Submitted on November 5, 2001 by Ward Olsen

Question: I've had conflicting opinions about the use of nonlaminated Dacron for a biradial genoa. We cruise on Lake Michigan in a Tartan 31 with a roller furled genoa. I would like a nonlaminated sail. One company claims their 4.9 oz biradial genoa will be lighter, stronger, and more durable than a heavier cross cut sail (claim their sailcloth strong in 2 directions.) Another says only a crosscut will last. What do you think? Thanks. Ward

Answer: Hi Ward, As you are finding out, there are lots of ways to build any sail. Radial sails are built using warp oriented fabrics and cross cut sails are built using fill oriented fabrics. "Warp oriented" means the fabric is stronger in the warp direction- the direction along the long edge of the rolled out piece of cloth. Fill oriented fabrics are strongest in the fill direction- the direction across the rolled out piece of cloth.

At North Sails we stay away from using warp oriented woven polyester fabrics for cruising sails. Warp oriented woven fabrics are less stable than fill oriented fabrics. This is especially true in heavier weights. When the fabric is woven the warp yarns are set up on the loom first and the fill yarns are woven through the warp beam and then packed in place. To make a heavy fabric, larger or more fill yarns are packed into every inch of the fabric. While the mass of the fill component can be increased by packing the fabric more tightly, the mass of the warp component is limited because there is no way to pack it more tightly. So the heavier the fabric is, the more fill oriented it naturally becomes. 5.0 oz dacron is generally considered the heaviest warp oriented fabric that is stable enough to build a decent sail out of. Your Tarten 31 headsail falls into the 5.0 oz range so a sailmaker that is used to working with warp dacrons will be able to get good results.

Radial sail construction works best with very stable fabrics. When the long, triangular radial panels are cut out of the fabric roll, each panel will have a couple of inches right down the middle where the yarns in the fabric are continuous and perfectly oriented. The cut edges of the panel will be way off the perfect orientation so the fabric will shift, or stretch more there. Fabric distortion in the direction between the warp and fill yarns is called bias stretch. The narrower the panels are cut, the less of a bias stretch problem the sail will have, but the longer the sail will take to assemble. Laminated fabrics work better for radial construction than woven fabrics because the mylar layers of the laminate help keep the fabric package from shifting. To stabilize woven sailcloth in the bias direction the fabric is treated with heat and pressure to lock in the weave and then some kind of resin finish is applied after that. The biradial sail will look smooth as long as this finish is in tact, but as the sail ages the finish of the fabric will break down and the sail will become progressively more misshapen. The radial seams are double the strength of the fabric in the single layer parts of the sail so the seams don't stretch as much as the surrounding area. When the radial dacron sail is pretty worn out it developes an oyster shell pattern to its surface.

Of course, a cross cut dacron sail will wear out as well. When the cross cut sail ages the draft location will migrate aft towards the middle of the cord sections and the sail will get deeper, just like the radial sail. But the surface of the sail will stay smoother. So even though both sails might be "blown out" the cross cut sail will look a little better.

A North Sails representative will reccommend that you buy a cross cut dacron sail, or a radial sail made from one of our Soft NorLam (laminated mylar and woven polyester) products. The radial sail will be a triradial cut, meaning the panels orient out of all three corners rather than biradial, where the panels orient out of just the head and clew. Thats the way we do it. Your other sail maker is no doubt skilled at making sails using warp oriented dacron. If that is his recommended cruising headsail product he will have figured out ways to maximise the fabrics strengths and reduce the effects of bias stretch. You won't be making a bad decision either way.

Regards,
Dan Neri

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