are closer to a giant genoa than they are to an asymmetric spinnaker. In order to utilize a bigger proportion of the a-sail's area, it wants to measure as close to a genoa as possible while still measuring as a legal spinnaker. North PRG member Dave Hirsch developed the mold for the sail pictured here. It is a fully legal IMS asymmetric, which should be legal in most other classes. He developed the mold and managed to get a much flatter shape than previously, yet still produce a shape that flies without the leech flapping. The sections are evenly shaped with a mid stripe camber around 20% compared to a light genoa at around 16% and a Gradient Code 1A around 23%. The Zero has more vertical curvature than a genoa and a much more twisted flying shape. The luff needs to set relatively straight and the leech has a small positive fan or 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.

Sizing
Sizing for these sails is pretty straight-forward. The luff is as long as the rig configuration allows. This is determined by measuring the distance from the halyard point at the head to wherever the tack will set. Ideally, the tack point is a strong fixture on the boat and forward of the headstay. These sails are not set on conventional poles and extreme caution should be used with regard to setting these on sprit style poles. Zeros exert a lot of upward and compression 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 be allowed for. The leech is molded close to the maximum allowed. The foot is somewhere close to half the luff length, but the exact length can be governed by the rules the sail must live within. Girth limited sails may need shorter foots. Area limited sails might need longer foots if there is a minimum area that must be met.

Structure
At the present time, most Code Zero shapes have too much curvature and twist to be molded at 3DL. As paneled sails there are a variety of fabric styles available. Most fabrics that will make a good light genoa can be applied here.

Currently, NLS 55S (a Spectra scrim laminate from North Cloth) is the most popular style for everything from Volvo 60's and Transpac Turbo Sleds to 40+ footers. KH 70SX and Cuben Fiber have been used in several applications. The 1.7 Norlam has been used on Farr 40's, the ILC 46 shown here and several smaller PHRF sails. The Stormlite is heavier and more elastic but more durable than the 1.7 Norlam and both are considerably less expensive than the KH 70 and the NLS. Cuben fiber is very well suited to these sails structurally, but a bit hard to swallow economically.

An exclusive panel layout has been developed that draws on both genoa and gennaker structures. Joiners are spread out vertically and the bottom belly section has a VeJo. There are several "warps" to control panel alignment. Unlike a genoa, the luffs and leeches of these sails can see considerable load so they are built without cloth step-ups. Patching is gennaker style and uses a combination of radials and strips on luffs exceeding 18m.

The luff rope is oversize Vectran, spliced into the head ring and laced to the tack ring. Since these sails need to set with their luffs very straight, reducing the stretch of this rope is critical. Leech and foot cords are adjusted at the clew through cleats.

Setting, Sailing and Dowsing
There are two methods of handling Code Zeros. First and preferred is one of the line drive furlers. These allow the sail to be rolled and stowed in a spinnaker box bag. Hook up the corners, hoist and release the furling line while they sheet it in. If you are not using a furler, the best alternative is lots of good stopping yarn, although zippered snakes seem to be making a comeback in some areas.

Before setting the sail for the first time, make sure the luff cord is tensioned much tighter than you think would ever be needed. Tie it off as short as possible as it is easier to ease than tension. The sail should set with minimum cloth tension on the luff.

Note how difficult it would be to use a furler in this setup!

The tack (or furler) should be shackled to the tack point or to a "snout line". While the Racing Rules of Sailing removed the need to set spinnakers on a pole, some rules may still have this restriction (IMS is not completely clear). In this case, raise the butt end and lay the pole tip next to the tack with a guy lead through the beak and snapped into the tack ring. Apply slight tension to the afterguy and the rule is met.

Some rig builders advise the use of a 2:1 halyard in order to reduce the compression load on the spar. Either way, the halyard needs to be very low stretch and very strong. The sheeting point is the spinnaker sheet block, although the use of 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, but without one, the best method seems to be bearing way off and lots of hands. Spiking off the tack and using the gap between the boom and mainsail foot to smother the sail also works well.