Aerodynamics of the slot

Discussion in 'Sailboats' started by chabrenas, Feb 4, 2009.

  1. chabrenas
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    chabrenas Mike K-H

    Can anyone point me to good modern articles or professional papers addressing the aerodynamics of the headsail slot?

    Recently, I came across one (mid '70s, Texas, I think) by a yachtsman & professional aerodynamicist which refuted the classical explanation that the jib accelerated the flow across the back of the main. He claimed almost the opposite: a headsail decreases the flow through the gap between forestay and main, but the lift and stalling point of the headsail are modified, allowing the boat to point higher.
     
  2. PI Design
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    PI Design Senior Member

    You are talking about Arvel Gentry, I think. His work is seminal and totally correct - you won't find a better source. Circulation theory backs up his claim, which he also proved by doing full scale tests on his boat.
     
  3. daiquiri
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    daiquiri Engineering and Design

  4. Steam Flyer
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    Steam Flyer Junior Member

    Yep, Arvel Gentry and C.J. Marchaj are the two best sources I know of.

    When you think about the "classic explanation" of the jib accelerating air across the suction side of the mainsail, you know in a moment it -must- be wrong... exactly backwards, in fact.

    If you look at simple physics of it: the high-pressure (slower flow) of the jib is mated to the lower-pressure (faster flow) side of the main. The jib isn't "accelerating" anything with regard to the main.

    OTOH the sails are operating in each other's upwash & downwash; plus the combined air flow off the jib into the mainsail tends to keep attached flow thru a wider range of conditions.

    Nontheless... it has often seemed that if the jib+main combination was more efficient than a cat or una rig of the same square area, then all the development classes wouldn't be shrinking the jib at every opportunity to throw more sail area into the mainsail. How many iceboats have jibs? How many C-Class cats?

    FB- Doug
     
  5. chabrenas
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    chabrenas Mike K-H

    Thank you, everyone. Yes, it was Arvel Gentry's work that I saw. Very neatly done.

    I had never seen the WB Sails simulations - that was the kind of thing I was looking for. Excellent.

    I was lucky enough to have been at Southampton University when Tony Marchaj started his postgrad research (using an old 6-metre and a small wind tunnel) and chatted with him two or three times. In those days, computer programs were submitted on punched paper tape...

    Where are the main sailboat aero- and hydrodynamics research centres today? The Wolfson Unit appears to have evolved from the seed planted by Marchaj's work, and I guess the big sailmakers do a lot of computer modelling.
     
  6. daiquiri
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    daiquiri Engineering and Design

    It looks to me like their screen captions have been created with Cosmos Floworks. Nice and simple general-purpose CFD software, delivered as a plugin for Solidworks. It is very easy to use, even if you are not an expert in aerodynamics. Of course, it takes a minimum of knowledge of fluid dynamics to interpret the results.
    If you can get this software you might try experimenting different sail configurations by yourself. ;)
     
  7. chabrenas
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    chabrenas Mike K-H

    Thanks, Daiquiri. Maybe if I speak nicely to Bernard Charlés he can let me have a copy....
     
  8. daiquiri
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    daiquiri Engineering and Design

    Bernard Charles Ecclestone or Bernard Charles of Dassault Systems?
    The first one will probably send you the invoice based on number of words you will pronounce. :D ;)
     
  9. chabrenas
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    chabrenas Mike K-H

    Bernard Charlés of Dassault Systèmes
     
  10. gggGuest
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    gggGuest ...

    To the best of my knowledge all the development classes are *not* shrinking the jib. The extreme performance of the Ice boat and thus the angle at which it must meet the apparent wind makes it a special case, and offwind the Cogito C Class rig has two slots. The unrestricted skiff classes have never gone for single sails, and although they have recently been permitted in the International Canoes, the impression is at the moment two sails seem to be working better.

    There's quite a bit on this, including the relative sizes of sail which have been found to be best in the 18ft Skiffs, in Bethwaite's new book "High*er* Performance Sailing.

    I suspect the pros/cons are mainly related to the angle of incidence of the sails, which means that the size of the boat, efficiency of foils and the speed which it reaches relative to wind strength would be factors.
     
    Last edited: Feb 10, 2009
  11. sailor2
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    sailor2 Senior Member

    Well, it all comes down how you define the word efficiency in case of sails.
    Most would use L/D in which case a single wing element without slots always come on top. If you mean max drive for a given area, important for c-cats at least downwind, that's more like effectiveness than efficiency, defining it the opposite way is just bad semantics.

    So if the above is about effectiveness instead of efficiency, I have no reason to disagree. But the post you replyed to mentioned efficiency.
     
  12. Tcubed
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    Tcubed Boat Designer

    Slots , apart from breaking up sail plan into small enough to be manageable areas, serve the function of increasing the angle of stall and hence the Cl max, at the expensee of max L/D ratio.

    Hence the predomination of una rigged ultra high performance sail vehicles, which are always sailing at high V/W ratios ie small angles of attack and at vector angles where the most important faactor is L/D.

    In other boats obtaining high Cl max can be more pertinent.
     
  13. Steam Flyer
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    Steam Flyer Junior Member

    Yeah, what he said ;)

    I was trying to figure out how to explain what I undersand of the situation, "Tcubed"s post said it way better and shorter.

    The only thing I can add is that in light air, -all- boats are sailing at high ratio of boatspeed to windspeed, but low-aspect ratio sails seem to generate more power.

    FB- Doug
     
  14. Tcubed
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    Tcubed Boat Designer

    Yes that is a crucial and often neglected obs. that all boats , even high disp monohulls if well designed and sailed are at high V/W at low W. The reason though at that point for low AR is purely from the practicality of trying to get as much thrust as possible from the existing spars.

    Therefore telescoping spars are very advantageous.

    Look at , for example traditional topsail rigging arrangements for Essex smacks, Bawleys, and Bristol channel pilot cutters. I am not talking aabout modern imitations of the types, but rather the original housing topmast configurations.

    Dixon Kemp's "manual of sail and boat handling" is an invaluable tome on this, and gives no doubt as to the level of sailing understanding of two centuries ago.

    Also John Leather's books are excellent .
     

  15. chabrenas
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    chabrenas Mike K-H

    Good point, Tcubed. And, of course, old gaff-rigged boats would be glad to be rid of both the windage and the weight aloft when not setting topsails & topmast staysails. Solid wooden spars are heavy.

    However, a modern rig intended for windward work in light airs should be tall on two counts:

    - because of the wind velocity gradient
    - because of lower tip vortex losses on a high aspect ratio airfoil

    Thames raters and US scows were good examples of designs optimised for sailing in light airs.
     
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