Theory and intuition of trimming sails with cunningham

Discussion in 'Laser Talk' started by Master Landlubber, Oct 15, 2008.

  1. Master Landlubber

    Master Landlubber New Member

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    Common understanding seems to be that pulling tension to cunningham moves the sail's draft forward. It also opens the leech, flattens the main and removes the wrinkles along the luff but they are not relevant in my question.

    Why does pulling cunningham make the sail cloth behave like this. Intuitively one could think the opposite, that the instead of moving the sail's draft forward pulling cunningham should move draft aft away from increased tension.

    Can anyone explain or maybe provide a link to an applicable sail shape theory.
     
  2. fracisco

    fracisco New Member

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  3. 49208

    49208 Tentmaker

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    Supplementing the above - when you tighten the luff beyond the relaxed state of the cloth, it starts to stretch - As the luff stretches, cloth from the middle of the sail is pulled forward by the stretching /additional tension.

    With the Laser cloth being what it is, the effect is more then what you would see in a firmer, lower stretch fabric
     
  4. Master Landlubber

    Master Landlubber New Member

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    Thanks for your replies fracisco and 49208 but they don't quite give me the insight I am looking for.

    fracisco: The descriptions in http://www.uhh.hawaii.edu/~sailing/U...unningham.html tell what pulling cunningham does and why to do it but not really the mechanics why pulling the cunningham leads to moving the sail's draft forward.

    49208: I understand what you tell about added luff tension - > stretching and moving cloth from the middle of the sail forward. However I still don't see how this should result in moving the sail's draft forward. I can easily think that it should result in opposite direction.

    I think it is obvious that adding tension reduces the draft. But the question begging for answer remains: what is the mechanism how cunningham tension moves the location of the maximum draft?

    I find it complicated to think how a triangle cloth behaves when one of the three sides is fixed to a straight line (mast) and all of the corners are fixed to points. It would be easier to understand the behavior if the cloth were square and all sides were free i.e. not fixed to straight lines and initially the tension in each side were equal. In this case the maximum draft would be in the middle of the cloth. Now if we add tension to one of the sides, the overall draft of the cloth would obviously be reduced and the side tensioned would become less curved than the other sides and so, I think, the draft would move AWAY from that side, not towards it. Am I thinking wrong already with my simplified square cloth model or does it behave differently to what a triangle sail cloth does, or is it the cutting of the shape and mast bending that makes the trick, or stretching of the cloth, or something else?
     
  5. Alistair426

    Alistair426 New Member

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  6. Master Landlubber

    Master Landlubber New Member

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    Alistair426, thanks for the link. That did provide one explanation.

    "... the Cunningham control tensions the front of the luff of the sail, making it closer to the tension of the leach, so making the leach relatively less tight and more prone to open. Another way of looking at it would be to say that the front of the sail has pinched some of the tension from the leach."
    I have an other one related to what 49208 told "cloth from the middle of the sail is pulled forward".

    I will explain. First, the sail cut and the resulting curvature to the cloth. On the first diagram on left we have the sail before inserting the mast. Think of the dashed line as a painted line on the sail. It is a straight line before inserting the mast but it gets curved by the wind when on mast and the cunningham is loose.

    Then, the impact of pulling cunningham, diagram 2. Let C a point in a horizontal section of the sail where that section meets the painted dashed line, D denote the point of maximum draft and L the point in luff where the point C will move after pulling cunningham tight. The move from C to L can be broken to two dimensions, forward to C' and beam to C''. If we observe the movement of the maximum draft point separately for C -> C' and C-> C'' we can see that for the C -> C' the maximum draft point moves to D' and direction is forward from D but for the C-> C'' maximum draft point moves to D'' and direction is aft from D. If we assume that the sail is relatively flat, not as curved as in my diagrams, then we can assume that the impact of the move C -> C' is bigger than the impact of the move C -> C'' which explains the original question why pulling tension to cunningham moves the sail's draft forward.

    Any other explanations or comments ?

     

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  7. LooserLu

    LooserLu LooserLu

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    I am not convinced about that. Think new about that movement of "D" to "D'' ". Begin with: "D" moves into the same direction like "C'' ".
     
  8. Nicko

    Nicko New Member

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    My understanding is that the sail has built in draft at a certain point in the sail.

    As wind pressure increases, the low pressure zone on the leeward side of sail, just aft of maximum draft, increases.
    This stretches the fabric and pulls the draft backwards towards the leech of the sail . The cunningham is tighten to prevent this happening.

    So it is not to bring the draft forward in a static situation, rather it is to prevent the draft moving backwards by prestretching the sail fabric , a bit like tightening a guitar string, the tighter it is the harder it is to pluck.
    Cheers!
    Nicko
     
  9. Master Landlubber

    Master Landlubber New Member

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    LooserLu: Maybe my description was incomplete. Don't think the point D in my diagram as a painted point in the sail. Instead, it is the location of maximum draft - think of the highest point in a hill. So I believe that my diagram shows correctly the location of the new draft maximum, or hill top if you wish, at D''.

    Nicko: I probably understand what you mean but that explanation can be challenged. Let's take into consideration that the wind force and cunningham force are in different directions and the conlcusion is opposite.
    With high wind pressure the order of cause and effect is : pressure -> force in perpendicular direction to sail -> tension -> stretch -> bigger draft in aft -> maximum draft moves aft. The more flexible the cloth the bigger the draft increase.

    Whereas in tightening the front part of the sail: force in parallel direction to sail (-> tension -> stretch) -> smaller draft in front -> maximum draft moves aft. Here tension and stretch are inside the parenthesis because they kind of resist the shape change. The more flexible the cloth the smaller the draft change.
    I think I made my previous reply to Alistair426 too early. After a second though I would say the Roostersailing article is good, it explains how cunningham tension impacts on the sail shape, but I don't see it providing any explanation HOW cunningham tension impacts as it does. As a matter of fact the paragraph I was quoting could be interpreted to imply that higher cunningham tension moves draft aft, which of course is not correct. I hope I'm not confusing you now. If you got confused, forget this paragraph - the Roostesailing article is good,but it does not give me an answer to my original question.

    Where are we now? I'm quite happy with the explanation I figured out this morning and described with the diagrams in my previous message. But I'm sure that I'm not the only person who has been wondering the question and I'm curious to read how other people have thought it.
     
  10. 49208

    49208 Tentmaker

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    Your diagrams are good, except you need to add one more, which would represent the sail on the Laser mast when it is bent (more or less to match the luff curve you have in the left most diagram).

    I can't think of any conditions where sailing upwind where the mast is straight - it's either bent to match the luff curve, or beyond (when you start to get the diagonal wrinkle from clew to mid luff)

    So, apply cunningham in your leftmost diagram, and you will be stretching the luff and pulling additional cloth from leech to luff. The effect is not so much a change in the amount of depth (represented by your curves) , but where the max depth occurs along the curve.

    This effect is quite different from mast bend, which adds/removes depth
     
  11. Master Landlubber

    Master Landlubber New Member

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    This is a very good point and I had missed it. I have to think more about the mechanism here, how the cunningham tension changes the sail shape in this case. Anyone with a good explanation?

    I don't quite see how stretching luff here pulls additional cloth from leech to luff nor how it impacts more the max depth location instead of amount of depth. Can you elaborate.
     
  12. peterl

    peterl New Member

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    Hi, I'm not sure if this really answers your question or not, but you can see this effect if you take a piece of cloth, say a small towel, and pull along one edge of the cloth by holding a corner in each hand. You will then see material get pulled in the perpendicular direction to form a "bag" along the edge you stretched.

    I read about this demonstration in some sailing book whose name escapes me at the moment. If I ever remember the title, I'll let you know.

    - Peter
     
  13. Master Landlubber

    Master Landlubber New Member

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    I followed 49208's advice and made one more diagram to show what happens to cross sections of a sail when Laser mast is bent I came to a conclusion that pulling tension to cunningham does NOT always move the sail's draft forward. This may sound a bit unorthodoxy but on the other hand my sketches fully support other sail shape changes that we want to achieve by pulling cunningham, namely opening the leech and flattening the sail.

    I draw the sail cross section changes at two hights, low and high. In the higher level the impact of pulling cunningham is bigger, dramatically bigger, due to several reasons. First, the twist boosts the impact of cunningham pulling in two ways, the slack that can be pulled is bigger and the direction of tension is more in parallel to the sail. Second, because the overall cross section length is shorter the relative leverage of cunningham is bigger (lenght of the cross section on the right of the dashed line when compared to the length of the cross section on the left side).
    At the higher level this results in very strong flattening of the sail and opening of the leech when cunningham is pulled. At the lower level the flattening is much smaller.
    At all hights the draft seems to move aft, not forward.

    Can anyone point bad assumptions or serious mistakes in my thinking?

    What about practice, can anyone confirm or deny is this how the sail really works? My own experiences on Laser sailing are limited and I can not really tell how the sail shape changes when the mast is bent and cunningham is pulled.
     

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  14. Merrily

    Merrily Administrator Staff Member

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    The details are too much for me before finishing my morning cup of tea. All I can say is that when you pull on cunningham, in practice, the draft moves forward.
     
  15. 49208

    49208 Tentmaker

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    Your last diagram may be a little overboard and while it's possible to pull the cunningham on so hard with so little mast bend that you get either a straight entry for the first couple of inches or a vertical fold of cloth in the sail, that is not a fast shape. I think with proper cunningham tension the direction of the arrow in location C is more towards the mast especially when we consider the pressurized shape (ie, what happens to the sail when the force of the wind is applied).


    I think the diagram I have attached may be helpful

    We haven't brought bias stretch of the cloth into the discussion either, but it's the bias stretch that allows the luff to be stretched so much, and the weave of the cloth is distorting under this load (and this distortion can be permanent, one reason why Laser sails wear out so fast in hvy air. If anyone is paying attention, you can add some life to your sail by making sure you free up the cunningham in between heavy air races and/or anytime you ease the vang)


    See if the following excerpts help you at all:
    http://books.google.com/books?id=ax...&hl=en&sa=X&oi=book_result&resnum=9&ct=result
    http://books.google.com/books?id=rR...=X&oi=book_result&resnum=1&ct=result#PPA51,M1
    http://books.google.com/books?id=D9...=X&oi=book_result&resnum=5&ct=result#PPA14,M1

    View the last video on this page:
    http://www.regattaactiveimages.com/2006/ClinicMaterial/VideoSailControls/tableofcontents.htm

    Lastly, on an unstayed mast like the Laser, the cunningham can flatten the sail if pulled on super hard, but cause of that is partly due because the extreme load on the luff of the sail is being transferred as compression on the mast, which causes the mast to bend more. More mast bend = flatter cross sections. You won't see this effect on stayed masts that have some sort of mast bend control (check stays, swept back shrouds etc)
     

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  16. Master Landlubber

    Master Landlubber New Member

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    Thanks 49208, good stuff.

    You are right. My diagram was intentionally a simplification.

    It is. Now I understand that my previous diagram was only telling part of the story. It had omitted the major fact that pulling cunningham not only directly pulls along the straight line from the cunningham grommet to mast head but it pulls the whole sail and when the mast head is fixed this pulling results in the whole sail moving forward.
    I am happy you told me this thing again while I didn't get it first time. Sometimes understanding requires repetition and support by a picture.

    This I don't understand. Could you explain more?

    Thanks also for the links. If not providing direct help to my case they did confirm some related thoughts I have had.

    If I have time and energy I may redraw my diagrams in coming days taking into account the everything I have learnt.
     
  17. 49208

    49208 Tentmaker

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    The standard dacron/nylon sailcloth weave has thread running in 2 axis (0 aka warp and 90 degrees aka fill) - In simple terms, one thread runs in the direction of the seams in a sail, the other runs at approx 90 degrees to the seam.

    When you pull on the cloth in either of those directions, the material does not elongate much (low stretch) because you are basically pulling directly against either the fill or warp threads

    However, if you pull on the cloth anywhere from 10-45 degrees off either axis (aka Bias), the cloth will elongate much more as there is no thread directly running in that direction. What happens is the actual weave distorts from a square and/or rectangle towards a parallelogram shape as the threads try to pick up the load.

    So, that's the theory - Now to apply to a Laser sail (a full rig sail, not a Radial) - if you look at the sail, the seams should be right around 90 degrees to the leech.

    How come ?

    Then if you look at the seams as they meet up with the luff tube, what do you see as far as the angle ?

    And if the pull of the cunningham is parallel to the luff tube, based on the theory above, what is happening to the cloth in the sail along the luff ?
     
  18. MasterMike

    MasterMike D22

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    SO much engineering...

    Here's an interesting and intuitive thing to try:
    Go out on a light wind day, enough for a slight heel. Sail on a beat with enough water to go in a nice straight line, relative to the wind, for some time. Adjust one sail control after another while looking up into the sail to watch it change shape (I layed in the cockpit and looked straight up the sail). The try combinations of controls. Adjust them to their max.

    This did a lot to explain to me all the reading about sail controls. I understand you want to know the physics of why it happens, but it is very interesting to just watch it happen.
     

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