Mainsail and Mizzen The mainsail is measured with width at the top (HB) and four widths found on the leach between head and clew: at 1/4 (MGL), 1/2 (MGM), 3/4 (MGU) and 7/8 (MGT) leech heights. Points on the leech are found by folding the sail to find equal distances between clew and head or between two adjacent measurement points. The height of the mainsail luff (P) and mainsail foot (E) are measured on the mast and boom as part of the rig measurements. The basic formula for mainsail area is
The mainsail measured area is calculated by the simplified trapezoid formula above, dividing the luff in amounts of 1/4, 1/2, 3/4 and 7/8. However, the mainsail rated area is calculated by using the actual heights on the luff from the tack point to the points where mainsail widths are measured. The amount of roach will therefore proportionally increase the rated area from the measured one by giving the exact area and centre of effort height for square top and other high roach main sails.
If there is a batten above the 7/8 (MGT) leech point, then the top width (HB) measurement is increased taking in account effect of a batten positioned this high on the mainsail.
Mizzen measurements are the same as for the mainsail, with all corresponding measurements abbreviations having "Y" as a suffix.
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