Set draft position with luff tension
Once you've set the overall depth of the sail, the next step is to position the area of maximum draft. In most wind conditions, your goal is to locate the draft about 50% of the way from luff to leech. This is usually done with cunningham tension.
The cunningham applies tension to the luff of the main, and this controls draft position. Tighten the cunningham to move the draft forward; ease it to let the draft move aft (see below). In general, the more you bend the mast, the tighter you need to pull the cunningham to get the draft in the right place. You'll also have to pull the cunningham harder on an older main, because a sail's draft moves aft with age.
In light air, keep the cunningham quite loose. A few "speed wrinkles" along the lower luff won't hurt. Remember you're using the cunningham to control draft position, not make the sail look pretty. In light air, you may even have to lower your main halyard (especially downwind) to get the proper luff tension.
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