Why sail twist matters:
- Thread starter Brian S
- Start date
Here's the main reason the sailmaker builds twist into the sail. The wind speed you feel on your neck at deck level is not the wind speed at the top of a 50' mast. The wind speed at the top of the mast is 50% to 60% stronger. If twist wasn't built into the sail you wouldn't be able to control the sail. The boom vang allows you to vary the amount of twist.
This shows wind SHEAR, or wind direction varying by altitude. It is very common on warm days over very cold water.... like Lake Superior. We often have 45 degrees of shear for entire legs of a race. To help deal with this we have tell-tails every 10 feet on our vertical and cap shrouds.Here is an image of a display at the Exploratorium in San Francisco showing how the wind direction varies at different heights above ground. This shows how important proper main sail twist is to sailing effectively.
And you are correct, you generally induce huge twist to match it; or failing that you allow a section of the sail (top or bottom) to twist totally off (almost luffing) and you sail with whats left.
The primary sail trim control for mainsail twist is the boom vang -- if you have one. The secondary twist control, if you don't have a boom vang, is the mainsheet.
That's pretty much it. Upwind in moderate breeze most performance minded sailors will turn the vang off, because they want full control of twist with the mainsheet and don't want the vang stopping desired lift (twist).
Off the wind or upwind in strong breeze, the vang comes on to limit twist when the boom is let out to widen the AOA. Or pop it open if you need to dump pressure in a hurry!
Maybe I should buy your book. Oh wait. I already have it. Maybe I should read it... again!
Must disagree.. I rarely use vang for upwind sailing, unless it's to support boom (rigid vang) in light air. But hey, I grew up on beach cats... where the Cunningham and mainsheet are most used controls and the boom never goes outside the range of the traveler.
Here's a well known sail trim video put out by North Sails that may help new and old salts alike. The first section is about the main sail trim.... https://www.youtube.com/watch?v=nwuaCy_sHD0
Here's a well known sail trim video put out by North Sails that may help new and old salts alike. The first section is about the main sail trim.... https://www.youtube.com/watch?v=nwuaCy_sHD0