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How Does Sail Twist Work?

We get a lot of great questions through our Ask the Expert portal and sometimes they make for great articles. Quantum's David Flynn explains how sail twist works and why you need it in answer to Danielle P.'s great question.


I know sail twist is important, but can you help me understand why?

- 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.

 

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The Discussion

Matt Lechner

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.

Matt Lechner

December 6, 2017

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