"Code Zero" (A0) Asymmetric Spinnakers
Part genoa, part asymmetric reacher... is the A0 (aka "Code Zero") right for you? 

In the opening leg of the 1997-98 Whitbread Around the World Race EF Language unveiled a “secret weapon.” It was a new close-reaching asymmetric developed by North sail designers dubbed the “Code Zero,” and it vaulted EFL to an early lead the team would never relinquish. A decade later, North “Code Zero” (A0) asymmetrics are versatile members of mainstream inventories... not only used by boats that spend a lot of time close reaching in light air, but also in windier conditions and wider angles.

What they do
Code Zero asymmetrics fit effectively into the crossover gap between a genoa and the ubiquitous 3A asymmetric reacher. They provide additional power at approx. 40-degrees AWA in true wind speeds under 10 knots; conditions that are typically slow with conventional sails. Code Zeros have also proved effective for reaching in 15-25 knot winds at 80 to 90 degree apparent wind angles. Most boats sailing offshore can put a Code Zero to good use.

Code Zero Design
Code Zeros are closer to a oversized hybrid genoa than they are to a typical asymmetric spinnaker. For racers, a Code Zero wants to measure as close to a genoa as possible while still qualifying as a legal spinnaker. They are designed with a flatter shape than other asymmetrics, yet still fly without the leech flapping. The draft sections are evenly shaped with a mid-stripe camber of around 20%. A typical light genoa’s mid camber is around 16% and a V-Series 1A asymmtric’s mid camber is around 23%. The Zero has more vertical curvature than a genoa and carries a much more twisted flying shape. The luff needs to set relatively straight and the leech has a small positive fan (roach). Wind tunnel testing indicates this configuration increases forward force with a lower side force and center of effort than a fuller sail with more normal edge profiles.

Code Zero design can be affected by limits placed on them by rating rules, handicaps or class rules. Typically, that means:
  • The leech must be less than 95% of the luff in length.
  • Mid girth must be at least 75% of the foot length. (This is in the RRS, as well)
  • Additionally, asymmetrics are limited to a maximum width (girth limited) or to an area through a formula using luff, leech, foot and mid girth
A Code Zero luff should be as long as the rig allows. This is determined by measuring the distance from the raised halyard point to wherever the tack will set. Ideally, the tack point is a strong fixture on the boat forward of the headstay. These sails are rarely set on conventional spinnaker poles and extreme caution should be used setting a Code Zero on retractable sprit-style poles. Zeros exert a lot of upward force in order to fly with the luff as straight as possible. If the sail will set on a furler (recommended), clearance between the drum and the headstay must also be accommodated. The leech is designed close to the maximum allowed. The foot is usually close to half the luff length, but also can be governed by handicap, rating or class rules. Girth-limited Code Zeros may need shorter foots. Area limited sails might need longer foots if there is a minimum area requirement that needs to be met.

Code Zero asymmetrics have been developed using the University of Auckland Twisted Flow wind tunnel. Here, a Code Zero is being tested for a Volvo Ocean Race team.

Code Zero sail handling
There are two methods of handling Code Zeros. The preferred is a roller furling luff, which allows the sail to be rolled and stowed in a spinnaker box bag. When raising, simply hook up the corners, hoist and release the furling line as the sail is sheeted in. An alternative method is lots of good stopping yarn or zippered snakes.

When setting the sail, make sure the luff cord is tensioned more tightly than you think would ever be needed. Tie it off as short as possible as it is easier to ease than add tension later. The sail should set with minimum cloth tension on the luff.

Some rig builders advise the use of a 2:1 halyard in order to reduce compression on the spar. In any case, the halyard needs to be very low stretch and very strong. The sheeting point is usually the spinnaker sheet block, although a tweaker might be required. Leech cord tension is very sensitive and can span a wide range. When tensioned to stop the flapping, expect to see considerable curl in the leech. This is the inevitable result of the amount of leech fan required to meet the minimum mid girth restriction.

Takedowns with a furler are fairly simple. Without one, the best method is bear off with lots of hands. Spiking off the tack and using the gap between the boom and mainsail foot to smother the sail also works well.

Which North downwind sail is right for you?

A Code Zero is a cross between a genoa and an asymmetric reaching spinnaker.

Most fabrics suitable for a light genoa can be applied to a Code Zero, including Spectra, polyester and lightweight Cuben Fiber laminates.