Directly against the wind?

Discussion in 'Sailboats' started by Windmaster, Jul 29, 2010.

  1. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor

    Fast is good, faster is better....

    I read these threads just to "check in" on the progress of the argument. I was stunned by folks saying that there wasn't any "use" for going downwind faster than the wind.... I suppose that person things 20 to 30 MPH is plenty fast enough and that we ought not to expend energy building faster transport... ah well.

    Regarding going up wind faster than the windspeed, I see no reason at all that the machine JB and the gang have built can't do that quite easily with the correct shaped prop and gearing. Clearly, the device has demonstrated that it can harvest a great deal more energy from the wind than a static sail. Equally clearly, a sail boat can go upwind at speeds approaching 1/4 to 1/3 of the windspeed with all sorts of parasitic drag by tacking (a slow way of going directly up wind). A prop/screw like JB's with some sort of better attachment to the water than a keel could easily be faster upwind on the water, and will obviously be faster than the wind upwind on land (where traction from tires is far more effective and efficient that a prop in the water).

    To date, no one has given a concrete reason that all the same principles that allowed JB's machine to go downwind far faster than the wind don't apply going upwind as well. Indeed, going upwind you're moving into undisturbed air rather than spinning the prop in air that has already been disturbed (which happens traveling downwind prior to the device moving faster than the wind). Thus, it should accelerate even faster than the downwind device - IMHO.

    Beau
     
  2. Windmaster
    Joined: Nov 2006
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    Windmaster Senior Member

    Clearly, you don't know the history of this, going upwind directly into the wind predates going downwind faster than the wind by at least a few hundred years, the idea has been traced back as far as the 1300's but not achieved until last century.
    I myself have been doing it since 1995.
    Going directly upwind is the sailors' dream because a normal sailing boat cannot do it.
    If you can go directly upwind then you can sail on every point of the compass, very useful.
    By contrast, going directly downwind faster than the wind is only useful on a very narrow range of headings (a few degrees) and only achieves something faster than can already be achieved at a slower speed (slower than the wind).
    However, I don't want to disparage the achievement, it's a great brainteaser (to some) but with a little reflection it's quite simple really.

    If you think going downwind faster than the wind has a practical purpose, please explain exactly what the purpose is.

    So to summarise:
    Going upwind directly into the wind - very useful, never exploited, very relevant at the moment because of energy and environmental concerns. Has been dreamed of for hundreds of years.

    Going directly downwind faster than the wind - a wonderful trick to amaze the audience, (but quite easy to do really) - little practical use, never even thought of until the last few years of this century.

    To say "going downwind faster than the wind proves it's possible to go upwind directly into the wind" is putting the cart before the horse, since the second predates the first by many, many years. However, it is true that the one proves the other.

    What is interesting is the lack of clear thinking by the audience between the two different states, and how every thread about upwind becomes inevitably diverted into one about downwind.

    I'm not wanting to denigrate Spork's and JB's achievements here, just trying to get back to reality amongst all the hype.
     
  3. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor

    Well, I guess my irony didn't work in my earlier post. Here's what I was attempting to say in much plainer language.

    1) Going faster is almost always better than going slower. Thus, going downwind faster than the wind is better than going downwind slower than the wind. This seems obvious because folks have spend millions on improving the speed of sailing craft (and other craft for that matter) for centuries. Therefore, it's hardly "of little practical use". If that were true, many of the innovations that have improved downwind speed would also be "of little practical use", something that's clearly not true.

    2) Going faster than the wind as exhibited by JB craft is based upon a principle that is completely independent of the relative wind direction. This means that the principles upon which it is based will work no matter what direction one is going. One simply need rotate the propeller to face into the wind and the device will move. The point I made earlier is that going faster than the wind is the interesting issue, not simply going up wind. Also, it seems clear that it will be easier to go upwind at all, and moreover greater than the true wind speed, as the propeller won't be in it's own disturbed air during initial acceleration.

    3) The history of who attempt what, which came first, and who gets credit for what are all utterly irrelevant to the questions of physics. While loads of people attempt to claim they were first etc.... JB and his team have steadfast never claimed any of that and have always given other credit in everything I've read and the public presentations I've attended. One of the nicest things about physics is that it's either right or wrong and it completely free of the ego of who did what first. Of course some of the physicists, both professional and amateur, fail to accomplish this lack of ego.

    4) I completely agree that this entire field is fraught with hyperbole and various forms of rants that are based on all manner of opinion, with little or no regard to measurement of experimentation, let along theory.

    I hope this make my earlier point more clearly. I'll stay away from Irony in the future.

    Beau
     
  4. Windmaster
    Joined: Nov 2006
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    Windmaster Senior Member

    True, but my point was that going directly upwind cannot be done in any other way, whereas, going downwind can be done and has been done in a slower way. Thus going upwind directly is unique, whereas going downwind is not unique and the JB craft is only an improvement.


    I'm trying to decipher that paragraph. It may indicate that you are unclear about how the DDWFTTW craft works. I think even the builders have never claimed it would work in any wind direction relative to the heading of the craft. With some gearing changes it would work directly upwind, but in that case the propeller would be working as a turbine driving the wheels. Whereas going directly downwind the wheels are driving the propeller (the other way round). Rotating the propeller (and it is a propeller - not a turbine) would not be any use, since the propeller is the driving device and needs to be aligned to the wheels at all times, it would not be any good pushing the craft sideways!

    Also, if it so "easy" and "obvious" that you can go directly upwind faster than the wind I wonder why no-one has ever done it?
     

  5. tspeer
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    tspeer Senior Member

    DDWFTTW is technically limited to a few degrees only because people think it is impossible. The restriction to a few degrees from DDW is to ensure that the boat isn't being operated FTTW on a broad reach, which everyone knows is possible and commonplace by high-performance sail craft.

    A practical boat that could achieve DDWFTTW would not be restricted as to its heading. In principle, it could sail faster than the wind on any heading. That would be a huge improvement in performance over conventional craft.

    Of course, it would need to have variable pitch for the rotary sail and the water propeller, in order to have adequate efficiency for the different advance ratios and to be able to switch roles. Morphing blades that can change their twist angle would be very useful for optimizing performance. The rotary sail would also need to be rotated about a vertical axis to get the proper alignment of the plane of the sail to the relative wind. A transmission that can vary the relative rpm of the sail and prop would also be required.

    The rotary sail would act like a windmill upwind and as a propeller downwind. the prop would act like a propeller upwind and a turbine downwind. On reaches, there would be some point of sail at which it would be best to operate as an autorotating sail and not drive the prop at all, with the prop feathered or folded. Above that point it would probably be best to employ some combination of conventional sailing and windmill operation, and below that point it would be best to employ a combination of conventional sailing and water turbine operation. Just like when motorsailing a conventional sailboat, there would be a smooth transition of conventional sailing to driven propulsion as the boat approached either DDW or straight upwind.

    By conventional sailing, I mean the use of aerodynamic vs hydrodynamic side force instead of pure axial force from the rotating machinery. The prop could also act as a rotary keel if it could be rotated about a vertical axis. However, this may imply a much larger prop than would be efficient for propulsion or turbine operation, so the hydrodynamic side force may have to come from a conventional keel to get the necessary span efficiency with a reasonable parasite drag.
     
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