Lifted from mysailing.com.au written by Dave Flynn of Quantum Sails.
Technically, twist is “the change in the angle of attack from the bottom of the sail to the top.”
Twist is necessitated by the changing speed of the wind, hence changing angle relative to the boat, as you move away from the water. The drag induced by the water slows the wind near the surface, shifting it relatively further forward by comparison with the faster flowing wind further aloft. This effect is exaggerated at lower wind speeds.
In the real world, it means that the leech of a sail must open up to some degree as we move from bottom to top.
In mechanical terms any time the aft end of the boom is allowed to rise, (easing the mainsheet or boomvang), twist is increased. The same length of fabric is now strung between two points that are closer together, so the leech of the sail opens up. Conversely, pull down on the clew and twist is reduced, closing off and rounding up the leech. A tight, round leech creates power and forces the boat to point, but can also cause airflow to stall, or overpower the boat (create too much helm and heel). A twisted leech profile promotes airflow in light air when it is hard to get air to stay attached, and in heavy air the flatter, more open sections depower the sail and help keep the boat on its feet.
On all boats, but particularly on the new breed of fast sailboats which often rely on bigger mainsails and smaller foretriangles, having the right amount of mainsail twist for the conditions is perhaps the single biggest key to upwind boatspeed. A competent mainsail trimmer can get you in the ballpark, but the true boatspeed virtuoso understands, feels, and can implement the changes on a moment-to-moment basis that make the difference. In a very real sense, they are driving the boat as much as the helmsperson. That’s why you often see them hunched over, (usually directly in line with the view of the helmsperson), as they ply their trade. They are looking at the same inputs to guide their sense of feel and dictate the appropriate reaction: angle of heel, jib telltales, boatspeed, waves, and wind angle.
Changing wind, changing mode.
In general terms, you can think of mainsail twist in three modes. In light air use extra twist and an open leech to promote attached flow and aid in acceleration. The top batten will be open, pointing three to ten degrees to leeward from where the boom is pointing, and the top telltale should flow aft. Sailshape in light air will be full, so it is important to keep the leech open and twisted to keep the sail from stalling. Position the boom on the centerline with the traveler, once twist is set, for maximum power and pointing.
In medium conditions the boat should be moving well, so leech tension can be increased and twist reduced. This will force pointing. Overall sail shape will be flatter, so there is less danger of stall, and if the boat is up to speed, it is okay to reduce twist to the point at which the top telltale stalls (disappears behind the leech). The traveler will drop so that the boom doesn’t get above centerline, and it will be lowered further to control heel as necessary. Using the traveler to control helm and heel in moderate conditions allows the trimmer to use twist to balance speed versus pointing.
In heavy air, control of heel is paramount. More twist will help keep the boat upright. The boat will typically have to sail at wider angles (foot) to have the power necessary to blast through waves and this will generate more heel. In smooth water, the helmsperson can “feather” more, or let the inside telltales lift in puffs. The overall sail shape will be as flat as possible which will also help induce twist and open the leech. The traveler, which is great for fine tuning balance in moderate conditions, usually does not provide enough gross change to handle big puffs, so twisting the entire sail with the mainsheet works best. I typically pull the traveler up a couple of feet above the leeward coaming and play the sheet to control heel. In windy conditions, use the boom vang to help augment the mainsheet.
On a moment-to-moment basis, the game is simple, the mainsail trimmer is constantly trying to reduce twist (trim harder), as long as the boat is up to speed and heel is under control. The goal is to point as much as speed and helm allow. Keep in mind the golden rule: speed first, and then try to point.
The thought process.
Target boatspeed is 7.2, twa 38. Out of the tack, mainsheet is eased at least until top telltale is flowing or until heel is under control. Pull the traveler up with your other hand as you ease the mainsheet if you need power. Speed has turned at 5.8 and is building. Sheet harder as the speed comes back up to 7.2, lowering the traveler as necessary with other hand. Up to speed, full trim. Hmmm..seems like this tack is a little more into the waves than port was, can’t sheet quite as hard. Getting slow, ease a half inch of sheet for more twist. Skipper is pressing for speed but there is a little too much heel, ease some more. Speed climbs rapidly, too rapidly… over target, sheet harder as helmsperson feathers up. Set of waves coming, let’s press and build speed over target, ease mainsheet, a little extra heel is okay but not too much. Through the waves, back hard on the wind, sheet harder. Good angle, good speed, no big waves, sheet harder still to make the driver work.
Oops, carried away, hard to build speed, ease a fraction. Big wave, ease a bunch over the top as driver bears off to avoid the slam. Stay eased until speed is coming back then gradually sheet harder. And the game continues…
Of course, don’t think you’ve got this totally wired yet. Not only is getting twist correct a dynamic, ever changing proposition, it’s a little different on every boat.
There is no one magic combination of twist versus boom position (traveler) that works for every boat. Each reacts a little differently.