..... but for the C-> C'' maximum draft point moves to D'' and direction is aft from D. ..
Any other explanations or comments ?
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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
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.
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.
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
SO much engineering...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 ...