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Reefing is a sailing manoeuvre intended to reduce the area of a sail on a sailboat or sailing ship, which can improve the ship's stability and reduce the risk of capsizing, broaching, or damaging sails or boat hardware in a strong wind. Modern sailboats often combine reefing and furling of sails, as shown in fully furled Genoa headsail of the Bavaria 36 in the image at right.
There are three common methods of reefing: conventional, roller, and jiffy. The latter two make sail-handling easier and allow reefing to be done with fewer crew members. In any case, the most important point is to reef before you need to. The basic rule is to reef as soon as you wonder if you ought to. In any case, a boat should be reefed if the wind is likely to cause it to heel beyond 25 degrees.
Sails may have built-in alternative attachment points that allow their area to be reduced. In a mainsail, one to four horizontal rows of cringles or lines, called reef points, may be placed above the foot of the sail. Rolling up the foot of the sail and tying these reef points to the foot of the sail forms a new tack and clew, reducing the sail's area. More than one row of reef points increases options for possible sail area.
To reef the sail of a Bermuda rigged sloop: 1. The boat should be brought head-to-wind (in irons). 2.Lower the jib and main sail. Leaving the jib up allows it to turn the boat away from the wind. Lowering the main just enough to reef runs the risk of having the wind change directions, blowing the main outboard and filling the sail. Set the boom in the boom crutch and pull the sheet tight. 3. Lash the tack reef cringle to the boom. Pull the leech reef cringle away from the mast, stretching the sail taut along the boom. Lash the leech cringle to the boom, and then to the end of the boom. Then tie the remaining reefs. Always reef the sail to itself, not to the boom, or the strain may rip out the reefing cringles. Tie the reefs with the reef (square) knot, or with the slippery reef knot. 4. Once the sail is reefed, raise the main, stow the boom crotch, raise the jib, and bear off. If the reefing is done while moored or in sheltered waters, the sail to be reefed need not be lowered entirely. One crewman must pull the reefing line as another crewman lowers the sail.
Roller reefing involves rolling or wrapping the sail around a wire, foil, or spar to reduce the sail's exposure to the wind. The mainsail is wrapped around the boom, which contains a mechanism in the gooseneck that rolls in the sail—or special hardware inside the boom or mast is used to reef the sail by winding it around a rotating foil. These latter systems are known as mainsail furling systems.
Furling systems can be controlled with lines led to the cockpit or some other safe position, so that the sail can be reefed without going to the deck, as long as the system works as intended. Roller reefing also allows a more gradual reefing than conventional or jiffy reefing. The downside is that the furled sail seldom has an optimal shape and that replacing or repairing the sail is more difficult.
Jiffy reefing, also called slab reefing or single line reefing, is quicker and easier than conventional reefing or conventional roller reefingcitation needed and involves folding the sail in sections, or slabs, along the boom. One or two reefing lines placed through the reef cringles at the sail's luff and leach edges are used to pull those points down tight to the boom, creating a new tack and clew for the sail. Reefing lines can be led back to the cockpit, and crew members can perform reefing without going on deck in heavy weather. In jiffy reefing there is no need to tie to the boom at the reef cringles on the sail. The equipment for jiffy reefing is often integrated with Dutchman flaking, a furling technology that flakes (or folds up) the sail on alternate sides of the boom rather than on a messy pile on one side of the boom.