gybing center boards

Discussion in 'Multihulls' started by warwick, Jul 4, 2012.

  1. idkfa
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    idkfa Senior Member

  2. Silver Raven
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    Silver Raven Senior Member

    Gooday 'idkfa' - There is much conjecture about what you have said above.

    You have 'posted' a link where - again you have made some 'statements' rather than put forward an 'opinion' held by you.

    Your 'opinion' was challenged by Tom Speers.

    Along with - I am sure - many, many others in this 'forum' discussion - I would put to you that what you are saying is just not as 'correct' as you seem to 'state'

    Your statement - lift varries with angle of attack and is not much affected by thickness or shape - - is just not true.

    This just is not what the best of the best designers in the world are currently thinking & the very efficient maxi monohulls & multihull also do not reflect your 'opinion' ! !

    Please do much more research before attempting to confuse - some in this place that are genuinely trying to learn - for it is my humble belief that your a long way off the mark as far as the current - up to date design facts are showing the rest of us. Ciao, james
     
  3. Gary Baigent
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    Gary Baigent Senior Member

    During a broad reach, since the boat will be relatively flat on this point of sail, whether, as Paul has suggested, it would be an advantage to lift the leeward (lifting to windward) asymmetric board and have the windward (lift to leeward) board down. This is opposite to what you would do hard reaching or beating. There would probably be more ventilation of the windward foil but if the boat could be kept flat, there might be some gains (to leeward) being made. I'm sure this has probably been tried ... but haven't heard about it.
     
  4. warwick
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    warwick Senior Member

    Is there anything that can be taken from Paul Larsons Vesta sail rocket with its ventilating foils or is the speed difference to great to compare.
     
  5. RHough
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    RHough Retro Dude

    There are several things at work here that seem to have people confused.

    Symmetric sections have lower drag for the same cordwise thickness distribution at zero lift.

    At angles of attack where lift is produced the cambered section will have lower profile drag for the same lift.

    Producing lift also produces induced drag.

    Induced drag is many times greater than profile drag.

    When two foils are working to produce lift, changing the profile will have a very small effect.

    Changing the area will have greater effect.

    Changing velocity will have the greatest effect.

    At an angle of attack in the 2-3° range the difference in profile drag between a symmetric foil and a symmetric foil with a flap (trim tab) will be very small indeed.

    The reason that gybing boards are used is not to alter the angle the foil is working at. It is to change the angle that the hull moves through the water. No extra lift can be had to make the boat "point higher". It cannot happen. Lift to windward is always the same. It is equal to the drag to leeward due to windage and sail forces. The theory of gybing boards says it is possible that hull drag due to leeway angle can be reduced so that the foil/hull/rig combination has a better Lift/Drag ratio.

    The hull will "point" lower with a gybed board, but the course relative to the wind *might* be higher if drag reduction can be achieved.

    There is no way that a boat with a gybing board will appear to sail at a higher pointing angle. The physics of sailing will not allow it.

    In the range of speeds that sailboats operate in. Lift and Drag theory and formulas are exactly the same for sails in the air and foils in the water. The only difference is the density.
     
  6. Silver Raven
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    Silver Raven Senior Member

    Gary - maybe you "haven't heard about it" but you have 'seen it'. Go back to USA 17 & watch with great care all the sailing footage. You will see many times - that very 'thing' happening - that is - climbing up on the course - while pulling the bow(s) away. That's how they won the final race - after starting 100's & 100's meters behind - then finishing - way in front - after the very first leg - to windward. Look at the rudder angle & compare it to the course being sailed. If you look carefully - you'll see it. That 'helmsperson' kept - pulling the helm away & the boat kept accelerating up to & above the course 'lay-line'. Their 'course-made-good' - directly to windward was truely phenomenal & all the guru's were saying a lot about - just what was happening - at that time. Some of them - even knew what they were talking about.

    Part of the 'ultimate' answer is in - 'prue boat-speed' - the other part is in - bearing-away a tad - lower than 'course lay-line'- to be able to 'lift-up' to the course 'lay-line' but doing it - 'fully powered-up'.

    I'm sure as hell - not good at explaining it - but I can get on board & make it happen.

    Keep sailing & enjoying everybody. Ciao, james
     
  7. HASYB
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    HASYB Senior Member

    Very nice thread,
    We worked the phenomena downwind sailing the round and flat bottomed dutch boats/yachts/ships with the leeboards. It only worked in relative flat water and a small wind-window/course since the leeboards otherwise tends to shear of. Sometimes a wooden wedge brought relief. The wedges were also you used to change the angle of attack sailing upwind.
    I was a bit surprised when finding the daggerboard-case of the Newick 8.5 rectangular and thought of ways to "fixing" the board. Can't wait to experiment now when she goes back in the water hopefully this month.

    cheers,
     
  8. rapscallion
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    rapscallion Senior Member


    I wish you lived closer to me, I would love to take you sailing on my cat!.
     
  9. rapscallion
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    rapscallion Senior Member

    See attached. It's from the G32 owner's manual. the manual does not claim the boat will point higher due to the gybing boards, but will get the hull to pass through the water straight, not at the leeway angle. After rereading this I may be pointing too high, because I do have leeway upwind. I'm just a few degrees lower than the monohulls (B32, J80, ect.) but I have 8 to 9 knots of boatspeed upwind... When the boards are fixed my leeway is very noticible compard to the monohulls, about 10 degrees or more.
     

    Attached Files:

  10. smallyachtsailr
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    smallyachtsailr Junior Member

    So it seems that using a gybing board allows the hull to move straight through the water, reducing induced hull drag that's normally caused by crosswise flow over the hull. Presumably then, gybing boards might help more on a boat with fine bows or hard chines that sit down in the water (such as a W-17?)

    Having some knowledge of aero- and hydrodynamics, I agree with RHough's summary--it sounds correct from what I've learned. He's right about lift and drag theory being essentially the same on a sailboat for both the sails and the underwater hull and foils. Until you get up to mach 0.8-0.9, air acts pretty much as an incompressible fluid.
     
  11. DCockey
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    DCockey Senior Member

    Is the same definition of "pointing" being used by everyone in this thread? Three definitions of pointing I've seen used.
    1. Angle of the centerline of the boat / direction of the bow relative to the wind direction. This is the easiest to measure, just compare what a compass aligned with the centerline of the boat is reading compared to the wind direction, and seems to be the most common in general use. For most purposes it is also the least useful.

    2. Angle between the course of the boat / direction the boat is moving relative to the wind. The difference between this and 1 is the leeway angle of the boat. This is more useful for most purposes, and can be the most important if trying to squeeze by to windward of a buoy, pier, anchored boat, etc.

    3. Velocity made good to windward, VMG. How fast the boat is traveling in the direction of the wind. This is usually the most useful when going upwind. Optimum VMG may occur at larger angles than the minimum of 1 or 2.
     
  12. DCockey
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    DCockey Senior Member

    For the side force a board of the same depth but with more area and thus lower aspect ratio will operate at a lower leeway angle.

    Lots of experimental data that lifting surfaces of similar shapes behave the same in water as in air at low Mach numbers. The presence of a free surface can modify the characteristics of a lifting surface if the lifting surface is sufficiently close to the free surface.

    idkfa is correct. The slope of the linear part of the lift vs angle of attack curve for lifting surfaces of even relatively low aspect ratio lifting surfaces is insensitive to thickness ratio and profile shape as long as the thickness ratio is in the range (or less) typically used for centerboards, daggerboards, keels, rudders, wings, etc. Thickness ratio and profile shape do affect how large the linear range is and the maximum lift. I suspect much of the confusion about "lift" and thickness and profile is due to confusion between the lift a given angle of attack and overall maximum lift. Have a look at the numerous experimental curves in Theory of Wing Sections.

    The major effect of assymetry is to shift the lift vs minimum drag point. For the same thickness and thickness distribution an asymmetric foil may develop higher maximum lift. But a symmetric foil with a different section shape and/or thickness may develop higher maximum lift. Lift vs drag is a somewhat different story. For given lift a airfoil with the appropriate amount of assymetry will have lower drag than airfoils with the same thickness and thickness distribution with different amount of assymmetry or no assymmetry (symmetric). This is the reason most airplanes have assymetric airfoils. Have a look at the experimental curves in Theory of Wing Sections.
     
  13. brian eiland
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    brian eiland Senior Member

    Sometimes I just prefer a more 'pictorial representation' to the technical analysis in formal textbooks.....like this
    History of Airfoil Development
    http://adg.stanford.edu/aa241/airfoils/airfoilhistory.html

    "The earliest serious work on the development of airfoil sections began in the late 1800's. Although it was known that flat plates would produce lift when set at an angle of incidence, some suspected that shapes with curvature, that more closely resembled bird wings would produce more lift or do so more efficiently. H.F. Phillips patented a series of airfoil shapes in 1884 after testing them in one of the earliest wind tunnels in which "artificial currents of air (were) produced from induction by a steam jet in a wooden trunk or conduit." Octave Chanute writes in 1893, "...it seems very desirable that further scientific experiments be be made on concavo-convex surfaces of varying shapes, for it is not impossible that the difference between success and failure of a proposed flying machine will depend upon the sustaining effect between a plane surface and one properly curved to get a maximum of 'lift'."

    I seem to remember that the Wright Bros could never get their plane to fly until they got that wing of theirs to flex enough to give a good amount of camber,....lets see, isn't that an aysmmetric foil? ....that's providing the greater lift per wing area at the smaller angle of incidence. ...That's just the way my mind works.
     
  14. philSweet
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    philSweet Senior Member

    My comments inside <<these>>

    You've introduced a lot of red herrings with that post. I thought the thread was running along better than most.

    Sorry if my pedantry upsets you, SR.
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    Quoting Silver Raven:
    "Go back to USA 17 & watch with great care all the sailing footage. You will see many times - that very 'thing' happening - that is - climbing up on the course - while pulling the bow(s) away. That's how they won the final race - after starting 100's & 100's meters behind - then finishing - way in front - after the very first leg - to windward. Look at the rudder angle & compare it to the course being sailed. If you look carefully - you'll see it. That 'helmsperson' kept - pulling the helm away & the boat kept accelerating up to & above the course 'lay-line'. Their 'course-made-good' - directly to windward was truely phenomenal & all the guru's were saying a lot about - just what was happening - at that time. Some of them - even knew what they were talking about.
    Part of the 'ultimate' answer is in - 'pure boat-speed' - the other part is in - bearing-away a tad - lower than 'course lay-line'- to be able to 'lift-up' to the course 'lay-line' but doing it - 'fully powered-up'."

    Silver Raven James,
    I was talking about twin asymmetric boards in a monohull where the "wrong" windward one could be left down while sailing offwind at a broad angle - that "maybe" could crab you lower and get you to the bottom mark faster - and then again, it might ventilate if the mono heeled too much and would create more drag and slow your performance.
    How I saw Dogzilla sail downwind so much better than Alinghi, seemed to be that the advantage was made by very skilful wing trimming, powering up the wingplan so that the helmsman could initially head up, then bear away all the wile keeping the central hull flying - at the same time the wing was flattened with the acceleration - and more speed allowed a better scoop downhill. Whereas Alinghi was dragging windward hull, bearing up, lifting hull clear, running out of power and flopping the hull down again. The difference was the full wing was much superior to the wing mast/soft sail setup ... and little to do with the foils and nothing to do with the leeward foil being lifted and relying on the windward one to take you downhill - which would be impossible on the trimaran's configuration. Just imo, of course. Others here may have observed it differently.
     
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