Sail: transportation: watercraft
A sail is used to propel a sailboat or assist a motorboat, or may be used to steady a working vessel. It is generally made of reinforced mylar or fabric, but sails have been made of aluminum, plywood, hide, leaves, and other materials. The sail is rigged to develop lift from the wind, and transfer the force to the hull.
The two primary methods of sails for getting lift from the wind are to block the wind or to bend the wind. The traditional square-rigged boat efficiently blocks the wind, giving it excellent downwind speed. Fore-and-aft sailing rigs are more efficient at bending the wind, allowing them to sail well at an angle upwind. Blocking the wind allows the sail to develop force based on the velocity of the wind, its density, and the surface area of the sail. There is not consensus on the exact mechanism by which force is developed from bending the wind, but the force is based on the apparent velocity, and the aerodynamics of the sail (including the angle of attack of the chord of the sail, depth and position of maximum camber, and the induced drag.)
Most sails are designed to use both methods of gaining lift, but a boat's rig will tend to focus on one or the other depending on it's designed use, and its sails will be more efficient in one method or the other. In example, an America's Cup boat is very efficient going upwind (bending), while a full-rigged Ship is the extreme downwind rig (blocking).
When describing a triangular sail, the corners are all reinforced with patches, as they carry the strain of the forces on the sail. Each has a hole of some type, called a cringle, to which lines or shackles are connected to attach and control the sail. Their names are as follows:
- Tack
The lower, forwardmost corner. It is usually attached to the boat using a shackle or tack hook fitting. Bermudan mainsails are tacked to the boom or gooseneck, while staysails (including jibs) are usually shackled to a deck fitting or the stemhead, sometimes using a tether. - Head
The uppermost corner is generally called the head. The halyard is attached here, which is used to hoist the sail up and to tension the leading edge of the sail. On Bermudan mains, a large and stiff headboard is usually attached, which supports additional sail area like a gaff or other yard does. - Clew
The lower, aftermost corner. On unboomed sails, the sheets attached directly to the clew and both trim and spread the sail. On boomed sails there is usually an outhaul line or car which is used to tension the sail, and the sheet is attached to the boom to adjust the trim.
Each edge of the sail also has a number of elements, and of course even more specialized language. Around each edge is a tape called the tabling which is carefully matched to the sail material so it will not deform the sail's shape as it stretches in the wind.
- Luff
The leading edge of the sail cuts the wind and indicates when the sail is stalling, such as when the pointing too close to the wind. The luff is often attached to a stay or mast using shackles, boltrope or slug in a slot, a lacing, hoops or parrels, or slides in or on a track. A sail downhaul may be added to an attached sail luff to make it easier to dowse the sail. For many staysails, however, the luff is just a smooth edge tensioned by the halyard and possibly a tack downhaul. - Foot
The lower edge of the sail is called the foot, and may include a rounded portion called foot roach if the sail is not attached to a boom. Boomed sails may be attached by the same methods as the luff, or the sail might be loose-footed and attached only at the tack and clew. The outhaul controls the tension of this edge. - Leech
The aftermost edge is responsible for releasing the wind smoothly to avoid causing turbulence and, thereby, induced drag. A leech cord inside the tape can help with the shape of the edge, as will a vang and the sheet lead. On Bermudan mainsails especially, but also other triangular sails occasionally, additional sail area may be added as a rounded bulge of the leech called roach. Roach is usually supported by battens, either short ones sewn into pockets or full-length yards attached to cars at the mast like a junk rig. (But unlike the junk, the battens cannot vang the leech for trim.) Traditionally a yacht's ensign is hoist 2/3 up the leech, or might be sewn there.
A four-sided bending sail adds an upper edge called the head. If the upper edge is attached to a spar it will have some form of outhaul but will most likely be laced to the yard. Otherwise the head will be a smooth tape, probably acting at least in part as an extension of the luff. The extra corner, at the juncture of the head and the luff, is called the throat, while the corner of head and leech is called the peak.
On the other hand, four-sided blocking sails, commonly called square-sails, have a different use of the jargon.
- Head
The head is generally laced to the yard. The tabling includes a head rope and head holes for lacing, and large earings at each end. - Leech
The square sail has two leeches, and the edge cutting into the wind like the luff of the bending sails will be called the weather leech (or just the luff on non-archaic boats.) Each leech will include cringles for the reef-tackle, and ends in a clew cringle. The leech liner is a patch added to take the chafe from the reef tackle. - Foot
The foot is concave, with the clews sometimes much lower than the middle of the sail. The footrope is a strong element of this edge, to prevent the stretching which might split the sail.
A sail is sized to a boat based on how much wind can be expected where the boat sails, how much the boat masses (displaces), and what the underwater shape of the hull might be. The most common issue is not enough wind, so a larger sail than is seamanly is usually flown. And reefing is how a sail is made smaller to deal with more wind than the boat can use. On traditional triangular sails, the sail is partially lowered and a portion of the foot is tied in a reef using reef nettles or sail ties, the tack and clew may be held down to the boom using a reefing line, a tack hook, or lashing. Roller reefing is hardly a new technology, and may be found on staysails rolling in the luff, and on mainsails rolling in either the luff or the foot of the sail. Some bending sails are brailed to reef them, such as on large sprit rigs and certain gaff rigs like the Brixton Trawler. On a square sail the clews and foot are hauled up toward the yard using the reef-tackle to haul in on the buntlines, with the reef tied in using the nettles on the reef band.
References:- Edwards, Fred; Sailing as a Second Language; International Marine Publishing Company; © 1988 Highmark Publishing Ltd.; ISBN 0-87742-965-0
- Marino, Emiliano; The Sailmaker's Apprentice: A guide for the self-reliant sailor; International Marine/Ragged Mountain Press; © 1994 International Marine; ISBN 0-07-157980-X
- rootbeer277; Fabulousness: Editing On-The-Fly; (requested ms not provided but is currently owed favours-of-choice)
- Rousmaniere, John; The Annapolis Book of Seamanship; Simon & Schuster; © 1983, 1989 John Rousmaniere; ISBN 0-671-67447-1