POPIS GENY
The most obvious characteristics of a genoa are its size and shape. We measure genoas by the length of their LP, or luff perpendicular. To construct an LP, draw a line from the sail's clew to its luff, intersecting the luff at a right angle. The length of the LP divided by J (the distance from the forestay to the front of the mast) equals the overlap of the sail. LP divided by J = Overlap (%) On IOR boats, the largest headsails usually have a 150% overlap; No. 2s have a 130% overlap; No. 3s have a 98% overlap, and so on. Most PHRF boats are allowed 155% genoas without a penalty.
The sails that are less than full size are used in heavier winds once the maximum amount of force for a given boat has been reached. Beyond this point, maximum sail area simply overburdens the boat, and it's better to reduce drag by changing down to a smaller jib. This will improve the lift-to-drag ratio (one of the best indicators of upwind performance) by holding lift at a maximum while lowering drag. Note that smaller headsails are almost always shorter on the foot, but still nearly full hoist. The reason for this is that a high-aspect-ratio sail is more efficient. The sailmaker preserves the maximum wing span of the boat's air foils, but shortens their width to the limits of construction technology.