An asymmetrical spinnaker is a sail used when sailing between about 90 and 165 degrees from the angle of the wind. Also known as an "asym",[1] "aspin",[2] or "A-sail"[3] it can be described as a cross between a genoa jib and a spinnaker. It is asymmetric like a genoa, but like a spinnaker, its luff is unstructured; its leading edge is allowed to float freely, unencumbered by an internal wire or hanks attaching it to a stay. Unlike a symmetric spinnaker, the asymmetric does not require a spinnaker pole, since it is fixed (tacked) to the bow or bowsprit.[4] The asymmetrical spinnaker has a larger camber than a genoa and a Spinnaker Mid-Gerth (SMG) -- also called Spinnaker Half Width (SHW) -- measurement greater than the length of its foot (a genoa is a pin-head sail so its mid-gerth dimension is shorter than its foot).
An asymmetrical spinnaker generates more lift at larger angles of attack than a genoa, providing the boat with more power when the apparent wind speed is dropping.
It can be also carried at higher angles to the wind than a conventional spinnaker. Since the apparent winds are higher at small angles, the boat should sail faster than it would with a conventional spinnaker. This may not, however, result in faster progress to a downwind destination, since less camber means that it an asymmetrical will not sail as deeply downwind as a conventional spinnaker.[1] This is demonstrated by a Polar diagram (sailing) showing the theoretical predicted boat speeds and carrying angles of a symmetrical vs. an asymmetrical spinnaker.
Polar diagram comparing the speeds and carrying angles of symmetrical with asymmetrical spinnakers when sailing
Due to its geometry, an asymmetrical spinnaker does not require the use of a spinnaker pole.[1] On cruising sailboats, the sail is often tacked to a bow pulpit, and can be known by other names, like "cruising chute" or a gennaker. In this duty, it is often paired with a Spinnaker chute for simpler or short-handed setting and retrieving.
When attached to a permanent bowsprit or "prod", an asymmetrical spinnaker can be much larger than a conventional sail (or a pulpit-connected asymmetrical), since it can be carried well forward of the boat and down to deck level.[1] Many modern sailboats have retractable bowsprits to enable this expansion.
Two sailboats flying asymmetrical spinnakers beam reaching in light wind
If the spinnaker is mounted to a bowsprit, it is 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.
Rigging is different from conventional spinnakers.[5] Like a jib, the asymmetric has two sheets and no "guy". Since there is no spinnaker pole, there is no longer need for a pole topping lift or a pole downhaul. An adjustable tack-line can be eased to allow the tack to lift, enabling the sail to also lift and its shoulders to rotate for deeper downwind angles. The asymmetric is simpler to gybe than a conventional spinnaker since it only requires releasing a sheet and pulling in the other one, passing the sail in front of the forestay.
An asymmetric spinnaker is particularly effective on fast planing dinghies and ultra-light displacement boats as their speed generates an apparent wind on the bow allowing them to sail more directly downwind.
History and Impact
In the 1960, catamaran sailors discovered that it is faster to sail downwind on a series of broad reaches with efficient airflow across the sail rather than directly downwind with the sails stalled. This technique had developed to the extent that in bar conversation at the end of one season Andrew Buckland observed that 18 Foot Skiffs had sailed all season (1982/83) without pulling the spinnaker pole back from the forestay and that all the systems could be simplified by eliminating the pole and setting the spinnaker from a bowsprit. The concept quickly evolved to a sail with a loose luff. Buckland collaborated with sailmaker Julian Bethwaite to rig and sail the first prototypes.[6][7] The first modern keelboat to incorporate a retractable bow sprit and an asymmetric spinnaker was the popular J/Boats J/105 designed in 1992.[8]
The concept has spread rapidly through the sailing world and has inspired the development of "code" sails, a family of hybrid sail designs that often combine a structured luff with positive mid-gerth that are used for very fast beam reaching and higher angles and may be furled.[9]
Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the water, on ice (iceboat) or on land over a chosen course, which is often part of a larger plan of navigation.
A jib is a triangular sail that sets ahead of the foremast of a sailing vessel. Its tack is fixed to the bowsprit, to the bows, or to the deck between the bowsprit and the foremost mast. Jibs and spinnakers are the two main types of headsails on a modern boat.
A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface.
A jibe (US) or gybe (Britain) is a sailing maneuver whereby a sailing vessel reaching downwind turns its stern through the wind, which then exerts its force from the opposite side of the vessel. Because the mainsail boom can swing across the cockpit quickly, jibes are potentially dangerous to person and rigging compared to tacking. Therefore, accidental jibes are to be avoided while the proper technique must be applied so as to control the maneuver. For square-rigged ships, this maneuver is called wearing ship.
A staysail ("stays'l") is a fore-and-aft rigged sail whose luff can be affixed to a stay running forward from a mast to the deck, the bowsprit, or to another mast.
A genoa sail is a type of large jib or staysail that extends past the mast and so overlaps the main sail when viewed from the side, sometimes eliminating it. It was originally called an "overlapping jib" and later a genoa jib. It is used on single-masted sloops and twin-masted boats such as yawls and ketches. Its larger surface area increases the speed of the craft in light to moderate winds; in high wind, a smaller jib is usually substituted, and downwind a spinnaker may be used.
A spinnaker is a sail designed specifically for sailing off the wind on courses between a reach to downwind. Spinnakers are constructed of lightweight fabric, usually nylon, and are often brightly colored. They may be designed to perform best as either a reaching or a running spinnaker, by the shaping of the panels and seams. They are attached at only three points and said to be flown.
A mainsail is a sail rigged on the main mast of a sailing vessel.
Running rigging is the rigging of a sailing vessel that is used for raising, lowering, shaping and controlling the sails on a sailing vessel—as opposed to the standing rigging, which supports the mast and bowsprit. Running rigging varies between vessels that are rigged fore and aft and those that are square-rigged.
Sail components include the features that define a sail's shape and function, plus its constituent parts from which it is manufactured. A sail may be classified in a variety of ways, including by its orientation to the vessel and its shape,. Sails are typically constructed out of flexible material that is shaped by various means, while in use, to offer an appropriate airfoil, according to the strength and apparent direction of the wind. A variety of features and fittings allow the sail to be attached to lines and spars.
A gennaker is a sail that was developed around 1990. Used when sailing downwind, it is a cross between a genoa and a spinnaker. It is not symmetric like a true spinnaker but is asymmetric like a genoa, but the gennaker is not attached to the forestay like a jib or genoa. The gennaker is rigged like a spinnaker but the tack is fastened to the hull or to a bowsprit. It has greater camber than a genoa. This is optimal for generating lift at larger angles of attack. An early form of gennaker was the "gollywhomper", used briefly in the 1870s.
A fractional rig on a sailing vessel consists of a foresail, such as a jib or genoa sail, that does not reach all the way to the top of the mast.
A wingsail, twin-skin sail or double skin sail is a variable-camber aerodynamic structure that is fitted to a marine vessel in place of conventional sails. Wingsails are analogous to airplane wings, except that they are designed to provide lift on either side to accommodate being on either tack. Whereas wings adjust camber with flaps, wingsails adjust camber with a flexible or jointed structure. Wingsails are typically mounted on an unstayed spar—often made of carbon fiber for lightness and strength. The geometry of wingsails provides more lift, and a better lift-to-drag ratio, than traditional sails. Wingsails are more complex and expensive than conventional sails.
The 3000 is a racing sailing dinghy crewed by two persons with a trapeze for the crew. Launched in 1996 as the Laser 3000, the 3000 was developed from the Laser 2, using the original Frank Bethwaite-designed planing hull combined with a new designed self-draining deck by Derek Clark. Clark also re-designed the rig, using spars and sails from premium proprietary sources and replacing the symmetric spinnaker of the Laser 2 by a larger asymmetric spinnaker (gennaker). The gennaker is chute-launched and retrieved using a single halyard line, and is set on a retractable bowsprit. Helm balance and handling were improved using a shorter-footed mainsail with two full-width battens giving a larger roach. A mast with conventional spreaders replaced the now-unusual diamond arrangement of the Laser 2.
High-performance sailing is achieved with low forward surface resistance—encountered by catamarans, sailing hydrofoils, iceboats or land sailing craft—as the sailing craft obtains motive power with its sails or aerofoils at speeds that are often faster than the wind on both upwind and downwind points of sail. Faster-than-the-wind sailing means that the apparent wind angle experienced on the moving craft is always ahead of the sail. This has generated a new concept of sailing, called "apparent wind sailing", which entails a new skill set for its practitioners, including tacking on downwind points of sail.
The Johnson 18 is an 18-foot (5.5 m) sailing dinghy designed by Rodger Martin. Although it is no longer produced, Johnson Boat Works fabricated over 100 of these sport boats before closing its doors in 1998.
Forces on sails result from movement of air that interacts with sails and gives them motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Similar principles in a rotating frame of reference apply to windmill sails and wind turbine blades, which are also wind-driven. They are differentiated from forces on wings, and propeller blades, the actions of which are not adjusted to the wind. Kites also power certain sailing craft, but do not employ a mast to support the airfoil and are beyond the scope of this article.
A sail is a tensile structure, which is made from fabric or other membrane materials, that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may be made from a combination of woven materials—including canvas or polyester cloth, laminated membranes or bonded filaments, usually in a three- or four-sided shape.
The following outline is provided as an overview of and topical guide to sailing:
Julian Bethwaite is an Australian, Sydney-based skiff sailor and sailboat designer. He wrote one chapter of his father Frank's book, Higher Performance Sailing.
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