A spinnaker is a special type of sail that is designed specifically for sailing downwind (with the wind behind the boat). It resembles a parachute somewhat in both construction and appearance, and when deployed, it fills with wind and balloons out in front of the boat. It is constructed of very lightweight nylon fabric, and often using a bright color.

A spinnaker is a type of airfoil and does generate lift, contrary to a 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 with 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.

Types of spinnakers

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There are mainly two categories of spinnakers, symmetric and asymmetric depending on whether a plane of symmetry exists for that particular sail.

Symmetric Spinnakers

The symmetric one is the most classic type, running symmetrical alongside the boat with a guy running from each corner of the sail, the windward one stabilized with a spinnaker pole. The spinnaker pole must be moved in each jibe, and is therefore quite difficult for beginners to use. However, it can be sailed in all downwind wind directions. Symmetric spinnakers develop most of their lift on the top surface, which makes them far less prone to broaching than other modern sails, and thus suitable for running straight downwind. When correctly set, the leading edges of a symmetric spinnaker (which is most of the top half of the sail) should be nearly parallel to the wind, so the flow of air over the leading edge remains attached. The balloon-like shape allows the maximum amount of attached flow over the lee side of the sail. The symmetric spinnaker also requires care when packing, since the three corners must be available on the top of the packing.

Asymmetric Spinnakers

The asymmetrical spinnaker is a more recent approach to the spinnaker. Its modern-day origins may lie with the Sydney Harbour 18 foot skiffs and date from at least the late 1980s. The tack of the sail may be attached at the bow like a genoa but is more frequently mounted on a bowsprit. It has two sheets very much like a genoa but no guy. It is very easy to jibe since it only requires releasing one sheet and pulling in the other one. Asymmetric spinnakers are unable to sail directly downwind and so instead the boat will sail a zig-zag course downwind, gybing at the corners. An asymmetric spinnaker is particularly effective on fast planing dinghies as their speed generates an apparent wind on the bow allowing them to sail more directly downwind. It is also particularly useful in cruising yachts in the form of a cruising chute. Here the ease of handling is important and it is less likely to be used with a bowsprit.

Objects with Spinnaker Shape

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In Portsmouth, UK, the construction of the Spinnaker Tower was completed in 2005, and the tower opened to the public on 18 October.

External links

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• Spinnaker How-To

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