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",^[1] "aspin",^[2] "A-sail",^[3] or gennaker, 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 and floats 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 a bowsprit.^[4]

When attached to a long bowsprit or "prod", an asymmetrical spinnaker can be larger than a conventional spinnaker, since it can be carried further forward of the boat than is possible with a conventional spinnaker pole and the foot of the sail can extend to deck level.^[1] Many modern sailboats have retractable bowsprits to enable this expansion.

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 smaller 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 since more power can be generated. This may not, however, result in faster progress to a downwind destination. Since an asymmetric spinnaker is positioned to leeward and behind the mainsail, and since it has a flatter shape than a conventional spinnaker, an asymmetrical may not sail as deeply downwind.^[1] This is demonstrated by a Polar diagram (sailing) showing the theoretical predicted boat speeds and carrying angles of a symmetrical vs. an asymmetrical spinnaker.

On cruising sailboats, a modestly sized asymmetrical spinnaker can tacked to a centerline bow pulpit, anchor roller or a furled headsail, and can be known by other names, like "cruising chute" or a gennaker. In this duty, it is often paired with a Spinnaker chute or "sock" for simpler or short-handed setting and retrieving.

If the spinnaker is mounted to a bow pulpit or short bowsprit, it may not be possible to fly the spinnaker and the jib at the same time, since the spinnaker will be shadowed by the jib. In this duty, the jib should be dropped or furled when the spinnaker is in use.