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    Sail & Sail-Plan
    Experiments and Background Information







    Experiments

    Background Information

    Definitions

    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.

    A sail-plan is a formal set of drawings, usually prepared by a naval architect. It shows the various combinations of sail proposed for a sailing ship.

    Topics of Interest

    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 For purposes of commerce, sails have been greatly superseded by other forms of propulsion, such as the internal combustion engine.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 sails in a fore-and-aft arrangement: the mainsail and the jib.

    Other sail powered machines include ice yachts and windmills.

    History of sails: The ships built at around 10,000 BCE were just crude log rafts or dug-out canoes. The earliest known ships were papyrus reed boats built by the Egyptians around 4,000 BCE. The Greeks and Phoenicians had begun trading by ship around 1,200 BCE. The Arabs had invented the sail in about 2,000 BCE in an attempt to establish trading routes around the Persian Gulf. The Chinese had designed sails around 3,000 BCE, and can still be seen on traditional wooden ships sailing off the coast of Vietnam in Ha Long Bay. Square sails mounted on yardarms perpendicular to the boat's hull are very good for downwind sailing, and dominated in the ancient Mediterranean and spread to Northern Europe, while being independently invented in China and Ecuador. Although fore-and-aft rigs have become more popular on modern yachts, square sails continued to power full rigged ships through the Age of Sail and to the present day. Triangular fore-and-aft lateen sails were invented independently in the Mediterranean (possibly from the square sail through the lugsail), the Indian Ocean, the Pacific (from V-shaped sails) and Peru, and continue to be used throughout the world. During the 16th-19th centuries other fore-and-aft sails were developed in Europe, such as the spritsail, gaff rig, jib/genoa/staysail, and Bermuda rig, which give better upwind sailing ability.

    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 may be set merely to trap the air as it flows by. Sails acting in this way are aerodynamically stalled. In stronger winds, turbulence created behind stalled sails can lead to aerodynamic instability, which in turn can manifest as increased downwind rolling of the boat. Spinnakers and square-rigged sails are often trimmed so that their upper edges become leading edges and they operate as airfoils again, but with airflow directed more or less vertically downwards. This mode of trim also provides the boat with some actual lift and may reduce both wetted area and the risk of 'digging in' to waves.

    The other way sails propel the boat occurs when the boat is traveling across or into the wind. In these situations, 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 although it acts largely horizontally.

    On a sailing boat, a keel or centreboard helps to prevent the boat from moving sideways. The shape of the keel has a much smaller cross section in the fore and aft axis and a much larger cross section on the athwart axis (across the beam of the boat). The resistance to motion along the smallest cross section is low while resistance to motion across the large cross section is high, so the boat moves forward rather than sideways. In other words it is easier for the sail to push the boat forward rather than sideways. However, there is always a small amount of sideways motion, or "leeway".

    Forces across the boat are resolved by balancing the sideways force from the sail with the sideways resistance of the keel or centerboard. Also, if the boat heels, there are restoring forces due to the shape of the hull and the mass of the ballast in the keel being raised against gravity. Forward forces are balanced by velocity through the water and friction between the hull, keel and the water.

    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. 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 asymmetrical 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).

    Sails were classically made of hemp or cotton. They are now made from polyesters (Dacron and PET film), sometimes reinforced with crystalline hydrocarbons (Kevlar and Spectra™). Some large, lightweight sails are made of polyamides (nylon).

    Ropes were classically made of manila, cotton, hemp, or jute; papyrus (in ancient Egypt) and coir have also been seen. They are now made of stainless steel (301), galvanized steel, polyester (Dacron), polyamides (nylon), and sometimes crystallized hydrocarbons (Kevlar and Spectra).


    A sail-plan is a formal set of drawings, usually prepared by a naval architect. It shows the various combinations of sail proposed for a sailing ship.

    In all sail plans, the architect attempts to balance the force of the sails against the drag of the underwater keel in such a way that the vessel naturally points into the wind. In this way, if control is lost, the vessel will avoid broaching (turning edge-to-the wind), and being beaten by breaking waves. Broaching always causes uncomfortable motion, and in a storm, the breaking waves can destroy a lightly-built boat.

    The architect also tries to balance the wind force on each sail plan against a range of loads and ballast. The calculation assures that the sail will not knock the vessel sideways with its mast in the water, a capsize and possible sinking.

    In English, thanks to the British Admiralty, all sail-plans call a sail by the same name, no matter what their sail-plan. Once a sail is named, its ropes have standard names according to their use. Once a sailor learns the standard names for the sails, he knows the terms for all the parts on any sail-plan.

    The standard terminology assumes three masts, from front to back, the fore-mast, main-mast and mizzen-mast. On ships with fewer than three masts, the tallest is the main-mast. Ships with more masts number them. Some barks have had as many as twelve masts

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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