Sail

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For other uses, see Sail (disambiguation).

A sail is any type of surface intended to generate thrust by being placed in a wind — in essence a vertically-oriented wing. Sails are used in sailing.

Contents 1 History of sails 2 Sail aerodynamics 3 Use of sails 4 Parts of the Sail 5 Sail Types 6 Sail Construction 7 Advances in Sail Materials and Manufacture 8 See also 8.1 Types of sails 9 External links

[edit] History of sails

Sails were invented in the ancient age, and was the most important source of propulsion during the age of sail.

[edit] Sail aerodynamics

Sails propel the boat in one of two ways. When the boat is going in the direction of the wind (i.e. downwind - see Points of sail), the sails merely act to catch the air as it flows by. This method of propulsion is simple enough that it really doesn't need any additional explanation — the wind simply blows the boat downwind and the sails act more like a parachute than like a wing.

The other way sails propel the boat occurs when the boat is traveling across the wind or into the wind. In this situation, the sails propel the boat by redirecting the wind coming in from the side towards the rear. In accordance with the law of conservation of momentum, air is redirected backwards, making the boat go forward. This driving force is called lift.

(Often, Bernoulli's principle is invoked to explain the action of sails and other airfoils, but this explanation is often based on the incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge.)

The lift can be resolved into two components; forward force and sideways force (see vector space for an explanation of this principle). These forces are acting against opposing forces generated by the hull and the keel

On a sailing boat, a keel or centreboard is used to convert this lateral force into forward movement of the boat with some sideways leaning, or 'heel'. The forward motion is created because the shape of the keel has a much smaller cross section in the fore and aft axis and a very large cross section on the athwart axis (across the beam of the boat). The resistance to motion along the smallest cross section is "easy" while motion across the large cross section is "hard". Forces across the boat are resolved by balancing the sideways force by the mass of the ballast in the keel being raised against gravity by the boat heeling, while forward forces are balanced by velocity through the water and friction between the hull, keel and the water. In other words it is easier for the sail to push the boat forward rather than sideways.

[edit] Use of sails

Sails are primarily used at sea, on sailing ships as a propulsion system. For purposes of commerce, sails have been greatly superseded by other forms of propulsion, such as the internal combustion engine. For recreation, however, sailing vessels remain popular.

The most familiar type of sailboat, a small pleasure yacht, usually has a sail-plan called a sloop. This has two fore-and-aft sails: the mainsail and the jib.

The mainsail extends aftward and is secured the whole length of its edges to the mast and to a boom also hung from the mast. The sails of tall ships are attached to wooden timbers or "spars".

The jib is secured along its leading edge to a forestay (strong wire) strung from the top of the mast to the bowsprit on the bow (nose) of the boat. A genoa is also used on some boats. It is a type of jib that is larger, and cut so that it is fuller than an ordinary jib.

Fore-and-aft sails can be switched from one side of the boat to the other, in order to alter the boat's course. When the boat's stern crosses the wind, this is called jibing; when the bow crosses the wind, it is called tacking. Tacking repeatedly from port to starboard and/or vice versa, called "beating", is done in order to allow the boat to follow a course into the wind.

A primary feature of a properly designed sail is an amount of "draft", caused by curvature of the surface of the sail. When the sail is oriented into the wind, this curvature induces lift, much like the wing of an airplane. Modern sails are manufactured with a combination of broadseaming and non-stretch fabric (ref New technology below). The former adds draft, while the latter allows the sail to keep a constant shape as the wind pressure increases. The draft of the sail can be reduced in stronger winds by use of a cunningham and outhaul, and also by increasing the downward pressure of the boom by use of a boom vang.

Other sail powered machines include ice yachts and windmills.

Sail construction is governed by the science of aerodynamics.

[edit] Parts of the Sail

Main article: Parts of a sail

The lower edge of a triangular sail is called the "foot" of the sail, while the upper point is known as the "head". The lower two points of the sail, on either end of the foot, are called the "tack" (forward) and "clew" (aft). The forward edge of the sail is called the "luff" (from which derives the term "luffing", a rippling of the sail when the angle of the wind fails to maintain a good aerodynamic shape near the luff). The aft edge of a sail is called the "leech".

Modern sails are designed such that the warp and the weft of the sailcloth are oriented parallel to the luff and foot of the sail. This places the most stretchable axis of the cloth along the diagonal axis (parallel to the leech), and makes it possible for sailors to reduce the draft of the sail by tensioning the sail, mast and boom in various ways.

Often tell-tales, small pieces of yarn, are attached to the sail. They are used as a guide when trimming the sail.

An alternative approach to sail design is that used in Junks, originally an oriental design. It uses horizontal sail curving to produce an efficient and easily controlled sail-plan. [1].

[edit] Sail Types

Modern sails can be classified into three main categories: Mainsail, Headsail, and Spinnaker or downwind sail (also termed Kite). Special-purpose sails are often a variation of the three main categories. Most modern yachts including bermuda rig, ketch and yawl boats have a sail "inventory" which usually includes more than one of these types of sails. Although the mainsail is “permanently” hoisted while sailing, headsails and spinnakers can be changed depending on the particular weather conditions to allow better handling and speed.

Mainsails as the name implies are the main element of the sailplan. A "motor" as well as a rudder for the boat, mainsails can be as simple as a traditional triangle-shaped, cross-cut sail (see Sail Construction below) and as complex as a quad-lateral, full-batten, exotic material beast such as those sported by America's Cup Class boats. In most cases, the mainsail isn’t changed while sailing although there are mechanisms to reduce its surface if the wind is very strong (a technique called reefing). In extreme weather, a mainsail can be folded and a trysail hoisted to allow steerage without endangering the boat.

Headsails are the main driving sails when going upwind (sailing towards the wind). There are many types of headsails with Genoa and Jib being the most commonly used. Both these types have different subtypes depending on their intended use. Headsails are usually classified according to their weight (that is, the relative weight of the sailcloth used) and size or total area of the sail. A common classification is numbering from 1 to 3 (larger to smaller) with a description of the use for example: #1 Heavy or #1 Medium/Light. Special types of headsails include the Gennaker (also named Code 0 by some sailmakers), the drifter (a type of Genoa that is used like an assymetrical spinnaker), the screecher (essentially a large Genoa), the windseeker and storm jib. Certain Genoas and Jibs also have battens which assist in maintaining an optimal shape for the sail.

Spinnakers are used for reaching and running (downwind sailing). They are very light and have a balloon-like shape. As with headsails there are many types of spinnakers depending on the shape, area and cloth weight. Symmetrical spinnakers are most efficient on runs and dead runs (sailing with wind coming directly from behind) while asymmetric spinnakers are very efficient in reaching (the wind coming from the rear but at an angle to the boat or from the side).

[edit] Sail Construction

A sail might look flat when lying on the floor but once it's hoisted, it becomes a three-dimensional, curved surface, in essence an airfoil. In order for a sail to be "built", it has to be designed in a number of elements (or panels) which are cut and sewn together to form the foil. In older days, this was rightfully considered an art which was later complimented (and arguably overshadowed) by technology. With the advent of computers, sail manufacturers were able to model their sails using special computer-aided design (CAD) programs and directly feed the data to very accurate laser plotters/cutters which cut the panels from rolls of sail cloth, replacing the traditional manual process (scissors).

The key features that distinguish a "fast" from a "slow" sail are its shape related to the particular boat and rig and its ability to consistently maintain that shape. These two features rely mostly on the design of the sail (the way that the panels are placed with one another) and the sail cloth used. The traditional parallel-panel (cross-cut) gave way to more complex (radial) designs where the panels have different shapes for the top, mid, and lower sections of the sail depending on pressure of the air caused by its flow over the sail surface. Again aided by CAD and special modelling software the sailmakers use cloths of different weight, placing heavier cloth panels where there is more stress and lighter cloth where there is less to make savings in weight.

Older fabrics (especially cotton and low budget synthetic), have the tendency to stretch with wind pressure which results in distorted and consequently non-efficient sail shapes. Moreover, the cloth itself is heavy which also adds to the inefficiency. Synthetic materials such as Nylon and Dacron were followed by advanced sail cloths made from exotic material yarns such as kevlar, carbon fiber, spectra, PBO, and vectran. These materials were a breakthrough in sail technology as they provided the raw material in the manufacture of low-stretch, low-weight and long-life sail cloths. Manufacturers were able to use different weights of yarn to weave cloths with exceptional properties. In order to protect the yarns themselves from the elements (salt water and sunlight) and harsh use (flagging and chaffing), the woven cloth is "sandwiched" between two films of Mylar and placed in special ovens under pressure to bond into a single body, a process called lamination. Once the panels are sewn together, the sailmakers complete the sail by placing the finishing elements such as the leech and foot lines, protective patches in the areas where the sail will scrape against hardware (stanchions, spreaders), steel rings and straps at the tack and clew, cleats, batten pockets (if required) and sail numbers.

[edit] Advances in Sail Materials and Manufacture

In addition to advances in the exotic materials and consequent cloths themselves, manufacturers have also progressed the manufacturing process with the creation of glued and more importantly molded sails. Glued sails are regular panelled sails but instead of sewing the pieces together, the sailmaker uses a special, ultra-strong polymer glue which bonds through the use of ultrasound. In molding, a curved mold is designed and created in the optimum (three dimensional) shape of the sail that the sailmaker wants to produce. A film of Mylar is placed on the mold and a special gantry hovers over the film laying the yarns based on instructions of a computer that has the model of the sail. Once this is done, a second sheet of Mylar film is placed on top and the whole mold (with the sail) is placed in a vacuum oven which causes the materials to bond (curing). The result is a smooth sail which is lighter and has a wider effective wind range (the minimum and maximum wind speed that the sail can withstand and be effective).

Molding initially targeted high-end competition boats because of the costs of the sails produced but has steadily moved on to cover cruising yachts although panelled (woven) sails account for the majority of sails (racing or recreational) used around the world. The concept of molded sails was introduced by Sobstad Sails with its Genesis line but did not maintain consistent product performance. North Sails introduced its successful 3DL product line which also resulted in a legal battle with Sobstad. Variations of the molding sailmaking process are used by other leading sail manufacturers such as Quantum with the Fusion-M line and Doyle Sailmakers with the Stratis line. Other sailmakers are producing lines which make use of molding concepts although not necessarily the production process itself such as the UK-Halsey TapeDrive line.

[edit] See also

• Sailing
• Cruising (maritime)
• Points of sail
• Sail-plan
• Rigging
• Wing
• Rudder
• Fin
• Solar sail
• Sail twist
• Sail Class Markings
• Marine canvas
• Baggywrinkle

[edit] Types of sails

• Flanker
• Lugsail
• Ringtail
• Royal
• Skysail
• Wingsail
• Turbosail
• Rotorsail

[edit] External links

• The quest for the perfect sailshape