UK Sailmakers' Encyclopedia of Sails
4.3 - Spinnakers
The curved sides of a spinnaker make the computation of area a complicated process. It is therefore an almost universal practice in the sail-making industry to compute the area as if the sail were a rectangle. We follow that practice, and the area shown on a price list is the product of the luff length times the maximum width. The actual area will vary from about 70% to 85% of the rectangular area, de-pending on the type of spinnaker.
The length of the luffs and the maximum width are computed from the I and JC dimensions in accordance with a formula which is common to almost all of the rating rules for handicap racing. This formula yields dimensions which have worked well in practice for many years. For that reason, we use those dimensions in building spinnakers, even if the particular customer has no intention of racing his or her boat. For those interested, the maximum width equals 1.8 times JC, and the maxi-mum luff length equals 95% of the square root of the sum of I squared and JC squared.
Three basic spinnaker types are available: Tri–radials, Fatcats and Matrix Cuts. All three have radial heads and radial clews. The Tri–radial and the Fatcat both have horizontal panels in the center section, separating the head from the radial clews. These horizontal panels are cut with curved edges to permit proper control of the draft of the spinnaker.
The Fatcat differs from the Matrix and Tri-Radial in that it is designed to be very flat for close reaching. The break point between the bottom of the head panels and the top horizontal panel occurs much lower in the sail, and the total surface area of the sail is less, especially in the head. As a consequence, a Fatcat usually has fewer horizontal panels than a standard tri-radial for the same boat. It is normally made of stronger, heavier material, and it doubles nicely as a "chicken chute" for heavy weather running.
The Matrix Cut is a variation of the tri-radial. It omits the horizontal panels altogether; the radial clews are extended up to the middle of the sail, where they meet the head. The full radial panel layout allows for precise articulation of cloth threadline to match primary sail loadings. Each panel is shaped individu-ally by our computer cutting machines to distribute shaping smoothly and evenly, with a minimum number of horizontal seams. Fewer seams make a smoother surface for the wind to flow over. The Matrix creates the lightest, strongest spinnaker for a given cloth weight. The rocked vertical panels ensure a distortion-free sail, even when close reaching.