Inside the Asymmetrical Spinnaker
by Sandy Goodall
Asymmetrical spinnakers (or aspins) have become so popular in the last few years that most sailors are aware of what they are and how to use them. But there are many versions of these sails, and sailmakers are constantly experimenting with different shapes and sizes in cruising and racing aspins-for all-around use or for sailing in very specific conditions.
It would be difficult, if not impossible, to get much of a consensus from competing sailmakers, regarding the "best" or "correct" size or shape for this type of sail. However, here are some of the things that I have discovered by designing and using aspins on a wide variety of racing and cruising boats over the last several years.
One of the biggest attractions with aspins is that because of their simplicity, they require less gear and fewer crew to handle than symmetrical kites, and that makes us more likely to use them more often. They make downwind sailing much more enjoyable, not to mention faster, and are extremely versatile sails in terms of achievable angles.
Some of the early predecessors to modern aspins were simply large, deep, lightly built (nylon) headsails, hanked to the forestay. These "fat drifters" were able to pull a boat off the wind surprisingly well, and because they were so easy to set and use, they were popular, particularly with cruising sailors. Slowly, the concept evolved, and once the luff was released from the forestay, the sail became a true "flying" sail-a spinnaker rather than a headsail.
Then came the search for a more effective size and shape. The best size and shape has a lot to do with the intended use. Racing boats will have several different aspins, of different sizes, shapes and materials, for different wind angles and wind speeds. But for all-around use, I think many sailmakers would now agree that a good all purpose aspin will have a luff length that is as long as possible, without the sail dragging in the water when it is hanging straight down in no wind.
Foot length has been, and still is, a topic for debate. Many sailmakers have marketed an all-around aspin design where the foot was approximately 160% of the boat's J measurement. I have found that with a modern (somewhat flatter) cross section shape, these sails work very well with a foot length of 180% of the J, just like the traditional symmetrical kites. Wider seems to want to collapse in light airs, while narrower can feel a bit underpowered.
Girth at half height also varies greatly. Aspins for racing One Designs, or where the aspin must replace a dedicated running symmetrical, often have a midgirth that is significantly larger than the foot girth. But the resulting shape has definite limitations regarding how "hot" you can reach. The fact is, the more midgirth you are trying to fly, the more camber (depth) is required to support those shoulders, so a "wide" sail usually becomes a "full" sail.
For best all-around performance, without having a whole quiver of different shapes and sizes to draw from, I have found that an aspin with max luff, foot length of 180% of J, and a midgirth of the same width as the foot, or just slightly narrower than the foot, works surprisingly well. At first glance, such a sail might seem a bit narrow at the midgirth, but this means that the sail is not carrying such large "shoulders" that it needs to be full. Under closer reaching angles, the sail can be trimmed with a tighter, straighter luff and the cross section shape will flatten accordingly. I am always amazed at how high these sails can be carried effectively, and yet still project out to weather when running deeper angles.
Clew height is another variable. As the sail is eased, the leech tension is reduced and the sail leech twists away to leeward. A good way to effectively retain some "downward" pull on the leech is to have the clew cut fairly high--typically at or near boom height.
Combine these basic parameters with a nicely rounded entry and a fairly flat exit, and you have a sail that can be trimmed through a surprisingly wide range of flying shapes, depending on the angle you are trying to sail. Tack height, luff tension, sheet tension and twist all have a significant effect on the resulting flying shape.
A "true" radial panel layout (no crosscut panels) handles the widely varying load paths these sails can have under different conditions. For light air use, of course, the .5 oz kites will fill sooner, but for all-around use, .75 oz or 1.5 oz (depending on boat size) delivers a more robust sail that can tolerate a bit more abuse (fun!).
Add a modern snuffer, such as the ATN, and you can be sure to avoid any anxiety connected to setting, jibing or dowsing the sail, even short handed. Now all you have to do is decide whether to rig your sheets for "inside" or "outside" jibes!
Next Month: Rigging and Handling Your Asymmetrical Spinnaker
About the Author: Sandy Goodall, FX Sails Head of Design, has 30 years sailmaking experience and is the former technical director and head of design for Elvström Sails Denmark.