Set twist with mainsheet tension
We learned in "The Basics" that twist refers to the changes in a sail's chordlines from bottom to top. A sail needs to be twisted because of wind gradient, which moves the apparent wind aft as you get higher off the water.
On a mainsail, twist is controlled by the amount of mainsheet tension, as well as the amount of vang. The mainsail leech is our best indicator of how much the sail is twisted. The front of the main is certainly a poor measure of twist because it sits in the confused airflow of the slot and directly behind the mast's turbulence.
To set proper twist, trim the mainsheet until the top batten is parallel to the boom. If you have a long top batten, your goal is to make its aft end parallel to the boom. Using the angle of the top batten isn't exactly measuring twist (because the angle of the batten is different than the angle of the chordline), but it's a good guide.
When the sheet is eased, the main has a very twisted shape, with the top batten falling off to leeward. As you trim the sheet, the top batten angle narrows until it is parallel with the boom. Trimming harder will take away all the twist, close the upper leech, and make the top batten poke to windward.
The best average setting for the top batten on everything from Lasers to 12 Metres is parallel to the boom. This is one of the golden rules of mainsail trim.
With the batten in this position, the top batten telltale should stream aft between 50 and 90% of the time. This telltale, attached to the aft end of the top batten and extending 8 to10 inches beyond the leech, indicates whether the upper leech is stalling. When the leech is stalled, the telltale curls around to leeward of the main (see right). Twisting the main more will open up the leech and re-establish flow.
When to bend the rule
Rules, of course, were made to be broken. On a masthead boat, you can sometimes trimthe sheet hard enough to tighten the upper leech and poke the top batten slightly towindward. This is fastest in medium air and smooth water, when you can point high andmaintain speed. This works with a masthead rig because the full hoist genoa steers theairflow around the lee side of the upper leech and reduces the danger of stall. On a fractional rig, the upper mainsail has no genoa to steer flow, so its upper leech may need to be opened up slightly by easing the sheet. There are a number of times when you may want to twist the main enough to let the top batten fall off slightly to leeward. In achop, after tacks and in light air, ease the sheet to open up the leech slightly andprevent stall.
» Set depth with mast bend and outhaul tension