The primary purpose of a traveler is in controling the twist in the mainsail. The further that the traveler car is moved to windward the more horizontal the pull and the more twist that is induced into the sail. The closer to being directly under the boom, the more vertical the pull and the greater the download on the sail and so there is greater tension in the leech of the sail and the less twist is experienced by the sail. Controlling twist is a critical part of optomising performance. Some degree of twist is beneficial, especially in light to moderate winds.



To begin with, if you look at a sail in section (cut horizontally through the sail) it is a wing. Even very efficient wings have an 'incident angle'; and 'slip angle'. In other words, a wing (or a sail) needs to be placed at an angle to the wind to work. For any given wind, and any given sail, and any given boat in a given condition, at any given spot on the sail, there is an optimum angle of attack in order to achieve the best performance. What I mean by 'best preformance' is an increase in forward drive while as well as minimizing of drag, and more importantly reducing the amount of sideforce which is responsible for creating heeling and leeway .



What further complicates all of this is that the wind at the top of the sail is actually different than the wind at the bottom of the sail. Called ''gradient effect'', in light to moderate winds the wind speed typically increases the higher you get above the surface of the water.



Visualize gradient effect this way, there is friction between air and water and between air and air. Because of this friction, at the surface of the water there is a (barrier) layer of air that does not move at all relative to the water. Next to this layer of air is another layer of air that moves slowly over this stationary barrier layer. That layer feels the friction of the barrier layer and the friction of the layer above it that is being motavated by the ambient wind. Each higher layer moves a bit more quickly compared to the layer below until at some point up in the air there is a point at which the air moves at the speed of the ambient winds and does not feel the affect of the barrier.



In light air, this gradient affect can be dozens of feet deep. At very much higher wind speeds the whole gradiant effect, from barrier to free flowing wind, is only a couple inches deep. Most of us typically sail in windspeeds where the effect is somewhere in between those extremes but typically taller than the average mast height even in a moderate wind.



In a sailboat, this means that the boat feels more true wind speed at the masthead than it does at the deck. Because of the way that apparent wind works, when close reaching or beating, the higher wind speed at the masthead produces higher apparent winds at the masthead that are also more abeam to the boat than the apparent winds that are felt lower in the sail. Getting back to your question, twist allows the sail to have differing attack angles as you move up the sail, each at a proper angle of attack relative to the apparent wind that it is passing over the sail at that point.



If you eliminated twist in light to moderate conditions, some of the sail will be over trimmed and/or some of the sail will be under trimmed for the conditions. Of course as windspeeds increase, gradient wind effect decreases. As a result as the wind increases in speed, twist should be reduced. This is done by lowering the traveller which should also be used in concert with reduction in camber (depth of the curve in the sail) achieved by increasing halyard, outhaul and sheet tension. In a really strong breeze the sail needs a comparatively flat camber and flaat angle of attack and so the sail should be bladed out. This means maximum halyard tension, outhaul tension, backstay tension, mainsheet tension. To further reduce the angle of attack the traveller is dropped as well. This will decrease weather helm and heeling.



Just for the record, jibs have twist as well. Twist in jibs is controlled by the jib sheet lead angles. Moving the jibsheet car aft tightens the lower sail and increases twist in the sail, moving the track forward pulls down on the leech and so decreases twist. On jibs you increase twist in really light air to open the slot and in really heavy air to reduce heeling.



The clues to the proper amount of twist comes from the teltales. On mainsails the leech teletales at the battens provide the best information. All of the teletales up the leech should be flying when the sail is set properly. When there is inadequate twist the teletale at the head will be stalled and sucked back into the sail. On some if not most mainsails, in moderate winds, some intermitant stalling of the upper teletale is the fastest way upwind.



On jibs, the luff teletales should all be flying and all of the teletales should ''break'' evenly. On small jibs (with battens), leech teletales are very helpful with sail trim as well.



One of the problems with battenless mainsails is that it is much harder to control twist without developing leech flutter. That problem, almost as much as the smaller sail area, is what kills performance in in-mast furling sails in lighter conditions. Sails with sunguards also can have problems with leech flutter and so often require greater leechline tension, which produces a hook in the leech of the sail and some loss in performance as well.



Regards,

Jeff