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A jibe or gybe is a sailing maneuver where a sailing vessel reaching downwind turns its stern through the wind, such that the wind direction changes from one side of the boat to the other. For square-rigged ships, this maneuver is called wearing ship.
In this maneuver, the mainsail will cross the center of the boat while the jib is pulled to the other side of the boat. If the spinnaker is up, the pole will have to be manually moved to the other side, to remain opposite the mainsail. In a dinghy, raising the centerboard can reduce the risk of capsizing during what can be a somewhat violent maneuver.
Jibing is a less common technique than tacking, since a sailboat can sail straight downwind, whereas it cannot sail directly into the wind and has to tack (or sail a zig-zag course at alternating angles into the wind). However, many sailboats are significantly faster sailing on a broad reach than running (sailing straight downwind), so the increased speed of a zig-zag course of alternating broad reaches can more than make up for the extra distance it takes over a straight downwind course. Jibing is also used commonly in races, which often use a triangular course marked with buoys; the most direct way of rounding a buoy may be to jibe. A jibe can generally be completed more quickly than a tack because the boat never turns into the wind, and thus a jibing boat's sails are always powered where a tacking boat's luffing sails are un-powered as the bow crosses into the direction of the wind.
When running (sailing nearly directly downwind) in a sloop, one may 'jibe' only the mainsail to the opposite side of the boat. This keeps both the main and jib exposed to the wind resulting in a more efficient use of wind. Setting the mainsail and the jib on opposite sides of the boat is often referred to as running "goose-winged", "gull-winged", or "wing-and-wing". When running wing-and-wing, a light spinnaker pole or whisker pole is often used to hold the clew of the foresail out to the windward side of the boat.
A jibe can be a dangerous maneuver in a fore-and-aft rigged boat because, as the direction of the wind crosses the boat's centerline, the "old" leeward side of the mainsail and boom suddenly becomes the new windward side, and the sails are always fully filled by the wind. The load on the sail and mainsheet can remain high throughout the maneuver, and if uncontrolled, the boom and mainsail can swing across the deck with high speed, striking and injuring anyone standing in the path of the boom or its tackle, as they sweep across the boat. An uncontrolled boom slamming to the limit of its range may also put excessive stress on the rigging, and can break the boom or standing rigging, perhaps even bringing the mast down. A jibe can also result in a sudden change in the direction of heeling, and can cause unexpected course changes due to the change in mainsail force changing from one side of the boat to the other.
Owing to the hazard associated with the accidental jibe, some designers are moving away from designs that require the use of booms. There are also several designers that have placed the mast in the back of the boat in a configuration termed mast aft. This enables them to greatly reduce the size of the boom or eliminate it completely.
A safe jibe can be accomplished by making sure the yacht has a boom vang that is tensioned to stop the boom lifting during gybing. In fresh winds, by: A. sailing nearly directly downwind briefly before and after the jibe and to make a small boat direction change when jibing, so that there is less heeling force and rounding up tendency during a jibe. This will allow the crew to deal only with attending to the sail changes for this time, while the helmsman focuses on sailing direction. B. In the heaviest gust the crew or skipper can sheet the boom in somewhat and force the boom across the boat by hand, and hold the boom in position (using a preventer or jibe-guard), as the direction of the wind crosses the centerline of the boat; then the mainsail can be eased out to its new sailing course.
After the jibe has been performed the course can be changed to higher points of sail (e.g. broad or beam reach) when the crew is relocated, and the helmsman can better control course changes.
Because of the inherent dangers in jibing (uncontrolled, the boom can travel almost 180° with great speed and lethal force), communication with the crew is important. The skipper typically uses three commands when jibing: “Prepare to jibe” (or "ready to jibe") indicates that everyone should remain clear of the potential boom travel and assigned crew should be in position to handle the sails (actually the sheets and boom). “Bearing away” (similar to saying “helm's a-lee” during a tack) is used when the rudder action is applied to change course. At this stage sail and boom handling should be performed and repositioning of the remaining crew should occur, both coordinated with the boats heading. Finally, “jibe-ho” is called when the boom is starting to come across (this is a reminder analogous to "duck" or "take cover"). (In small maneuverable boats "bearing away" and "jibe ho" can be, in effect, one command.) If the helmsman maintains control and good communication with the crew and takes responsibility for the evolution and exactly when the boom jibes, the whole jibe will go more smoothly, safely and under control. If the helmsman is in control, it is possible to slow or even stop the jibe if anything is going too slowly or wrong.
Accidental jibes may occur when sailing on a course that is running dead downwind if the wind catches the leeward side of the sail. When the wind direction crosses the centerline of the boat, without jibing, the point of sail is referred to as "by the lee" When sailing "by the lee" the outer edge of the mainsail is facing slightly into the wind. Rolling motion, slight changes in the boat heading or wind direction can cause an unexpected and surprising jibe, suddenly and forcefully flipping the mainsail to the opposite side of the boat. Do not sail "by the lee" except for brief durations (such as to avoid an obstacle), and only when keeping all crew clear of the boom swing and the arc of the mainsheet sweep. A crew member can be used to help hold the boom in place in smaller boats. When sailing directly downwind, unintentional jibes can also occur; diligent helmsmanship is required to prevent "by the lee" conditions and keeping clear of the boom sweep is advised. In larger stable boats, a preventer can help by keeping the boom held forward, preventing the boom motion of a jibe, especially in light winds. However, in high winds, the "sheeting in action" of a preventer can cause severe rounding up on the other tack. Smaller boats may find that a backwinded sail is more heel inducing than allowing a jibe. See broach, Chinese gybe and death roll.
When sailing in high winds, a small boat or dinghy can capsize shortly after a jibe due to helmsman error (loss of direction control, or suddenly rounding into the wind too far) or tripping over the centerboard. It is partly for this second reason that centerboards are often lifted while sailing downwind even in non-planing hulls, the main reason being that a centreboard/keel is not needed for sailing downwind and simply adds to the drag of the hull. Raising the centreboard reduces drag and increases the boat's speed.
As with most sailing training, it is particularly important to learn this maneuver in lighter winds, and practice in increase wind and faster maneuvers gradually.
Alternatives to jibing 
The process of turning a fore-and-aft rigged boat upwind and tacking through more than 180 degrees to avoid having to jibe on a downwind course. While tacking is much slower, and is sometimes derisively called "chicken jibe", this technique avoids the dangers of passing the boom across the boat under load. This the converse to the practice of wearing ship on a square-rigged vessel, in which the vessel jibes to avoid the problems that square-rigged vessels face when tacking. Despite the somewhat demeaning term, use a "chicken jibe" in heavier winds, or with inadequate crew or helmsman experience.
See also 
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- Rousmaniere, John, The Annapolis Book of Seamanship, Simon & Schuster, 1999, pp. 54-55, 101-102, 106-107, & 331
- Chapman Book of Piloting (various contributors), Hearst Corporation, 1999, pp. 231 – 233
- Herreshoff, Halsey (consulting editor), The Sailor’s Handbook, Little Brown and Company, 1983, pp. 34-37, & 160
- Seidman, David, The Complete Sailor, International Marine, 1995, pp. 47-49,53, & 110
- Jobson, Gary, Sailing Fundamentals, Simon & Schuster, 1987, pp. 41-45, 71, 77-78, 95, & 186