Spinnaker

From Academic Kids

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

Contents

Function

Contrary to popular belief, a spinnaker is an airfoil and does generate lift. 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.

Types of spinnakers

There are mainly two categories of spinnakers, symmetric and asymmetric depending on whether a plane of symmetry exists for that particular sail. 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 asymmetrical spinnaker is a new approach to the spinnaker, introduced in the 1993 America's Cup. It is an asymmetric sail, fastened like a genoa but much larger. It is very easy to gybe with, since it only requires loosing the windward guy and hauling in on the other one. Often is the lower corner extended from the boat to give the sail more wind. A big advantage with this asymmetrical spinnaker is the much reduced chance of broaching. Broaching usually occurs with symmetric spinnaker when a large blow makes the sail collapse and fill from the wrong side. This results in an extreme sideways force, the boat turns quickly towards the wind while heeling very, very much to the lee. A broach is very uncomfortable and dangerous, and sometimes leads to a complete capsize.

The symmetric spinnaker also requires care when packing, since the three corners must be available on the top of the packing.

Template:Sail Types

Objects with Spinnaker Shape

In Portsmouth the construction of the Spinnaker Tower is due to be complete in the Summer of 2005.

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