Spinnaker
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Bear of Britain, a Farr 52 with masthead spinnaker in front of [[Calshot Spit |
]] 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 brighly 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.
There are two main categories of spinnakers, symmetric and asymmetric depending on whether a plane of symmetry exists for that particular sail.
Symmetric Spinnakers
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420 class dinghies with 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
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RS K6 keelboat with an asymmetric spinnaker on a bowsprit. |
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.
In
Portsmouth,
UK, the construction of the
Spinnaker Tower was completed in 2005, and the tower opened to the public on 18 October.
*
Spinnaker How-To*
Flying a Spinnaker, Washington Yacht Club, with instructions on spinnaker use and how to avoid a broach.
