Spinnaker Totally Explained

Everything about Spinnaker totally explained

A spinnaker is a special type of sail that's designed specifically for sailing off the wind from a reaching course to a downwind, for example with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it's deployed, called flying. It is constructed of very lightweight, usually nylon, fabric, and is often brightly colored. It may be optimised for a particular range of wind angles, as either a reaching or a running spinnaker, by the shaping of the panels and seams. The spinnaker is often called a kite, or a chute (as in cruising chute) because it somewhat resembles a parachute in both construction and appearance. This shouldn't be confused with the spinnaker chute which is a hull fitting sometimes used for launching and recovering the spinnaker.

Operation

A spinnaker is a type of airfoil and will generate lift, if it's 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 spinnaker, causing a reduction in lift and reduced performance.
When running downwind in heavy weather or when hit by a gust, with or without a spinnaker, there may be a tendency for a roll of increasing amplitude to build up, known as the death roll. It has been shown that this is due to aerodynamic instability of bermuda rigs when running, which can be aggravated by gusts, waves, mainsail twist, daggerboard etc too far down, hull form, and the sailing equivalent of pilot-induced oscillation. Excessive heel leads to loss of rudder effectiveness resulting the boat slewing round uncontrollably in the direction opposite to the direction of heel. This is known as broaching. Aerodynamic instability when running can be countered by easing the pole forward slightly and over-sheeting the spinnaker somewhat to stop it swinging from side to side, by reducing mainsail twist using the boom vang, and by skilfully trimming the mainsheet. Luffing carefully onto a broad reach may help to retain control, as can moving everyone's weight as far aft as possible. Reducing sail should be considered.

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 controlled by lines known as a sheet and a guy running from the lower two corners of the sail. The windward line, or guy, is attached to the corner called the tack of the sail, and is stabilized by a spinnaker pole. The leeward (downwind) line is called the sheet. It attaches to the clew of the spinnaker and is used to control the shape of the sail. 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 sailing across the wind (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. When reaching, the sail camber allows only some attached flow over the leeward side of the spinnaker. On running the spinnaker 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

The asymmetrical spinnaker is a more recent approach to the spinnaker. Its modern-day origins may lie with the Sydney Harbour 18ft 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 frequently mounted on a bowsprit, often a retracting one. If the spinnaker is mounted to a special bowsprit, it's often possible to fly the spinnaker and the jib at the same time; if not, then the spinnaker will be shadowed by the jib, and the jib should be furled when the spinnaker is in use.
The asymmetric has two sheets, very much like a jib, but isn't attached to the forestay along the length of the luff, but only at the corners. Unlike a spinnaker, the asymmetric doesn't require a spinnaker pole, since it's fixed to the bow or bowsprit. The asymmetric is very easy to jibe since it only requires releasing one sheet and pulling in the other one, passing the sail in front of the forestay. Asymmetrics are less suited to sailing directly downwind than spinnakers, and so instead the boat will often 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 spinnaker or cruising chute, where the ease of handling is important and it's less likely to be used with a bowsprit. Various types of asymmetrics exist, and a common nomenclature classifies them by code from 0 to 6. Codes 1, 3, and 5 are reaching sails, and codes 2, 4, and 6 are running sails; the code 0 is a hybrid of genoa and spinnaker, designed to work like a genoa but classified under racing rules as a spinnaker.

Code 0 The code 0 asymmetric is a tight reaching sail, the most upwind capable of the asymmetrics. The luff is as straight as possible, and the sail is flatter than other spinnakers. Due to the flatness of the code 0, it's usually made with a wire luff for strength, and of a heavier, less stretchy fabric than normal for a spinnaker. Due to the tight luff and flat cut, the code 0 can be fitted for roller furling.
Code 1 The code 1 is a light air reaching sail, where the apparent wind angles at low speeds has a significant effect to create angles of less than 90 degrees.
Code 2 The code 2 is a medium air running sail, used for apparent wind angles over 90 degrees.
Code 3 The code 3 is a medium air reaching sail, used for apparent wind angles near 90 degrees.
Code 4 The code 4 is a heavy air running sail, used in the heaviest winds normally expected.
Code 5 The code 5 is a heavy air reaching sail, used in the heaviest winds normally expected.
Code 6 The code 6 is a storm sail, for running in storm conditions.

Spinnakers for cruising boats are starting to be patterned after the roller furling code 0 racing spinnakers, as they provide the easiest handling. North Sails, for example, offers three gennaker sails, based on the racing code 0 asymmetrics, with different sizes and cambers for varying angles and wind speeds. Other manufacturers offer similar cruising code 0 designs under different names, such as the screecher and reacher for upwind and downwind use respectively.

Flying the spinnaker

Since they're only used on certain points of sail, raising and lowering the spinnaker is a task that's 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 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're generally set up before setting sail, and left in place even when the spinnaker is stowed.
   Since symmetrics are downwind sails, they're never tacked, they're only jibed. When jibing a symmetric, the pole is moved to the bow, where the sail is detached, 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 detached from the spinnaker pole, and the guy is released. This allows the spinnaker 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. To keep the tack near the centerline of the boat, it may be attached to the forestay with a sliding collar (often riding over the furled jib on parrel beads or similar device). This allows the tack to slide up and down the forestay to adjust the luff tension. On racing boats, the tack of the asymmetric is often rigged to a retractable bowsprit, which increases the foretriangle area and prevents interference with the jib. As this trend becomes more popular in racing boats, it may result in similar adaptations to cruising boats as well.
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's 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.
Retrieving 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's packed into the turtle.

Dousing socks

The dousing sock, "spinnaker sleeve", 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 or the tack is released and the sock is pulled down, gathering the sail. The halyard is then dropped and the sail may be packed away.

Spinnaker chute

A spinnaker chute is usually a tube or aperture in the deck close to the forestay, for the launching and recovery of the spinnaker. They are most commonly found on modern dinghy designs, and updated older classes. To allow recovery of the spinnaker into the chute, one or more recovery patches are fitted to the spinnaker, to which the tail of the spinnaker halyard is attached or passed through. The spinnaker and its halyard thus form a continuous loop, passing through the chute.
If the spinnaker chute penetrates the hull and is required to be watertight, it takes the form of a hard tube sealed to the hull at both ends. If a watertight arrangement isn't required, a cloth tube may be used to contain the lowered spinnaker.

Etymology of the Word

The first boat to fly a spinnaker was called the "Sphinx". The friends of Sphinx's owner jokingly referred to the sail as "Sphinx's half-acre". It was later to be abbreviated as SPINNAKER.

Further Information

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