The setup shown uses end-boom sheeting with a fixed traveler. The photo nicely shows the way the traveler results in more sideways pull than center-boom sheeting would.
- Sheet with traveler and vang
- SheetAndVang_800.jpg (62.02 KiB) Viewed 5373 times
One part of the main sheet goes from the mainsheet swivel to the middle of the boom. This one is at the angle that center-boom sheeting would have. It contributes 1/5th of the total main sheet pull on the main. Why 1/5th? The reason is that the two parts at the end of the boom each have the same tension. They are part of a continuous line and that means that apart from small effects like friction in the blocks, the tension in each part must be the same. But they have double the lever arm, so they pull the boom with 2 x 2 = 4 times the force as the first part.
So the traveler forces the majority of the main sheet's force on the boom to be more sideways. This means that the sheet pulls less strongly downwards, which reduces the leech tension for the same sheeting angle. That leech tension can be added to by tightening the vang. The vang is not pictured, but its effect is to pull directly downward on the boom, no matter the sheeting angle. It''s contribution depends on the tension on the vang, which is not tied to the tension in the sheet needed to get to a certain sheeting angle. It's shown with an arrow in a different color to indicate that it is independently controlled.
As you can see, by using a vang with end-boom sheeting you have two independent controls for sheeting angle and leech tension. (Well not perfectly independent as the sheet always adds a bit of leech tension as well).
For example, if you let out more sheet, but don't adjust the vang, your sheeting angle would increase, but the sail shape would change only moderately, because most of the leech tension would be set by the vang. That makes depowering the mainsail in a gust more controlled.
In contrast, if you only have center-boom sheeting (and no vang) the sheet will add a lot of leech tension when the angle is small (sheet vertical) and less when you let out the sheet. That's not beneficial for your gust response.