Luff – “P” Dimension
Attach the tape measure to the main halyard (along with the head of the old mainsail if available). Hoist it under normal luff tension. If the old sail is not available, hoist to the maximum height you want your mainsail to stretch up to (better a little short than too long). If your mast has bands, the measure to the band. Then measure to the top of the boom itself. (Not the tack pin on the boom).
While the measure is still hoisted, swing it back to the aft end of the boom and measure (straight line) to where you want your clew ring to be located.
Foot – “E” Dimension
Lower the tape, attach it to the outhaul line along with the clew of your old mainsail if available. Pull the outhaul to the maximum point of draw. Take the measurement from there to the aft face of the mast (not to the tack fitting on the boom). If no old mainsail is available to pull it to where you want your clew to stop. (If there is a band on the boom, measure to it).
Luff & Foot Hardware
Note the shape and size of the slides, slugs, or bolt rope and how they attach to the mast. These are easily changed on mainsails but it is helpful to know anyway. Flat internal and external slides are measured in width while cylindrical slugs and bolt rope are measured in diameter.
Tack Pin Setback
Determine the location of the Tack Pin on your boom “gooseneck”. This is measured in inches behind the mast and above the boom and is used to locate the tack ring on your mainsail in the right place so it doesn’t pucker. This is not necessary for used or stock sails since they are already fixed. You can add shackles or lashings if needed to smooth it out.
Max. Luff – Max Hoist
Hook a long tape measure to the jib halyard (or top furling swivel on a furler see “Furler Hoist” below), as if it were the head ring of the sail. Hoist it all the way until it stops at the top. Measure down to the point you intend to attach the sail tack to. Allow a few centimeters for stretch or error (better a little short than too long).
Measure the horizontal distance between the tack fitting and the mast. This is the “J” dimension and will be the number the “L/P” is compared to for percent overlap.
Sheet Lead Angle
Be sure that the clew height for the jib or genoa you are ordering will allow you to sheet to the existing genoa track or you may need to add additional tracks. Extend an imaginary line from about mid luff through the clew to the deck. When the sail is sheeted in a flat you should be able to achieve even tension both down the leech and back on the foot. Otherwise, you will end up with a sail that is “Strapped” (tight) along one and loose on the other. If in doubt we recommend you “mock-up” the sail using line or tape measures and test the lead out.
If you have a furling unit, hook the tape measure up to the upper furling swivel (as if it were the head of the sail). Hoist the tape all the way up and measure down to the tack fitting on the furling drum. Also, determine the diameter of the luff tape needed. (Luff tape is the small bead that feeds into the head foil ).This can be measured on an old sail with calipers or a fine scale ruler. Luff tape size is provided with furling unit documentation too. The most common size is #6 (measures slightly over 3/16.”)
For racing, the spinnakers luff should equal the “I” dimension and the max foot/girth should not exceed 1.8 X “J”. For cruising spinnakers, the luff can be plus or minus 8% of “I” but cannot exceed maximum hoist on the halyard. Spinnaker foot should fall between 1.6 and 1.8 X “J”