Measuring Sail Area
Using a consistent method to calculate sail area makes it possible to compare boats.
The conventional way to find sail area is to calculate the area of the foretriangle (FT) between the mast, headstay, and deck; the area of the mainsail triangle (M) between the mast and the boom; and add them: SA = FT + M (= I*J/2 + P*E/2).
The area of an actual headsail is measured by striking a perpendicular from the luff to the clew and multiplying that length by the length of the sail’s luff. Under the International Offshore Rule, the base length for that perpendicular was J, expressed as LP = J. Headsails were then designated as percentages of LP.
Under the IOR, the normal maximum size for a genoa was 150%LP, meaning it had a clew-to-luff perpendicular of 1.5 x LP, or 1.5 x J.
Because the measured sail area uses the luff length and not I, a 100% jib has an area greater than 100%FT.
Note that the mainsail calculation doesn’t include roach, the area between the P*E/2 triangle and the sail’s leech. Under the IOR, permitted batten lengths limited roach to about 11 percent of M. Full-battened sails carry much more roach, and this is included in a builder’s published sail areas.