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| Sail Measurement Assistance Boat: Every boat has maximum sail dimensions that cannot be exceeded. They are defined as I, J, P and E, (also Py and Ey for Ketchs and Yawls). Unless you have existing sails to use as a reference, it is always best to physically measure the boat and not rely totally on manufacturers specifications. Occasionally rig sizes will vary even between the same make and size boat. Measurements can also be altered as a result of re-rigging. Main Sails: Attach a tape measure to the main halyard and hoist to the maximum or to the black band around the mast if available. Measure to the main tack fitting for the P measurement. The E measurement is the length of the boom measured from the tack fitting to the clew connection or the black band around the boom if available. 
Headsails (Spinnakers require I and J measurements): Attach a tape measure to the jib halyard (or top swivel of the furler) and hoist to the maximum. Measure to the deck sheer line for the I measurement. Also measure to the tack attachment point on the bow (or top of furler) for the maximum hoist measurement. Measure from the intersection of the deck and headstay to the front side of the mast for the J measurement. 
Tip: tie a line to the halyard along with the tape measure in the event the tape should break you can easily retrieve the halyard. Sails: Sails are defined by another set of terms. The luff is the leading edge of the sail. The leech is the trailing edge. The foot is the bottom edge. Jibs and Genoas have a Luff Perpendicular (LP) that is the perpendicular distance from the luff to the clew. The LP defines the sail size in terms of percentage increase beyond the J measurement as in the following examples. Headsails are designated as Number 1, 2, and 3.
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