Was Marchaj having us on?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Apr 12, 2021.

?

Did Marchaj know he was wrong when he claimed, on P199 in my post #63, that "A arrives ...before B".

  1. Yes, and therefore he was "having us on".

    100.0%
  2. No, he didn't understand that the air flows faster over the upper surface.

    0 vote(s)
    0.0%
  3. He was right, air flows travels over the respective surfaces at equal speed.

    0 vote(s)
    0.0%
  4. He confused A with B. (The pic shows B arriving at the TE before A!)

    0 vote(s)
    0.0%
Multiple votes are allowed.
  1. Sailor Al
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    Sailor Al Senior Member

    Actually that is precisely not within the human experience!
    It was an abstract thought experiment by a genius worried by a paradox that could never actually be experienced because it could only be actually observed if the train was travelling at a speed close to the speed of light.
    And that is not within the human experience.
     
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  2. Will Gilmore
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    Will Gilmore Senior Member

    You are right. His theories are not under attack. Only a hypothesis about the beginning of the Universe that was based upon his theories.

    I apologize for the misleading words.
     
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  3. messabout
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    messabout Senior Member

    I have a book titled :Yacht Racing, The Aerodynamics of Sails. Author Manfred Curry delved deeply into whatever it is that makes sails do what sails do. The original book was published in 1928. Curry explored aerodynamics at length. His book has some pretty good graphs and charts. He was influential enough to have the privilege of using the Messerschmitt wind tunnel for some of his experiments. Not all of his work has been validated but it was "in the ballpark" so to speak.

    Curry was doing his best to share useful information with other yachtsmen. He may not have been absolutely correct, but he was certainly not Having us on. Neither was Marchaj.
     
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  4. Sailor Al
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    Sailor Al Senior Member

    Does he have an explanation for the low pressure to leeward and high pressure to windward please? Does he use Marchaj's approach?
    Could you share it with us?
     
  5. Sailor Al
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    Sailor Al Senior Member

    Can you scan and post the pages describing the source of the aerodynamic force please?
     
  6. Sailor Al
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    Sailor Al Senior Member

    On my SVT thread, you have suggested that I have been messing about for my own amusement, or am a Flat Earther.
    Either way it's a bit harsh to ignore the third possibility.
    You offered Curry's book as a useful source, and despite two requests, you have declined to support that with any evidence. I don't have a copy and it's a bit of a hassle to obtain one.
    Could I request, once again, that you reproduce his explanation for the source of the pressure differences around a sail that generates the thrust and leeway/heel forces on a yacht?
     
  7. Boat Design Net Moderator
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  8. Sailor Al
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    Sailor Al Senior Member

    Thank you. I have since sourced a Kindle copy for Amazon.
    On the subject of copyright, I have been able to download many books from vdoc.com (but not this one, I had to pay for it). I am sure that many are still in copyright. How does that work?
     
  9. Sailor Al
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    Sailor Al Senior Member

    I have spent a very pleasant few hours dipping into this well written, thoughtful and informative book. Whilst my primary objective was to find his explanation for the source of the aerodynamic force, I was seriously distracted by Part II, Racing Tactics, which takes up the second half of the book. I will have to return to study that section in more detail.

    With a Kindle version, I was able to perform word searches and was delighted to see that despite its title "The Aerodynamics of Sails...", he made no reference to circulation, vortices, viscosity, Bernoulli, boundary layers - no-slip or otherwise, Navier-Stokes, or Kutta - Jukowski. He makes but a single reference to Prandtl, but only in the context of his wind-tunnel experiments.
    Of the three mentions of "drag" only one related to air (P121) and then it was used in describing eddies of air being dragged.
    His use of "lift" in the modern aero- or hydrodynamic sense is extremely limited and is applied in the context of wings although, on p. 182 he does suggest "a comparison of the yacht sail and the wing of an aeroplane is surely justified."

    However, returning to my initial search, I arrived at pp. 27-28 where :
    "Drawings III below disclose the reason for the different forces developed by the plane and the arched surface in a current of air." (My emphasis.)

    The drawing III which followed is explained with:
    "It is different in the case of arched surfaces; here the lines of flow are not torn but bent; this is equivalent to a gain of energy: a downward acceleration is imparted to the surface by the air, that is, the surface itself is lifted or forced upward to a greater degree. "(Again, my emphasis.)

    There is no explanation for how this downward acceleration contributes to the pressure differences and indeed appears to be the widely discounted "momentum theory" of lift.

    On further reading I alighted upon p. 110 where he states:

    "We know that the suction developed on the leeward side of the sail is to be attributed to the partial vacuum that actually exists there. The sail is pulled or sucked into this space, while the various currents of air in the neighborhood are naturally drawn into it from all sides and tend to fill it and thus to reduce the negative pressure."

    In a way this statement is circular: the suction is to be attributed to a partial vacuum. Isn't that just a restatement of the effect of a partial vacuum? Isn't its tendency to create suction the very essence of a partial vacuum?

    He assumes our familiarity with this by opening the sentence with "We know that the suction…". I have to wonder whether this was a deliberate attempt to "paper over" this knowledge gap.

    So my conviction that no sailing books actually explain the source of the pressure difference is further bolstered.
     
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  10. wet feet
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    wet feet Senior Member

    I do occasionally dip in to a couple of aviation related forums and they also have differences of opinion about what actually is going on.I think it is important to keep in mind that when Manfred Curry was writing,practical aviating had only been taking place for a little over twenty years.Marchaj had several times that amount of research available to him and in the intervening years,even more has been accumulated.What we do know is that sails work!Have you read Pierre Gutelle,Fabio Fossati and Frank Bethwaite?There is plenty to consider in their work,even if Fabio Fossati's book was severely damaged by a poor translation into English.
     
  11. Will Gilmore
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    Will Gilmore Senior Member

    A very nicely documented and written summation. I think, the assumption he is making with the statement, "We know..." assumes that the reader already attributes the creation of that vacuum to the medium moving in a direction across and against the surface of the sail. There is a windward side and a leeward side. The eddy effect is very simple to understand, even for a non-aerodynamicist like me.

    Great post.

    -Will
     
  12. Sailor Al
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    Sailor Al Senior Member

    Welcome to the thread.
    Yes, but I'm reading his 1945 edition. And I'm still looking for the benefits of the intervening 70 years' research into explaining how sails work! Marchaj certainly had no idea! (See my earlier posts on this thread.)
    Yes, and the principles have been implemented by soaring birds for at least 40 million years. Which is why I'm pretty sure we don't need a 3 year degree in aerodynamics to understand that principle!
    No, Yes and No.
    I am unable to access a digital copy of Gutelle (The design of Sailing Yachts) or Bethwaite (Frank Bethwaite: High Performance Sailing). Could I trouble you to delve into your copies and report on their explanations for the source of the pressure differences around a sail that generates the thrust and leeway/heel force that drives the yacht?
    (Fossati's explanation doesn't fail as a result of poor translation.)
     
    Last edited: Dec 23, 2022
  13. gonzo
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    gonzo Senior Member

    wet feet said:
    What we do know is that sails work!
    Your arrogance keeps on being beyond believable. Do you think the blood circulating through your body qualifies you as a surgeon?
     
  14. Sailor Al
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    Sailor Al Senior Member

    @wet feet
    I have been able to access Gutelle's book, but am still unable to get Bethwaite's.
    Gutelle's explanation for the lift is fascinating!
    After explaining that while air is compressible, this property is not of interest at subsonic speeds and expounding Prandtl's no-slip boundary layer and circulation theory with reference to Kutter and Zhukovsky, on p. 44 he reveals all with the surprise declaration:
    "Flow is therefore no longer symmetrical, and the reduction in speed on the front face causes pressure to rise, whereas pressure drops at the back where speed is increased. The difference in pressure between the two faces of the body gives rise to a force that acts perpendicularly to the direction of flow, namely lift..."
    • No attempt is made to explain how the reduction and increase in speed causes the pressure rise and drop. (Presumably he's invoking Bernoulli, but that is not declared.)
    • Nor is there any serious attempt to explain, beyond an airy "Flow is therefore no longer symmetrical" why the flow should be faster at the back face than the front.
    So the simple sailor remains perplexed about the source of the pressure difference that gives rise to the thrust and leeway/heel force from the wind on the sails.
     

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  15. Sailor Al
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    Sailor Al Senior Member

    Maybe time to remind you of the Forum Rules?
    "Please be polite to all members. Please respect those who take their time to give advice freely...."
     
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