Spinnaker!

spin‧na‧ker  [spin-uh-ker]

–noun Nautical a large, usually triangular sail carried by a yacht as a headsail when running before the wind or when the wind is abaft the beam.

A spinnaker is a special type of sail that is designed specifically for sailing downwind (with the wind behind the boat). The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of very lightweight, usually nylon, fabric, and is often brightly colored. The spinnaker is often called a chute, as it somewhat resembles a parachute in both construction and appearance, or a kite.

A spinnaker is a type of airfoil and does generate lift, contrary to popular belief. Since the lift and drag generated by the spinnaker both act to move the boat forward, the lift to drag ratio is unimportant. The goal then is to generate the maximum amount of lift possible with no consideration of drag. Because of this, spinnakers have extreme amounts of camber, making them nearly hemispherical in form. The large camber maximizes the low pressure on the downwind side of the sail, generating the lift.

A well designed spinnaker will have taut leading edges when filled; leading edges that curl in will both reduce the lift and risk a collapse of the spinnaker. A well designed spinnaker will also have a smooth curve when filled, with no bubbles or depressions caused by inconsistent stretching of the sail fabric. Any deviations from a smooth curve will cause the airflow over the leeward side of the sail to separate, causing a reduction in lift and reduced performance.

When sailing downwind, with or without a spinnaker, there may be a tendency for the boat to become unstable and suddenly turn up into the wind. This is known as a broach. Use of a spinnaker can increase this danger, as the forces involved are magnified by the large sail area. Care must be taken that the stern does not lift from the water, or steering control will be lost, likley causing a broach. Another cause of broaching with a spinnaker is a collapse of the spinnaker, which will radically change the balance of the boat as the large source of lift at the bow suddenly stops. Unlike broaches with other types of sails, a broach with a spinnaker is actually more likely when trying to reach than when trying to run, so bearing off the wind is the correct response to signs of an impending broach. Depowering the mainsail may also help restore the balance by moving the center of lift forward.