The Wind Powered Sail-less Boat

Discussion in 'Boat Design' started by DuncanRox, Oct 20, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

    JB
    If you make a nice big prop for your land vehicle I will devise a boat with the best possible chance of demonstrating it on water.

    If your prop will push a manned land vehicle then I am confident I can devise a hull and turbine that will demonstrate DDWFTTW on water.

    I have some old hulls and can make a water turbine in a few hours but the prop is really too big for a once-off demo.

    Rick W
     
  2. Guest625101138

    Guest625101138 Previous Member

    ttt

    A pushing prop is self-stabilising. A pulling prop is unstable.

    I expect the reverse is true for a turbine so the dragging turbine might dive. It is too early in the year to work it out.

    Irrespective the turbine does not need to have huge diameter. Remember it is operating at high velocity and low force. The conditions are more favourable for high efficiency that the prop. The turbine strut would not need to be very heavy.

    There is one small advantage with the boat. There is no static friction on the hull so it will move as soon as there is a breeze. The drive friction needs to be very low.

    Rick W
     
  3. ThinAirDesigns
    Joined: Dec 2008
    Posts: 127
    Likes: 6, Points: 18, Legacy Rep: 40
    Location: USA

    ThinAirDesigns Senior Member

    Let's talk about whether it's practical to make a prop that can be used for both. I can't think of a better use for our land cart prop when we are done with it than to put it to use on water.

    Not sure if the design would require compromises that would threaten the success of both projects. I'll Pm you.

    JB
     
  4. Guest625101138

    Guest625101138 Previous Member

    Spork
    This thread is an extremely good example of poorly understood and misused terminology from a physics/engineering/scientific perspective. Hence my nitpicking on your use of the term Betz Efficiency indicating that the limit of "efficiency" for a turbine is 16/27 (59%):
    http://winds-energy.blogspot.com/2007/09/rotor-efficiency-and-betz-limit.html

    Efficiency for a machine is defined as the ratio of work out to the work in. For a machine working at a steady rate it can also be determined by power out to power in.

    For a turbine, the input power is the thrust force applied to hold or move the turbine against the air stream times the velocity of the fluid flow through the blades. The power out is the shaft torque times the rotational speed.

    The key point is that a well designed turbine can achieve an efficiency up around 90%. This has NOTHING to do with the Betz limit. In fact to get high efficiency you need quite low velocity ratios so the proportion of energy extracted from the stream will be much lower than 59%.

    Rick W
     
  5. markdrela
    Joined: Jun 2004
    Posts: 307
    Likes: 30, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    A much more detailed DDW analysis is in the attached PDF.

    It includes figures showing predicted velocity ratios for a boat and a wheeled vehicle. As expected, the net power-transmission efficiency has a powerful effect on the speed. The drag has rather less effect than I expected, and so the analysis indicates that reaching the DDWFTTW condition (V/W > 1) with a boat looks "easier" than one might expect. As always, there's the possibility of some derivation error somewhere, so these conclusions may change.

    I'm not sure where an air prop of a realistic size falls on the CT' axis, although the peaks are rather flat which indicates the prop size is not super critical. It would be interesting to see where the YouTube machines fall on these plots.
     

    Attached Files:

    • ddw2.pdf
      ddw2.pdf
      File size:
      59.1 KB
      Views:
      607
  6. spork

    spork Previous Member

    Sorry Rick but I just plain disagree. Efficiency can be defined in a million ways. The denominator can be almost anything that makes sense. I think we all understand the nature of the Betz limit and how it comes about. I don't really care to debate definitions however.
     
  7. Guest625101138

    Guest625101138 Previous Member

    Spork
    You are right if you are not talking about mechanical efficiency.

    MECHANICAL EFFICIENCY is defined as WORK OUT / WORK IN.

    This is a fundamental to understanding DDWFTTW as it is a simple machine. If you grasp this concept then it is easy going from there.

    Rick W
     
  8. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 147, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    What differences have to made to the DDWFTTW cart to make it go directly into the wind (DUWFTTW?)

    I understand the prop and turbine exchange functions and I can imagine it would require prop and turbine changes for max efficiency.

    Can it be done by nothing more than a simple change of gearing? that would make a 2-way machine practical. Well, possible anyway.
     
    Last edited: Dec 31, 2008
  9. ThinAirDesigns
    Joined: Dec 2008
    Posts: 127
    Likes: 6, Points: 18, Legacy Rep: 40
    Location: USA

    ThinAirDesigns Senior Member

    Kayaker,

    Yes, simply reduce the wheel size on our cart and when the wind hits it it backs up *into* the wind directly.

    Of course to optimize it for upwind use requires a bit more than that -- at least one would likely want to reverse the prop, but I'm sure there are other things as well.

    JB
     
  10. Guest625101138

    Guest625101138 Previous Member

    The basic difference is the gear ratio. It is a matter of what has mechanical advantage. So changing the ratio can alter the direction of travel relative to the wind.

    If you go back to post #180 on this thread you will see videos clips of a little vehicle I made to operate between a moving board and a table. The vehicle is flipped to show both modes of operation. It has about 2:1 gearing with a little rubber band over pulleys.

    The resulting vehicle would be a compromise. The turbine would ideally be smaller than the prop for the best efficiency in each mode. Also the blades would be asymmetric in each mode but facing in different direction for each mode of operation.

    The differences between prop and turbine for the land vehicle may not be as significant as a boat because the vehicle drag at speed is much lower giving it higher speed potential so the prop gets into a decent operating regime.

    A hydrofoil might work out OK with a boat but it will need a good breeze to get flying.

    Rick W
     
  11. spork

    spork Previous Member

    It seems we're getting kind of specific now. I refered to the Betz efficiency limit. This is common enough - and it CAN be accurately described as an efficiency measurement. I think you know exactly what the Betz limit is and how it comes about, and I think you know that I know what the Betz limit is and how it comes about.

    I initially concieved of this device independently. It wasn't until a short time later I learned that others had not only conceived of, but built and demonstrated such vehicles. I've described how it works MANY MANY times to MANY MANY people. I've changed a lot of minds about whether and how it works. I've done the analysis, and I think it's quite likely that I've come up with more valid analogies both mechanical and otherwise, than anyone else to describe how and why this works.

    But for some reason it seems I'm cornered into debating the meaning of "efficiency" or now "mechanical efficiency".

    If you think most people will understand and accept DDWFTTW once they understand and accept mechanical efficiency I've got some very very bad news for you. In fact I can point you to some professors of Aero, Physics, and M.E. that I assure you you won't be convincing with any analogy, analysis, demonstration, or even through the definition of mechanical efficiency. Let me know if you'd like their names so you can take a crack.

    EDIT:

    Rick, JB suggested that I took your last post the wrong way. This is quite possible. If so, please forgive my tone in this post.
     
  12. Guest625101138

    Guest625101138 Previous Member

    If you go back through this thread and a few others on the same subject you will see that I have done my share of convincing others here on the physics involved. I have to say with mixed success. Some get the DDWFTTW as soon as you point out the difference between a turbine and propeller and that the turbine is in the water and the propeller is in the air. Others simply do not understand basic physics and cannot grasp it.
    http://www.boatdesign.net/forums/pr...-how-many-out-there-they-viable-14182-11.html
    http://www.boatdesign.net/forums/projects-proposals/another-idea-1289-5.html

    You will see that I have been so frustrated with some skeptics that I realised they are simply too narrow minded and maths challenged that I gave up until JB relit the thread.

    In terms of putting the argument to professionals it is very important to use precise language. If you do not you will get shot down. Hence my nit picking over mechanical efficiency. Anyone who grasps the concept of mechanical efficiency can grasp how DDWFTTW works. If they claim to understand mechanical efficiency and do not grasp this concept then they are frauds. They are simply restating something they have been taught without grasping the fundamental.

    One of the confusing aspects concerning precision of language is the difference between a turbine and a propeller. (Windmaster made a point on this.)

    My concern about linking the Betz limit to efficiency in this thread is that others will view it as the limit of efficiency of a turbine. This is NOT the case. I get the impression you understand exactly what the Betz limit is but I doubt that many others reading the posts would distinguish between the coefficient of performance of a turbine and the efficiency of the turbine.

    In normal turbine applications mechanical efficiency has no meaning as the turbine is fixed to the ground but, in the case of a DDWFTTW boat, efficiency of the turbine is paramount. The coefficient of performance is unimportant and will be quite low as it is not consistent with high efficiency.

    At least we are debating fine detail and not the viability. I believe it important to be 'kinda specific'. The performance of any DDWFTTW boat will be highly dependent on the mechanical efficiencies of the prop and turbine.

    Rick W.
     
  13. spork

    spork Previous Member

    I have a strong background in aero and am pretty sure my limited language skills are not to blame for the physicists, M.E.'s, and Aero professors who refuse to believe even the physical evidence and basic analogies I've offered.

    It's pretty hard to believe that the people in question don't even understand mechanical efficiency, but they have astonished me, so I guess anything is possible. I personally don't think that an understanding of mechanical efficiency is necessarily sufficient to get your head around some of the subtleties of this simple toy.


    I think the question as to when this cart's prop acts as a prop vs. a turbine (e.g. upwind case) is indeed interesting. I think it's even more interesting when you consider when a sail on an ice-boat acts as a turbine vs. acting as a prop. But I don't consider that to be quite so much an issue of language precision. I do think at the lowest levels it becomes a definitional (or even philosophical) question as you can look at the sail on an ice-boat on a 45 degree downwind tack as a propeller, while the vessel as a whole can be viewed as a turbine.

    Now you have me more than a little confused as the propeller on the DDWFTTW vehicle never behaves as a turbine (wait a minute - I see you're talking about the boat. You must mean the turbine beneath the water).

    It's not yet clear to me that we're even debating fine detail. I think we're stuck on semantics. The viability ceased to be a question for me a few years ago when I sketched out the basic vector analysis for an ice-boat and came to the realization that it could achieve downwind VMG faster than the wind. You might be familiar with the videos JB and I made. I think they establish pretty firmly where we stand on viability.
     
  14. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 189, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    I have asked for it. Conservation of mass and conservation of energy, throughout whatever control volume you want, for an standing still cart in an steady wind as initial condition and that cart at speed grater than wind as final condition.
    Thanks.
     

  15. spork

    spork Previous Member


    I've asked you more than once to point me toward those proofs you have specific problems with. Why will you not tell me which ones you feel are failing?

    It seems I will be unable to convince you that DDWFTTW is possible. I think I'll let Rick Willoughby take this one.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.