Add twist to your main sail for more power and speed in light air
NOTE – April 3, 2018
This Post is in development, needing fine editing and improvement of its images. It is published because I thought it was important for us all to have the opportunity to know about the subject of sail twist before the next outing at the pond.
Pima Micro Yacht Club
This post contains three items found on the Internet about main sail twist. Also, clicking the image above takes you to a document developed by Brian Chapman, a sailmaker that does of good job of graphically describing sail trim.
ITEM 1 – UNDERSTANDING MAIN SAIL TWIST
Author: Quantum Sails, April 2017 – this article appeared on the Sailing World website: https://www.sailingworld.com/understanding-mainsail-twist
Mainsail twist can have a large impact on speed and performance. Quantum Sails explains what mainsail twist is and how to use it to get ahead on the racecourse.
Technically, twist is the change in the angle of attack (of the sail surface relative to the wind direction) from the bottom of the sail to the top. Twist is necessitated by the wind speed changes, hence changing angle relative to the boat as you move away from the water. The drag induced by the water slows wind near the surface and shifts it relatively further forward, as opposed to the faster-flowing wind further aloft. This effect is exaggerated at lower wind speeds.
In the real world, twist means the leech of a sail must open up to some degree as we move from bottom to top.
Any time the distance between the clew and the head is shortened (easing the mainsheet or boom vang), twist increases. The same length of fabric is now strung between two points that are closer together, so the leech of the sail opens up. Conversely, pulling down on the clew reduces twist, closing off and rounding up the leech. A tight, round leech creates power and forces the boat to point, but it can also cause airflow to stall or overpower the boat (creating too much helm and heel). A twisted leech profile promotes airflow in light air when it’s hard to get air to stay attached. In heavy air, the flatter, more-open sections depower the sail and help keep the boat on its feet.
Having the right amount of mainsail twist is perhaps the single biggest key to upwind boat speed, especially on the new breed of fast sailboats that often relies on bigger mainsails and smaller foretriangles. A competent mainsail trimmer can get you in the ballpark, but the true boat-speed virtuoso understands, feels, and implements changes on a moment-to-moment basis, which makes the real difference.
In a very real sense, mainsail trimmers are driving the boat as much as the helmsperson. That’s why you often see trimmers hunched over, (usually directly in line with the view of the helmsperson!), as they ply their trade. They’re looking at the same inputs to guide their sense of feel to dictate the appropriate reaction: angle of heel, jib telltales, boat speed, waves, and wind angle.
In general terms, you can think of mainsail twist in three modes: light, medium, and heavy air.
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 3-10 degrees to leeward from where the boom is pointing, and the top telltale should flow aft. Sail shape in light air will be full, so it’s important to keep the leech open and twisted to keep the sail from stalling. Once twist is set, position the boom on the centerline with the traveler 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. If the boat is up to speed, it’s okay to reduce twist to the point at which the top telltale stalls (disappears behind the leech). The traveler will drop so 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 that 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 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.
Playing the Game
On a moment-by-moment basis, the game is simple. The mainsail trimmer constantly tries to reduce twist (trimming harder) as long as the boat speed is up and heel is under control. The goal is to point as long as speed and helm allow. Keep in mind the golden rule: speed first, then try to point. Hypothetically, here’s how the thought process would work on board for a target boat speed of 7.2, true wind angle 38 degrees:
Out of the tack, ease the mainsheet at least until the top telltale flows, or until heel is under control. Pull the traveler up with one hand as you ease the mainsheet (if you need power).
Suppose speed turns at 5.8 and is building. Sheet harder as the speed comes back up to 7.2, lowering the traveler as necessary. Once you’re up to speed, full trim. If the tack is more into the waves than port was, don’t sheet as hard. If slowing, ease a half-inch of sheet for more twist. If the skipper presses for speed but there’s too much heel, ease some more. If the speed climbs too rapidly over target, sheet harder as the helmsperson feathers up.
Suppose a set of waves comes. Press and build speed over target and ease the mainsheet. Extra heel is okay, but not too much. Through the waves, back hard on the wind and sheet harder. When you reach a good angle, good speed, and no big waves, sheet harder still to make the driver work. If you get carried away, ease a fraction. For a big wave, ease a bunch over the top as the driver bears off to avoid the slam. Stay eased until speed comes back, then gradually sheet harder. And the game continues…
Getting the correct twist is a dynamic, ever-changing proposition, and it’s a little different on every boat. There is no one magic combination of twist versus traveler that works for every boat, so be prepared to re-educate yourself on each boat and don’t be afraid to ask for help!
=========== End of Item ==========
ITEM 2 – I KNOW SAIL TWIST IS IMPORTANT, BUT CAN YOU HELP ME UNDERSTAND WHY?
Author: Discussion on Quantum Sails website; question answered by David Flynn of Quantum Sails (quantumsails.com), November 2017.
– Danielle P.
You are definitely not the only sailor who has asked this question! Here’s the lowdown on twist and why you need it. Twist is the change in the angle of attack from the bottom of the sail to the top of the sail and is caused by a change in wind speed, which changes angle relative to the boat the farther away you are from the surface of the water. The drag from the water slows the wind near the surface, shifting it further forward in comparison to the faster flowing wind further aloft. This effect is exaggerated at lower wind speeds. In practice, it means that the leech of a sail must open up to some degree as you look from bottom to top.
Any time the distance between the clew and the head is shortened (easing the mainsheet or boom vang), 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, which closes off and rounds up the leech. A tight, round leech creates power and forces the boat to point, but it can also cause airflow to stall or overpower the boat (create too much helm and heel). In light air, when it is hard to get air to create lift, a twisted leech profile promotes airflow. In heavy air, flatter and more open sections depower the sail and help to keep the boat on its feet.
Mastering the boom vang is an important sail control when it comes to getting the right twist for the right conditions. Click here to read another sailor’s question about the vang.
December 6, 2017. Twist on the mainsail also controlled by the traveller, and some jibs use a barber haul line to control twist I believe the term relates to the “twist” in the shape of the airfoil leading-edge of the sail(s), when there is “twist” in the sail the center of effort of the leading edge airfoil is farther aft than on the lower portions of the sail, hence the airfoil distribution is not uniform along a vertical type imaginary line – i.e. the airfoil is on a closer angle of attack to the wind at the bottom of the sail than at the top. Some twist is desirable in light to medium air as it can tend to give the sail a little belly to generate a more powerful airfoil. Too much belly and you lose windward ability however. Use of the cunningham hole on the mainsail can reduce the belly of the sail particularly in the sail’s lower half. Tightening up or loosening the outhaul also has an effect. The sea state also must be taken into consideration because if there is a sea or chop, the boat will need a little more power to push through and not hobby-horse so you arrange the sail adjustments to allow a slightly lesser angle of windward attack when the water is not flat. The boat’s tolerance for sailing with varying degrees of heel also enters into the equation, because some boats do best when sailed more or less flat, others do fine at greater angles of heel.
Also, if you vang down the main boom hard – be aware it will be lower to the deck as you tack, so try not to get clonked on the head if you do get knocked unconscious by a vanged-down boom, put your life jacket on immediately so that you do not float face down in the water when you slide off the deck.
=========== End of Item ==========
ITEM 3 – 3 REASONS TO TWIST YOUR SAIL OUT
NauticEd is the World’s Most Advanced Sailing Education and Sailing Certification Program.
PERFECTING SAIL TRIM AND TWIST
Here are three reasons you should be twisting out the sail towards the top of the mast
- Get rid of excessive heeling
- Match the wind at the top of the sail to the wind gradient
- Drum Roll … The most important … Change heeling force into forward force.
(1) Getting rid of excessive healing forces.
I’ve written on this topic a few times mostly because it is an important fundamental topic of understanding the forces on a sail.
In a right triangle, a force applied evenly over the surface can be considered to act in one place. This place is called the Center of Pressure and is the geometric center of the right triangle and is 1/3rd of the way up the triangle. It is found by crisscrossing the corners and midpoints.
The propensity to heel is called the Heeling Moment and it is derived from a multiplication of the wind force magnitude and the height of the Center of Pressure.
Thus, the heeling can be reduced by lowering the Center of Pressure. You can do this obviously by reefing but also by twisting out the top of the sail which changes the triangle shape.
Twisted mainsail lowers and moves the center of pressure forward
This also has the added effect of moving the Center of Pressure forward which reduces your weather helm.
(2) Match the wind at the top of the mast to the wind gradient
Wind velocity at the surface is less than wind velocity at the top of the mast due to friction of the surface on the wind. This is called Wind Velocity Gradient. In addition there is another effect called Wind Shear which is due to coriolus effect dependent on the distance from the equator and if northern or southern hemisphere. This wind shear creates a different direction of wind at the top of the mast than at the boom height because the wind is seen to twist as it slows down.
When you combine Wind Velocity Gradient and boat velocity you also get different apparent wind directions on the sail. This is best described in detail in our free basic sail trim course.
Because of this effect the wind at the top of the mast is more from an aft direction. Said to be “more aft”.
Wind Velocity is different in speed and direction between surface and top of mast.
When flying a sail then you already know to match the sail angle to the wind to make it most efficient. If the wind direction at the top of the mast is more aft the sail direction must change to be more let out moving up the mast. To achieve this you twist the sail out by allowing the aft of the boom to rise up. This loosens the leach of the sail and allows it to twist out at the top.
Adjust Sail angles up the mast to match apparent wind direction
Control your leach tension via the boomvang. Keep in mind also that your mainsheet will also control leach tension as well. If your mainsheet is in tight, loosening the boomvang will have no effect. When you let out the mainsheet this will deliver the leach control to the boomvang. You can then use the traveler to re-center the boom.
(3) Changing heeling force into forward force
This is not talked about much but it is the most important when thinking about making your boat go fast.
The force acting on the sail from the wind can be thought of as being approximately in the direction perpendicular to the battens. As the sail is twisted out in going up the mast the force then shifts from sideways unwanted heeling force to desired forward driving force by the nature of its direction.
This then is very important. As you know, not only should you always have your sails let out as much as possible just before luffing to fly the sails efficiently but you should additionally be looking up the sail and adjusting twist as much as possible to translate the resultant force to be forward acting rather that sideways acting. Increase twist until upper sail luffing occurs then tighten slightly.
As with point 2 above use the boomvang, the mainsheet and traveler as your controls.
=========== End of Item ==========