MEASURING HEADSAILS OR WHAT IS A 150% GENOA
We often receive sail inquiries which define the size of a genoa or jib according to luff length and foot length, or simply according to the LPG percentage where nothing is known about the J dimension of the boat the sail is to fit.
In order to be sure that we and our customers mean the same thing when talking about Jibs & Genoas, we have written the following brief guide to the terms LPG (OR LP) and 150% (or 100% or 130%).
The term 150% refers to the Longest Perpendicular Girth or LPG measured across the genoa from the luff to the clew. The measurement line is perpendicular to the luff which also happens to be the shortest
distance between the luff and clew of the sail. The measurement is taken from the forward edge of the luff tape, rope or wire to the point of intersection of the foot and leech of the sail.
This measurement is often expressed as a percentage of the "J" dimension on the boat. The "J" dimension is the horizontal distance between the forward face of the mast at deck level and the point of intersection of the center of the headstay with the deck. The accompanying diagram should help to visualize these measurements.
In order to find the sail area (in sq. ft.) of a jib or genoa you can do thus: [LPG (in feet) X luff (in ft.)]/ 2 = area ( in sq. ft.) It doesn't matter whether the clew is at the deck or 10' above the deck, the area will be unchanged as long as the LPG and luff measurement are constant. Thus the foot and leech dimensions may vary and the area remain unchanged.
The accompanying sketch shows the J measurement and what 100 & 150% Genoas usually look like.