Headsail size
The sailmaker defines the luff size with the LP measurement (from luff perpendicular = luff plumb). To determine the LP measurement, the plumb line is dropped through the clew at right angles to the luff. Dividing the LP measurement by the J measurement (distance between the leading edge of the mast and the forestay) gives the overlap of the luff in percent. For example, if a boat has a J-measurement of 3.0 meters and a LP-measurement of 4.5 meters, you will get a 150% genoa, which corresponds to a normal genoa I. 150% genoa means that this sail fills the foresail triangle by 150% or overlaps the mast by 50%. Generally used on yachts are the following designations for the headsails:
> Genua I = 150% J-measurement
> Genua II = 130% J-measurement
> Genua III = 110% J-measurement
> working jib = 85-90% J-measurement
Jib or Genoa?Generally, overlapping sails are referred to as a genoa and non-overlapping headsails are referred to as a jib. One quality characteristic of a sail can be derived from aerodynamic laws: the aspect ratio. It describes the ratio between luff and foot. As a rule of thumb, the slimmer and higher a profile is, the more effective the sail is, since a sail with a longer leading edge always generates more propulsion with the same surface area. This is also the reason why sailmakers always try not to reduce the length of the luff on smaller sails, if possible, but to shorten the foot first. However, even with the new weft-stable and high-performance sailcloth, this only works up to a luff-to-luff ratio of about 3.5 to 1. If this value is exceeded, the luff must also be shortened.
Source: Moritz Segel
