Join Date: Feb 2000
Location: Annapolis, Md
Thanked 235 Times in 186 Posts
Rep Power: 10
This is piece that I wrote on Gradiant Wind and Twist. Its a bit wordy but it explains what you are experiencing.
The phenomina that you are describing is called "twist". Controling twist is a critical part of optomising performance. Some degree of twist in light to moderate winds is almost always a good thing.
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 a 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 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 'gradiant effect', in light to moderate winds the wind speed typically increases the higher you get above the surface of the water.
Visualize this 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 motavated by the ambient wind. Each higher layer moves a bit more quickly comapred 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 affect can be tens of feet deep. At very much higher wind speeds this whole gradiant effect, barrier to free flowing wind, is only a couple inches deep. We usually sail where the effect is somewhere in between but typically taller than the average mast height even in moderate winds.
In a sailboat, this means that the boat feels more true wind at the masthead than it does at the deck. Because of the way that apparent wind works, 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 through.
If you eliminated twist in light to moderate conditions, some of the sail will be over trimmed or some of the sail will be under trimmed for the conditions. In light to moderate winds the traveler should be brought to windward, and the vang eased, allowing more twist. On some classes the boom may actually need to be brought slightly to windward in these conditions in order to be able to maintain both flow and pointing ability. This is less of an airbrake than it might seems since boats generally have a greater leeway angle in light winds.
Of course as windspeeds increase, gradient wind effect decreases and so as wind increases in speed, twist should be reduced. This is done by lowering the traveller and increasing halyard, outhaul and sheet tension. In a really strong breeze the sail needs a comparatively flat camber and 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.
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 increases 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 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 jibs, the luff teletales should all be flying and all of the teletales hould 'break' evenly. On small jibs (with battens), leech teletales are very helpful with sail trim as well.
One of the problems with battenless sails is that it is much harder to control twist without developing leech flutter. That problem as much as the smaller sail area is what kills performance in in-mast furling sails in lighter conditions.