An asymmetrical spinnaker is a sail used when sailing downwind. Also known as an "asym" or "aspin", it can be described as a cross between a genoa jib and a spinnaker. It is asymmetric like a genoa, but, the asymmetrical spinnaker is not attached to the forestay over the full length of its luff, being rigged like a spinnaker. The asymmetrical spinnaker has a larger camber than a genoa, making it optimal for generating lift at larger angles of attack, but the camber is significantly less than that of a spinnaker.
The asymmetrical spinnaker is a specialty sail used on racing boats, bridging the performance gap between a genoa, which develops maximum driving force when the apparent wind angle is between 35 and 60 degrees, and a spinnaker, which has maximum power when the apparent wind is between 100 and 140 degrees. Due to its geometry, the sail is less prone to collapsing than a spinnaker and does not require the use of spinnaker pole. The sail can benefit greatly and be much larger if the boat is equipped with a bowsprit. Some boats (e.g., the Melges 17), have retractable bowsprits for this sail.
A form of asymmetrical spinnaker is also used on cruising boats as being easier to handle than a symmetrical spinnaker and known as a "cruising shute".
Rigging is different from other spinnakers. Maximizing performance and effective sailing of asymmetrical spinnakers requires unique sail and boat trim. It is often paired with a Spinnaker chute.