UK Sailmakers's 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.
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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.
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