Asymmetrical spinnaker

An asymmetrical spinnaker is a sail used when sailing between about 90 and 165 degrees from the angle of the wind. Also known as an "asym", "aspin", or "A-sail" it can be described as a cross between a genoa jib and a spinnaker. It is asymmetric like a genoa, but like a spinnaker, its luff is unstructured; its leading edge is allowed to float freely, unencumbered by an internal wire or hanks attaching it to a stay. Unlike a symmetric spinnaker, the asymmetric does not require a spinnaker pole, since it is fixed (tacked) to the bow or bowsprit. The asymmetrical spinnaker has a larger camber than a genoa and a Spinnaker Mid-Gerth (SMG) -- also called Spinnaker Half Width (SHW) -- measurement greater than the length of its foot (a genoa is a pin-head sail so its mid-gerth dimension is shorter than its foot).

An asymmetrical spinnaker generates more lift at larger angles of attack than a genoa, providing the boat with more power when the apparent wind speed is dropping.

It can be also carried at higher angles to the wind than a conventional spinnaker. Since the apparent winds are higher at small angles, the boat should sail faster than it would with a conventional spinnaker. This may not, however, result in faster progress to a downwind destination, since less camber means that it an asymmetrical will not sail as deeply downwind as a conventional spinnaker. This is demonstrated by a Polar diagram (sailing) showing the theoretical predicted boat speeds and carrying angles of a symmetrical vs. an asymmetrical spinnaker.

Due to its geometry, an asymmetrical spinnaker does not require the use of a spinnaker pole. On cruising sailboats, the sail is often tacked to a bow pulpit, and can be known by other names, like "cruising chute" or a gennaker. In this duty, it is often paired with a Spinnaker chute for simpler or short-handed setting and retrieving.

When attached to a permanent bowsprit or "prod", an asymmetrical spinnaker can be much larger than a conventional sail (or a pulpit-connected asymmetrical), since it can be carried well forward of the boat and down to deck level. Many modern sailboats have retractable bowsprits to enable this expansion.



If the spinnaker is mounted to a bowsprit, it is often possible to fly the spinnaker and the jib at the same time; if not, then the spinnaker will be shadowed by the jib, and the jib should be furled when the spinnaker is in use.

Rigging is different from conventional spinnakers. Like a jib, the asymmetric has two sheets and no "guy". Since there is no spinnaker pole, there is no longer need for a pole topping lift or a pole downhaul. An adjustable tack-line can be eased to allow the tack to lift, enabling the sail to also lift and its shoulders to rotate for deeper downwind angles. The asymmetric is simpler to gybe than a conventional spinnaker since it only requires releasing a sheet and pulling in the other one, passing the sail in front of the forestay.

An asymmetric spinnaker is particularly effective on fast planing dinghies and ultra-light displacement boats as their speed generates an apparent wind on the bow allowing them to sail more directly downwind.

History and Impact
In the 1960, catamaran sailors discovered that it is faster to sail downwind on a series of broad reaches with efficient airflow across the sail rather than directly downwind with the sails stalled. This technique had developed to the extent that in bar conversation at the end of one season Andrew Buckland observed that 18 Foot Skiffs had sailed all season (1982/83) without pulling the spinnaker pole back from the forestay and that all the systems could be simplified by eliminating the pole and setting the spinnaker from a bowsprit. The concept quickly evolved to a sail with a loose luff. Buckland collaborated with sailmaker Julian Bethwaite to rig and sail the first prototypes. The first modern keelboat to incorporate a retractable bow sprit and an asymmetric spinnaker was the popular J/Boats J/105 designed in 1992.

The concept has spread rapidly through the sailing world and has inspired the development of "code" sails, a family of hybrid sail designs that often combine a structured luff with positive mid-gerth that are used for very fast beam reaching and higher angles and may be furled.