The shape of the genoa sail is determined by the draft and relative position of the draft. There are many other parameters; horizontal camber, twist, angle of attack, and vertical camber distribution.
Draft effects power, acceleration, and drag. Position of maximum draft %45 for mainsails, %40 for genoa sails.
Air flows smoothly over the surface. Beyond %50 over the length flow becomes a turbulent creating drag. Trim the sails aimed at reducing this drag.
Another factor is the vertical shape of the genoa sail is the twist that should match the more open apparent wind angle. The shape of the genoa and main sail replace not only in the position draft.
Genoa sails are generally deeper than mainsails and more difficult to adjust. Genoa is controlled by the halyard, the car position, sheet tension, and headstay tension. Most important is to get halyard tension correct. It is better to have less tension on the luft so that you in fact get some wrinkling. If wrinkling coming from the front of sail then fine-tune this after the sail has been set.
The genoa halyard is used to increase and decrease the tension on the luff of the genoa. Once the genoa is hoisted the halyard should be tensioned until the horizontal wrinkles just disappear. As the wind increases in strength halyard tension should be increased to help depower the genoa sail.
Halyard tension too loose, increase until wrinkles disappear.
Halyard tension too tight, decrease until folds disappear.
The next control is the car position incorrect place. For this, you take a point approximately 50% of the luft length of the genoa sail. Down through the clew and this will give your initial point car. Then put the sailboat into the direction that we want to sail and trim on the sheet. Then need to fine-tune the position of the car. We do this by looking at the telltales on luft of the sail if the telltales at the top is breaking before the bottom we move the car forward and inversely if move the car aft.
Sailmakers put telltales in the forward section of the genoa sail for monitoring the flow of wind across the sail. With the genoa sail trimmed correctly, the wind is going over both sides of the sail, and therefore both telltales are ‘streaming’ towards the back of the sail so sails produce maximum force.
The windward telltale (green) just starting to break from streaming. The solution is to pull the sail in more than bear away a little.
The wind on the leeward side is now stalled and the leeward telltales (green) will not stream. The solution is to let the sail out than head up.