Gybing is the act of turning a sailboat across the wind downwind. For example, if the wind is from the north and you are headed southeast, the wind is behind you on your port aft quarter and your sails are to starboard. If you want to turn to a westerly direction, you'll cross the wind directly downwind and the sails will be blown to the port side.
Here's what happens when you turn the boat in a gybe:
The mainsail and boom will swing across the boat from one side to the other by themselves. In a larger sailboat, or a small boat in a strong wind, this can happen very quickly and stress the rigging. The boom moving very fast can be dangerous to anything or anyone in its path. The mainsheet is then adjusted for the new point of sail.
In a boat with a head sail, the jib also will be blown to the other side. The currently used jibsheet must be released to let the sail move to the other side, and the other jibsheet is brought in to trim the sail to the new heading.
The Difficulty and Danger of Gybing
Gybing is more difficult than tacking, or turning across the eye of the wind, because the sails move from far out on one side to far out on the other side. If the wind is light, especially in a small sailboat, this may not be difficult. But a larger boat and even a smaller boat in a strong wind faces these difficulties and dangers:
Because the boom and even the mainsheet tackle can be heavy and moving very fast during a gybe, they may injure a crew in the way. More sailors are knocked overboard by gybes than anything else on a sailboat.
High stresses on the rigging and sails occur when the mainsail snaps into its new position across the boat. Rigging or the sail may be damaged, including breaking a shroud or stay-which could even cause a dismasting.
With a large jib bellied out ahead of the forestay, the sail may wrap around in front of the forestay during the gybe. Friction and the wind can pin the sail against the forestay, preventing it from coming out cleanly on the other side.
How to Do a Safe, Controlled Gybe
If you choose to gybe rather than harden sail and tack instead to the new heading, follow these steps for a controlled gybe:
Alert all crew that you will be gybing. Make sure crew are not where they may be struck by the boom or tackle. Have someone ready with the jibsheets.
Prepare for the gybe by tightening the mainsheet to minimize the distance the boom will travel during the gybe.
Tighten the jibsheet to prevent the sail from getting out in front of the forestay.
When everyone is ready, announce "Gybe ho" and turn the boat across the wind. The jib will back (be blown backward) and the mainsail and boom will swing across.
When the jib is back-winded, haul in the other jibsheet as the first jibsheet is released. Do this gradually and under control. Trim the jib with the new jibsheet. Stabilize the direction of the boat on the new heading.
Let out the mainsheet to trim the mainsail for the new heading.
Note: in a sailing dinghy with just a mainsail, the steps are the same as above, minus the jibsheet steps. In a small boat with little or no ballast, you must move under the boom to the other side of the boat during the gybe.
Prevent Accidental Gybes
When sailing downwind, there is always a danger of an accidental gybe due to a wind gust, a wave suddenly turning the boat, or a steering error. To prevent this, use a line to hold the boom in place so that it cannot move across the boat.
This line, called a preventer, can be rigged in various ways depending on the boat. It can be as simple as a dock line tied to the boom and a cleat or stanchion forward of the mast. Permanent preventers can be rigged from the boom on both sides, running forward to blocks at the rail and then back to the cockpit. Such preventers can be left in place, cleated tight in the cockpit on the lee side as needed and released on the windward side until needed.
Gybing Still a Danger with a Preventer
A preventer does not keep the boat from turning across the wind - it only prevents the boom from crossing the boat. Should the boat turn across the wind, the mainsail will back and it will be difficult to control or turn the boat, especially in a strong wind. It is therefore important to steer very carefully downwind and, when practical, to sail a broad reach rather than a run to avoid the risk of an accidental gybe.
Here's a video clip from MIT's sailing school showing how to gybe a small sailboat.