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. of Britain spinnaker.jpg|thumb|300px|http://www.bearofbritain.com/ 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 specificaly for sailing off the wind from a reaching course to a downwind . 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. It may be optimised for a particular range of wind angles, as either a reaching or a running spinnaker.
Operation A spinnaker is a type of airfoil and will generate lift, if it is flown at a reaching angle. 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, running spinnakers have extreme amounts of camber, making them nearly hemispherical in form. The large camber maximizes the drag. Reaching spinnakers have less camber as they operate partially with an airflow that generates 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, in a reaching spinnalker, 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. The large spinnaker and mainsail generate a large aerodynamic force, while the hydrodynamic force remains unchanged. A sudden increase in aerodynamic force due to a wind increase can cause imbalance between the aerodynamic and hydrodynamic forces, with the aerodynamic force tending to cause a broach, laying the mast over to leeward, resulting in loss of steering control. Unlike broaches without spinnaker, a broach with a spinnaker is actually more likely when reaching than when running. 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 effort forward. Types of spinnakers There are two main categories of spinnakers, symmetric and asymmetric depending on whether a plane of symmetry exists for that particular sail. Asymmetric spinnakers operate more like a jib, generating lift from the side, rather than the top like a symmetric spinnaker. This makes asymmetrics a better choice on reaching courses than symmetric spinnakers, which excel when running. While a fully equipped racing boat might have a number of spinnakers, both symmetric and asymmetric, to cover all courses and wind conditions, cruising boats almost always use an asymmetric, due to the broader application and easier handling afforded by the asymmetric. 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 quite difficult for beginners to use. However, it can be sailed in all downwind wind directions. Symmetric spinnakers when reaching, develop most of their lift on the forward quarter, where the airflow remains attached. When correctly set, for reaching the leading edges of a symmetric spinnaker should be nearly parallel to the wind, so the flow of air over the leading edge remains attached. On reaching the sail camber allows only some attached flow over the leeward side of the spinnaker. On running the spinaker is angled for maximum drag, with the spinnaker pole at right angles to the apparent wind. The symmetric spinnaker also requires care when packing, since the three corners must be available on the top of the packing. Asymmetric Spinnakers
Flying the spinnaker Since they are only used on certain points of sail, raising and lowering the spinnaker is a task that is often performed while under sail. Due to the size of spinnakers (the spinnaker is often double or more the size of the mainsail) this can be a difficult operation, since the sail will immediately catch the wind. Rigging the symmetric spinnaker Typically the symmetric spinnaker is packed in its own bag, called a turtle, with the three corners on top for ready access. The clews (lower corners) are controlled by lines called sheets, which lead from the clews back to the stern of the boat, and serve to control the sail position. Symmetric spinnakers have the windward clew secured to a spinnaker pole which is attached to the mast and holds the windward edge of the sail in position. Lines that control the spinnaker pole are called guys. In small boats, a single line may serve as a combination sheet/guy. The head (top corner) is attached to the spinnaker halyard, which is used to raise the sail up the mast. The lee guy is often referred to as the spinnaker sheet The spinnaker pole may be allowed to raise and lower with the force of the wind, or it may have lines attached to it to raise (the topping lift) and lower (the foreguy) the angle of the pole. If these lines are used, they are generally set up before setting sail, and left in place even when the spinnaker is stowed. Since symmetrics are downwind sails, they are never tacked, they are only jibed. When jibing a symmetric, the pole is moved to the bow, where the sail is detatched, and the opposite corner attached. This corner now becomes the windward corner. The guys are adjusted as before to set the sail angle on the new course. To retrieve the spinnaker, the windward corner is detatched from the spinnaker pole, and the guy is released. This allows the spinnker to collapse into the shadow of the mainsail, where the foot is gathered by a crewmember. The halyard is then lowered, and a crewmember gathers the sail and stuffs it carefully into the turtle, corners out, and ready for the next deployment. Rigging the asymmetric spinnaker Like the symmetric, the asymmetric is often stored in a turtle, with the corners on top for easy access. Unlike the symmetric, asymmetrics have the tack attached to the bow or a bowsprit (often retractable), and have two guys attached to the clew. The head of the sail is attached to the spinnaker halyard, which is used to raise the sail. The guys are passed to either side of the forestay, one to each corner; they may be passed outside the tack of the asymmetric, or between the tack and the forestay. The guy on the downwind side of the hull is used to set the angle, and the opposite guy is left slack. Often a tack line is used at leading edge to provide adjustable tension on the luff of the spinnaker, or it may be attached to the forestay with a sliding collar (often riding over the furled jib on parrell beads or similar device) allowing the tack to slide up and down. Jibing with the asymmetric is much less complex than the symmetric, due to the lack of the spinnaker pole. Much like a jib, all that is required is to change guys--however, since the asymmetric still flies in front of the forestay, the operation is reversed. The sheet is slackened, and the opposite guy is pulled in, which allows the sail to pass around in front of the forestay, and then be sheeted in on the new lee side of the boat. Retreiving the asymmetric is similar to the process for the symmetric. The guys are released, allowing the sail to collapse to the front of the boat. The foot of the sail is then gathered, and the halyard released and the head of the sail lowered, where it is packed into the turtle. Dousing socks The dousing sock, snuffer, or just sock, is a device used to make deploying and retrieving the spinnaker a much easier task. The sock is a long fabric tube with a ring in one end to hold it open. Since the spinnaker is stored in the sock, the first step is to set up the sock. Two lines are attached to the sock; one is attached to a bridle on the ring, for pulling the sock down, and one is up the inside, from the ring, through the top, and back down, for raising the sock; these lines may be two ends of the same line, to form a loop. The head of the spinnaker is attached the top of the sock and the ring runs down to the tack. The resulting bundle is stuffed into the spinnaker bag. The top of the sock will have provisions for attaching to the spinnaker halyard. The spinnaker is raised as normal, but with the sock in place the spinnaker is unable to catch the wind. Once the spinnaker is raised and the guys are ready to set, the sock is raised, releasing the spinnaker. The sock remains bundled up at the head of the sail while the spinnaker is deployed. To retrieve the spinnaker, the sheet is released and the sock is pulled down, gathering the sail. The halyard is then dropped and the sail may be packed away. | ||||||||||
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