sailquik said..This section from the document quoted above: "How sails work - north sails", seems to suggest that flattening and twisting the top of a sail, while lowering the CoE and reducing heeling moment, will also
increase induced drag, not decrease it as some of us have speculated.
Setting SailRecognize that the objective of the sails is to create force to pull the boat, but that there can also be a constraint on heel. At some point the stability of the boat or weight of the crew cannot keep the boat sailing at an angle that does not compromise performance, so just using the sails to produce the most force possible is not necessarily the fastest procedure.In lighter winds, when the sails are struggling to extract enough force from the wind to move the boat fast, the sails should be set such that every section along the height of the sail is working to produce high lift, especially the top sections in order to minimize induced drag. When the wind builds beyond a level that the sails' force causes the boat to heel too much, the sails' characteristics must be modified. There are several options.Reducing the amount of camber in the entire sail will decrease the amount of force the sail produces, as will decreasing the angle of attack of the entire sail. Implementing these adjustments over the entire sail may or may not be the best alternative for the windier conditions.They reduce the amount of force generated by the sail, but that force is still centered at a similar height. In order to reduce the heeling moment created by the sails to a satisfactory level, the amount of force may decrease to a level that does not pull the boat very fast anymore.Another approach is to reduce the lift produced by the top of the sail. Reducing the camber of the top of the sail, and/or reducing the angle of attack of the top of the sail through additional twist will affect the sail's force such that the remaining force is centered lower down. similar reduction in heeling moment as simply reducing the entire sail's force can be achieved through depowering the top of the sail, but while maintaining more total force to pull the boat. The force is centered lower as the bottom of the sail still trimmed in a fashion that generates substantial lift. This method has the compromise of deviating further from the desired elliptical spanload, as the lift distribution diminishes much more rapidly toward the top of the sail, and causes higher induced drag. The question becomes whether the remaining higher sail force offsets the additional drag component.A parallel situation occurs with airplanes. Airplanes are not designed to fly with the optimal spanload that yields minimum induced drag because the higher outboard load on the wing would require that the wing be made stronger, hence heavier, to carry that load. It is more efficient to build the airplane lighter and generate more lift on the inboard wing and accept a little more induced drag. This is the same tradeoff that a sailboat experiences in strong wind when heeling becomes a factor and results in a similar, less than optimal spanload in order to maximize performance. I was thinking will I post this link or not, because I knew that this old theory is inside text.I hope no one will read all text.
Sailquik you are now open new most complicated topic "ELIPTICAL VS BELL spanload distribution"
Author assumption is that eliptical spanload has minimum induce drag.
If you want have eliptical spanload ,head must produce lift, so this is his logic.
Problem is that eliptical spanload has minimum drag only if wingspan is limitation.
In other situation bell spanlod is more efficient.
It is very complicated topic and you must have good aerodynamic knowledge to understand this.
Nasa spend milions of dollars to research "new" bell spanload distribution in order to find minimum drag.
Al Bowers Nasa engineer is cheef of this expedition,everything start when he start watching birds wings with bell spanload which have milions of years of evolution.He try to solve problem with "adwerse yaw" without rudder,birds dont have rudder,then he find this phenomen,
Concept name is PRANDTL wing (Preliminary Research Aerodynamic Design To Lower Drag PRANDTL)
link:
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160003578.pdfbell spanload is also present at HECS wings, where test seagull wing cofiguration,
link:
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060045566.pdfAlbatros,seagull,swift,all migrations birds that stay in air long time have bell spanload.Rember swift fly 10months without land,they eat,mate,sleep in air, only land on breeding period plus milions years of evolution.
Look at his wingtips(primary feathers) no lift on it,if they produce lift feathers will be bent up.There is reason why nature design this.
our sails has bell spanload,especially with high downhaul trim,
page 32 ,figure 19link:
mauiultrafins.com/technology-2/windsurfing-mechanics/
Quite a nicely written article. Now I will have to do some research to find the research to back that up. 
If turbulence is directly coupled with wind shear, one would expect a corresponding decrease in wind shear over height in stronger winds over a relatively smooth surface.
