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How Does Sail Twist Work?

November 7, 2017

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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