The shape of the headsail, jib/genoa, should be similar to the shape of the mainsail for boat balance and speed, which are determined by wind speed and sea conditions. For his blog, the headsail will be referred to as the jib.
The shape of the jib sail like the mainsail is related to twist, the amount of draft (flat/full) and draft position. Twist refers to the top part of the jib falling off to leeward as a result of inadequate leech tension. The amount of twist in the jib should approximately mirror the amount of twist in the mainsail. “Draft stripes”, dark stripes on the sail parallel to the deck help you to identify the depth of the draft as well as the position of the draft in the sail. One way to identify if the jib is trimmed too tight and/or the draft is too far aft is to observe the amount of backwind in the mainsail. If the backwind extends further back in the mainsail than normal then the jib is over-trimmed and/or draft to far aft in the sail, thereby closing the slot and disrupting the flow of air between the mainsail and the jib.
The number of controls to shape the jib are less than the mainsail. The primary controls to shape the jib are the jib sheet, position of the jib lead blocks and jib halyard tension. The jib draft should be located approximately 30% to 35% aft of the luff measured along the foot of the jib sail. The draft stripes will help you estimate how far back the draft position is located. The amount of draft or fullness of sail is determined by wind speed and sea state. The position of the draft will move aft as wind speed increases, to counter this, tighten jib halyard tension causing draft position to move forward. In light air, draft position may be too far forward, so ease jib halyard tension causing draft position to move aft.
In light air, similar to the mainsail, you want a relatively full jib sail, a relatively deep sail draft in relationship to the cord. Draft depth increases power and this can be accomplished by easing the jib sheet, which reduces the length between the clew and tack, thereby increasing draft in the sail. Another effect, however, is the clew rises upward and outward reducing leech tension resulting in increased twist in the upper part of the jib sail. To counter this effect, move the jib lead forward which causes a downward pull upon the clew, thereby increasing leech tension and reducing twist. In high winds, you want a flat jib sail to reduce power and heel in the boat, so move the jib lead aft, which flattens the foot and eases leech tension resulting in twist in the upper part of the jib sail, thereby reducing power and heeling effect. Just like the mainsail, proper jib sail shape is determined by wind speed and sea conditions and jib sail shape is measured by twist, draft depth and draft position to balance the boat and maximize boat speed.