07/28/2004

D4 is a construction technique; it has nothing to do with the design of a sail. But after building our first batch of D4 sails I am quite impressed with the finished product.

For those not up on the current status of the Sailmaking industry the D4 process is what is know as a "membrane" sail, or commonly called a "string" sail. Conventional sails are made of fabrics that have yarns in pre-set directions, usually running a 0 and 90 degrees. The panels are laid out in the sail so that the yarns in the strongest direction run more or less in line with the maximum load in the sail. A traditional cross cut sail has panels that are laid out at 90 degrees to the leech. Because of the inherent way that weaving looms work all but the very lightest woven cloths have less stretch in the fill (90 degrees) than they do in the warp (0 Degrees). By using the cross cut layout the fill yarns run up the leech in the general direction of the highest load in a sail. However the loading in a sail is actually much more complex with stresses running in many curved "load paths" between all three corners in the sail. To try and pick up these loads in a Dacron sail sailmakers relied on very heavy fabrics with a lot of resin finish to try and control the stretch in non-threadline directions. Cross cut sails were fairly easy to design and construct because shaping was done along the edges of the horizontal panel.

With the introduction of computer design programs in the early 80’s sailmakers were able to vary the panel layouts much more radically to build sails where the panels more closely followed the load paths in the sail; and cloth manufacturers were able to build laminated fabrics that had the primary, low stretch, yarns running in warp. The tri-radial panel layout quickly became the standard. With panels that radiate out of each corner and run at varying angles up through the sail we could rely on the primary yarns to support most of the load in the sail and greatly cut down on the amount of off load line yarns. The result was much lighter sails that distorted less under load.

A membrane sail is the next step in that evolution. The sail is constructed of a shaped membrane, usually of mylar film but it can be of films with light weight woven taffetas on the outer sides for extra durability, supported by yarns laid on top that curve throughout the sail to follow the load paths in the sail. The yarn density throughout the sail is varied to match the load requirements in different parts of the sail. The advantages are a matrix of yarns accurately and evenly supporting all parts of the sail for less distortion and more stability; with a corresponding reduction in weight.

The D4 process was developed by Frasier Sails in Australia and recently purchased by Dimension / Polyant Sailcloth. We design each sail with our design software the same as we would any other sail we build. The sails are designed with 6 or 7 horizontal sections. We email the design offset file to the Dimension production facility in Australia. There the shaped sections are re-constructed in their computer program with seam allowances and the load map for the sail is generated. The lower film membrane of the sail is laid out flat on their construction floor and a computer driven overhead gantry system lays out the continuous yarn matrix on the film. The top film layer is put down and the sail is laminated with pressure rollers. The advantage to this system of membrane sail construction is ability to laminate the flat sail membrane with lots of pressure, resulting in a much better finished laminate. The computer then plots out the shaped horizontal section seams on the finished membrane.

When we receive the laminated membrane it comes complete with any necessary batten pockets, matrix batten pocket end reinforcements, leech reinforcement tapes, and finish corner reinforcements. We cut the membrane apart along the computer plotted section lines and bond the sections together with an air pressured glue gun. The batten pockets and batten ends are finished and bonded on along with any leech, reef, or corner reinforcements. While most of the corner strengthening is built in from the multiple yarns coming together in each corner there are light Kevlar reinforcements applied and then covered with a layer of mylar laminate. We also sew down two layers of Dacron on each side to provide the final base needed for attaching the corner rings. At this point the sail is finished like any other sail. Leech and foot tablings are applied, luff tape is sewn on, and the sail is finished with corner rings, headboards, and reef points as needed.

So far we have finished four D4 sails, 2 mains and 2 jibs, and have six more on order. I have sailed two regattas with a main on the ILC 40 "Tera’s XL" and have been very impressed with the sail. It is incredibly smooth and the measured shape came out right on the design numbers. The finished sail weighs 45 pounds which is 6 pounds (12%) lighter than the previous Carbon paneled main. In light air the sail is very responsive; in 20 knots the sail flattens easily and the leech stays flat and firm. It is a very easy sail to trim because the shape is so stable.

D4 sails are available in Carbon, aramid, Vectran, and Pentex. While high end racing sails are usually built with the yarns between two layers of film for the best strength to weight ratio they are also available with Dacron taffetas on either one or both sides. The dacrons on the outside add some weight but also adds lots of durability for use in large performance cruising sails.