Genoa sizing is a something that is often confusing to the average sailor. There are a number of different systems to describe sails. “#1”, “155%”, “Lapper” are some of the expressions that can be used interchangeably. We should probably start with answering the question “what makes a sail a genoa?” A genoa is an overlapping headsail. Overlapping meaning that the clew of the sail comes back past the mast.
Racing sails are generally given a number to describe the sail’s size. A #1 would be the largest genoa in an inventory, a #2 the next largest, and so on. A # 3 is usually the largest non overlapping sail in the inventory. This sail is also often called a blade.
A more precise description of the size of a genoa is given as a percentage. The overlap size is the LP of the sail divided by the J dimension of the boat. The “LP” is the luff perpendicular which is the shortest distance from clew to luff, which always works out to be on a 90 degree angle from the luff. A 155% genoa has an LP that is 155% of J. If your J dimension is 10’ then a 155% genoa would measure 15.5’ as the shortest distance from the clew to the luff.