UK Sailmakers' Encyclopedia of Sails
4.4 - Asymmetrical Spinnakers
For most sailors asymmetric spinnakers are specialty sails that fill in the performance gap between genoas and spinnakers. On the growing new breed of light-weight sport boats that carry retractable bowsprits, asymmetrics are the only chutes carried.
As the graph below shows, there is a gap between the apparent wind angle when a genoa is at the peak of its power and where a spinnaker is at the peak of its power. Asymmetrical spinnakers fill this gap much better than a genoa designed for reaching or a flat symmetrical spinnaker. Asymmetrical spinnakers have been in use since the 1970s when UK Sailmakers's Owen Torrey invented the Flasher — the asymmetrical pole-less cruising spinnaker. For over a decade they've been used on lightweight dinghies like Australia 18-foot skiffs and racing multi-hulls. But it has only been recently that asymmetrical spinna-kers have been legalized for racing on more mainstream boats.
The graph shows that the flatter asymmetrics, which fly closer to the boat, are best at tight angles, while fuller sails that lift and fly out away from the boat are better at the wider angles.
The graph shows some other interesting points:
There is a trade-off between pointing and power. Flatter sails can be carried at narrower angles, but ultimately they don't develop as much driving power as the deeper sails. Any sail produces greater power when eased slightly from its closest possible angle of trim. At wider angles the performance falls off gradually. At narrower angles the performance falls off quickly, particularly in the flatter sails.
Generally, most sailors will benefit from our All Purpose shape, which is a compromise between the flattest and fullest shapes.