Know-How

Downwind Sails for Cruising

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I’m a real fan of downwind sails because they add a lot of speed and fun. On my 40-footer I carry a 1.5-ounce symmetric spinnaker in a sock, a 75-ounce asymmetric, also in a sock, that I set on a collar around the headstay, and a 2.2-ounce Code 0 that I have mounted on a Harken furler. I use the symmetric when I have a good crew but leave it ashore when I’m sailing shorthanded. The Code 0 is my primary downwind sail when I’m sailing by myself, and the asymmetric serves as a backup. If there’s one other knowledgeable crewmember aboard, the asymmetric becomes the primary sail and the Code 0 becomes the backup.

Downwind sails differ from conventional headsails in two respects. First, headsails have a fairly flat shape similar to an airplane wing while downwind sails have fuller shapes optimized for greater apparent-wind angles. Second, downwind sails gain additional sail area with positive luff and leech curves—the area outside the triangle formed by the head, tack, and clew. The greater the luff and leech curves, the larger the sail will be. There are three general types of spinnakers: the traditional symmetric sail; the asymmetric “cruising” spinnaker; and the Code 0, or flat asymmetric “gennaker.” Table 1 shows how much power these sails can produce at various apparent-wind angles.

Sail Area
For a given mast height, a symmetric spinnaker will contain more sail area than either of the other two types. An asymmetric spinnaker will have roughly 15 to 25 percent less area than a symmetric spinnaker, and a Code 0 gennaker with the same luff length will have about 30 to 40 percent less. Table 2 shows the approximate areas of the sails I use on my 40-foot cruising boat.

Sail design
Symmetric spinnakers are designed with a width (measured from luff to luff) of 180 percent of the boat’s J dimension (the distance from the mast/deck intersection to the headstay). Because both luffs of the sail are the same length, the shape of the sail on either side of the centerline is the same—hence symmetric. There is a large amount of positive curve to the luff areas on a symmetric spinnaker, and the sail can be carried effectively with the apparent wind as far forward as 65 to 75 degrees.

Symmetric spinnakers require a spinnaker pole to fly correctly. The pole is the reason that the symmetric is the only one of the three types that can remain stable when sailing at apparent-wind angles larger than 135 degrees. The symmetric spinnaker requires the most lines and rigging and takes the most effort to set, gybe, and douse. This is a disadvantage of the sail, but when you want to sail downwind at deep (up to 180 degrees) apparent-wind angles, the symmetric is the best sail for the job.

Asymmetric spinnakers are built with a different shape on each side of the centerline, so the sail has a defined luff and leech with the luff longer than the leech. The maximum width of an asymmetric is usually slightly smaller than that of a symmetric, the area at the head of the sail has a flatter shape, and the total area is smaller. An asymmetric’s overall shape is fairly deep, or full, but the luff and the leech have different shapes. The figure below illustrates the difference in sail area between an asymmetric spinnaker, a Code 0, and a conventional genoa.

Because an asymmetric does not require a spinnaker pole, the sail’s tack can be set free on a downhaul line at the stem, bowsprit, or headstay with a collar or strap that goes around a furled genoa. The asymmetric’s designed shape allows it to be carried effectively at lower wind velocities and with apparent-wind angles that are as far forward as 45 degrees. An asymmetric will start to be ineffective once the apparent wind moves farther aft than about 135 degrees, although setting it in a wing-and-wing configuration with a whisker pole will extend its effectiveness at greater angles.

Gybing an asymmetric involves easing the sheet and allowing the clew of the sail go forward and around in front of the headstay. In most cases the best way to do this is with a second, or lazy, sheet that is attached to the clew of the sail and led forward around the headstay and back aft to a winch. During a gybe the luff and tack attachments of an asymmetric do not change. The sail basically turns inside out in front of the headstay.

Code 0 sails were developed from research done by competitors in the Volvo Ocean Race and the America’s Cup. A Code 0 is effectively a cross between a genoa and a spinnaker. Code 0 sails, although they are fuller than a genoa, are much flatter in cross-section than either a symmetric or asymmetric spinnaker. The luff curve of a Code 0 is slightly positive, but not as great as the asymmetric. The leech also has a positive curve, but again less than an asymmetric. In 5-to-10-knot winds a Code 0 sail can be effective in apparent-wind angles as far forward as 35 to 45 degrees and as far aft as 115 to 125 degrees. A Code 0 sail is gybed the same way as an asymmetric.

The sail has a rope or light wire on the luff that allows it to be roller-furled. It can’t be partially roller-reefed like a genoa—it’s either all the way in or all the way out—but roller-furling makes a Code 0 sail easy to set and douse. To set a Code 0, the furled sail—it’s sometimes called a snake—is hoisted, and once the halyard is made secure, you pull on the sheet and unroll the sail. To douse the sail, you ease the sheet, pull on the furling line, and the Code 0 rolls back around the luff and can then be lowered to the deck.

Code 0 sails should be set on a bowsprit or retractable extension, such as a sprit or prod. This moves its furling drum forward and clear of both the genoa roller-furling drum and the bow pulpit.

Sail selection
To pick the right off-wind sail or sails, you need to consider four criteria.

Ease of handling In general, the smaller the crew and the lower the experience level, the easier the sail must be to handle. A Code 0 sail, with its roller-furling capability, is the easiest to set and drop. If conditions require it, a rolled Code 0 can remain hoisted until conditions allow it to be lowered safely. And, of course, it is also easy to gybe.

An asymmetric spinnaker, when used with a spinnaker sock, or “snuffer,” is nearly as easy to handle as the Code 0. The symmetric spinnaker is the most difficult to handle because of its large size, pole, and extra rigging. It’s also more difficult to gybe. Although a snuffer removes some of the control problems when lowering the sail, it still requires the most skill and manpower.

Cost The cost includes more than the sail itself. It comprises the spinnaker pole and fittings, spinnaker sock, sheets, control lines, and perhaps a furling system. In terms of total cost, a symmetric chute is the most expensive, a Code 0 the second most expensive, and an asymmetric the least costly.

Performance If you are making long offshore passages and expect to do a lot of downwind sailing, a good inventory of downwind sails can take days off your passage time. If you are planning shorter passages, the extra speed you get from multiple downwind sails will be less relevant, and having just one downwind sail may be sufficient. If you plan to carry only one off-wind sail, an asymmetric will be the most flexible.

Storage Sails and their associated gear obviously take up space, so this is an important consideration. A Code 0 sail requires the least stowage space (when furled), followed by the asymmetric and the symmetric.

Design
Downwind sails can be made to reflect the target wind angle you want to sail. Smaller apparent-wind angles require a flatter shape; higher apparent-wind angles require a fuller shape for best performance.

Construction Symmetric and asymmetric off-wind cruising sails are often made of nylon fabrics in .75-ounce, 1.5-ounce, 2.2-ounce (sometimes called Force 9) weights.

Polyester fabric, such as Dacron, is often used for Code 0 sails in weights that can range from 1.9 ounce through 4.5 ounce. Mylar, polyester, and exotic fibers like Spectra and Vectran can be used in high-tech laminates for downwind sails. But unless you want a racing sail, my recommendation is to stick with a nylon fabric. It remains the most cost effective, even though it doesn’t hold its shape quite as well as polyester or laminated fabric. I’ve been caught by sudden wind gusts when cruising, and I’ve seen how nylon’s stretchiness allows it to absorb those gust-induced shock loads far better than a polyester or laminate.

For a cruising boat over 35 feet, or for any boat that sails shorthanded, 1.5-ounce cloth should be the minimum weight. In light air a 1.5-ounce sail may not fly quite as high or full as a sail made from a lighter-weight cloth, but it will fly a lot better and more efficiently than a heavy polyester roller-furling genoa. Nylon is more susceptible to UV degradation than polyester. That’s another reason to choose a heavier-weight and more durable nylon cloth. Many cruising sailors shun spinnakers; they think they’re too much work—or too dangerous. I respectfully disagree. The design of the sail and the equipment that controls them are exceptional. Add a couple of downwind sails to your inventory and spice up your cruising life.

Conclusion
• Consider a Code 0 if you want a sail that’s the easiest and safest to set, gybe, and drop.
• For good performance over a broad range of wind directions and velocities, consider an asymmetric. To make handling it easier, use a spinnaker sock.
• If you sail a lot with the wind well aft, the symmetric sail, albeit the most difficult to handle, is the best performer.


Chip Lawson has been a sailmaker and rigger for over 40 years; he is also a licensed private pilot, a diving instructor, and a veteran offshore cruiser and racer. He can be reached at chip@lawsonsails.com.

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