMG upwind & previously we were discussing VMG downwind.
     
  2. kerosene
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    kerosene Senior Member

    Brainfart in my part. You are right.
     
  3. Doug Halsey
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    Doug Halsey Senior Member

    If you want to correct it with an edit, I'll delete my comment.
     
  4. tlouth7
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    tlouth7 Junior Member

    I agree with what you have written here. My point was that the diminishing speed as a boat luffs above her best course comes not from reduced lift (because it is possible to keep the sail at constant angle of attack). Rather the reduced speed comes from the lift vector becoming close to perpendicular to the boat's heading, so the forward component drops off.
    I follow your logic, and it is a perfectly valid way of thinking about the situation. Where I differ is that you assume that the VMG upstream will be very small (compared to speed across the river). Given that even quite a modest yacht can achieve a 45 degree angle to the moving fluid (air) when sailing upwind, why should a top-end foiler not achieve similar angles sailing upstream?
     
  5. Doug Lord
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    Doug Lord Flight Ready

    In the second day of SAIL GP racing in Sydney, several of the F50 cats did 30 knots in a 10 knot breeze.
     
  6. sharpii2
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    sharpii2 Senior Member

    This all depends on how narrow the canal is and how long the boat is. If the canal is narrower than the boat is long, it may not be possible at all. If the canal is at least three times as wide as the boat is long, an ordinary sailboat should be able to sail up it. A high-performance foiling boat, under the same conditions, may not be. In order to do this, a sailboat needs short-tacking capabilities. A clunky old folkboat may do better than a foiling catamaran.

    No it is not. This is because, as I explained in my previous post, the boat is extracting energy from the river flow and not the air. Remember, we are expecting the boat to move in a direction relative to the land. And of the two fluids air and water, only the water is moving relative to the land.
     
  7. sharpii2
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    sharpii2 Senior Member

    Good. This is close to the speed I had in mind in my previous post. I assummed the boat would make good at about a 25 degree angle to the wind but would lose ground every time it changed tacks.
     
  8. sharpii2
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    sharpii2 Senior Member

    Because it is not sailing in the traditional way. It is using its keel as a sail and its sail as a keel. The two are not exactly inter-changeable. In this case, the keel will have to move through the water faster than the sail moves through the air. I'm not sure this is even possible, but I'm willing to be open to the chance it may be. If this were ever pulled off, it would certainly knock a few wanna-be physicists for a real loop. :-
     
  9. tlouth7
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    tlouth7 Junior Member

    But when you appreciate that the position/speed of the land is irrelevant to how a boat sails you can see that they are exactly interchangeable.
    This is definitely possible. For example any moderately fast skiff, cat or foiler on a broad reach will have STW > AWS.

    At the lowest point on the below polar diagram the 49er is doing 20kt in 25kt TWS at an angle of 160 degrees. This gives AWS = 9.25kt (from the cosine rule), far lower than the speed the keel (daggerboard) is moving through the water.
    life on the layline: 49er Polars from TacticalSailing.com http://lifeonthelayline.blogspot.com/2012/02/49er-polars-from-tacticalsailingcom.html

    Sadly the 49er can't quite make VMG downwind faster than TWS. At best its STW = TWS whereas the F50s are managing STW = 3*TWS.
     
  10. sharpii2
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    sharpii2 Senior Member

    I guess we will have to agree to disagree.

    I do not know of a single occasion, except dead downwind sailing with the wind, in which the wind is moving faster than the boat, where the keel is moving through the water faster than the sail is moving through the air. This of course includes apparent wind. For this Amazon experiment to work, this is what must happen. If the boat is "sailing" at 30 kts in still air, the keel is moving through the water at 30 kts / the sin of 25 degrees (mentioned in my earlier post), or 30 kts/0.926, or 32.4 kts. Meanwhile, the sail is moving through the air at only 30 kts.

    Do you agree?

    To make this situation easier to picture, suppose we made our own treadmill windmill cart. But our cart would have one significant design difference. The diameter of the wheels would much greater. They would be > 0.40 * the diameter of the air prop. The gearing ratio would stay at 1.0 just like the other windmill carts. Now the air prop will turn a lot slower in relation to the treadmill's surface moving under the cart. This is because the cart's axle will be turning slower. And if the air prop shaft turns at the same speed as the axle, the air prop shaft will be turning slower as well. Now the air prop will most likely not be turning fast enough to drive the cart fast enough to even keep up with with the treadmill's speed let alone surpass it.

    This is very similar to what we would be trying to do on the Amazon with a 10 kt current and no wind.

    And yes. The land is relevant. This is because we are trying to move relative to the land. If we were not, I would agree with you entirely.
     
  11. tlouth7
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    tlouth7 Junior Member

    I guess we will have to disagree because I lack the skill to explain my argument in a better way.
    I thought that my 49er example above (#54) illustrated just such a situation? The boat is moving through the water at 20kt with only 9.25kt of apparent wind.
    upload_2019-2-20_9-22-41.png
    I am afraid I don't follow what you are describing here, but for these boats sailing 32.5kt in 30kt AWS is definitely on the cards.

    I will not attempt to answer the windmill part because I think it will just confuse the issue. Here is a vector diagram for a possible scenario that achieves Artemis' goal. Note that SOG=AWS (but in the opposite direction):
    upload_2019-2-20_9-38-25.png
    Wikipedia has a nice section about sailing downwind faster than the wind, including a table for VMG at various AWA/TWA values:
    High-performance sailing - Wikipedia https://en.wikipedia.org/wiki/High-performance_sailing#Velocity_made_good_(VMG)
     
  12. Niclas Vestman
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    Niclas Vestman Junior Member

    Is it possible. Absolutely! For a high tech ultra low weight foiler, on a good day, without floating debries (unlikely in the amazon) and without thunder. Meaning, any practical applications like goods or person transport is rueled out. Just like it is possible to briefly power a lightbulb with static electricity. But it's just not very practical for people to start trying to cover their electricity needs by sitting and rubbing their favourite polyester pants, for examples to power their tv while watching SNL.
     
  13. tlouth7
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    tlouth7 Junior Member

    What an evocative comparison!

    I think that this video was a rather contrived attempt by Artemis to claim some "eco" outcomes from the presumably vast investment they have put into this project (it was obviously not pitched at a sailing audience). Clearly you would be better off with some fixed turbines in the river - either directly feeding into the grid or charging batteries that are used on electric drive cargo/pleasure boats.
     
    Niclas Vestman likes this.

  14. Angélique
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    Angélique aka Angel (only by name)

    The latter is easily to confirm or refute by doing a test, just like the first, whenever Artemis wants to substantiate their claim by actually doing the test they've suggested.

     
    Last edited: Feb 21, 2019
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