The short version
- The danger
- The boom. In a gybe it swings across the whole cockpit, full of wind, at head height. An accidental gybe does that with no warning.
- Prevent the accidental one
- Don't sail by the lee (past dead downwind) without meaning to. On a long run in a keelboat, rig a preventer.
- Control the planned one
- Gybe at full speed, sheet the boom in before it crosses, steer straight as it lands, and call it so every head is down.
- Or don't gybe
- When it's too windy to gybe safely, tack round instead (a "chicken gybe"). It costs a few lengths. A broken boom or a head injury costs the day.
Why the boom is dangerous
In a tack the sails flap as the bow passes through the wind, and the boom drifts across with no power in it. In a gybe the wind stays in the mainsail the whole way. When the stern passes through the wind, the wind gets behind the sail and throws it to the other side, the full width of the boat, in a second or less. The boom is a metal or carbon spar at head height. A boom across the head is one of the best-known ways to get hurt on a sailboat, and the mainsheet sweeping across the cockpit can catch a neck or an arm on the way.
The force grows quickly with the wind. A gybe that's harmless in 8 knots can break a boom, a vang (the line or strut that holds the boom down) or a person in 25. Respect it.
Accidental gybes and how to see them coming
An accidental gybe, sometimes called a crash gybe, happens when the boat turns past dead downwind without anyone meaning it to, and the wind catches the other side of the mainsail. The usual causes are a wave pushing the stern round, a wind shift, a moment of distraction at the helm, or a deliberate attempt to sail very deep.
Sailing by the lee means the wind is coming over the same side the boom is on. It's sometimes done on purpose, especially in dinghies, but it's the step before an accidental gybe. Watch for the signs:
- The jib goes limp or starts to cross to the other side on its own.
- The leech (the back edge) of the mainsail flicks toward you, and the top of the sail wants to fold over.
- The boom lifts and feels light, as if it wants to come in.
- The wind indicator at the masthead swings round to point at the boom.
When you see them, head up a few degrees, away from the side the boom is on, and the boat settles. In waves, steer actively. The stern gets pushed around as each wave passes under it.
Preventers
A preventer is a line that holds the boom out so it can't swing across if the boat gybes by accident. It's common on keelboats sailing long runs and offshore races, and rarely needed on a short club course where you'll gybe every few minutes.
- Attach it at the end of the boom, not the middle. Tied to the middle, an accidental gybe can bend or break the boom against it.
- Lead it forward to a strong point near the bow, then back to the cockpit, so it can be let go from a safe place, under load.
- Never tie it so it can't be released. If the boat is knocked over with the boom held out and the end of the boom in the water, you need to free it fast.
- Take it off before every planned gybe. Make "preventer off" part of the gybe call.
The vang is not a preventer. It holds the boom down, not out. Some boats use a boom brake (a friction device that lets the boom cross slowly) instead. Ask what's normal on your boat.
A controlled gybe in heavy air
The steps are the same as in how to gybe. In a breeze, these are what keep it under control:
- Gybe at full speed. The faster the boat goes, the less wind there is across the deck and the less load on the sail. In waves, gybe as the boat surfs down the face of one, not as it slows in the trough.
- Bear away fully first. The boom must cross from dead downwind, not from a reach.
- Keelboats: sheet the main in toward the centerline before it crosses, so it has a short way to travel, then let it run out quickly on the new side so the boat doesn't round up and heel over.
- Dinghies: pull the boom across by the mainsheet rather than letting it slam, and keep the vang on firmly. With a loose vang the boom lifts as it crosses, and the top of the sail can flip over while the bottom stays put, leaving the boat out of control.
- Steer straight as the boom lands. The boat wants to spin into the wind. Center the tiller, or steer briefly back the other way, to stop it.
- Keep the boat flat and get weight across fast. A boat heeled to windward as the boom lands can capsize to windward. One heeled to leeward rounds up.
- With a spinnaker up, ask whether you need it up for the gybe at all. Dropping it, gybing on main and jib, and hoisting again is slower, but it's a lot safer than a broach.
The chicken gybe: tack instead
When it's too windy to gybe safely, or the crew is tired, or you've already capsized once today, don't gybe. Head up, tack through the wind, and bear away onto the new gybe. Sailors call it a "chicken gybe", but it's good seamanship. The boom crosses as the bow passes through the wind, with the sails flapping and no power, exactly as in any tack.
What it costs: a loop of several boat lengths, plus the speed lost in a tack and in heading up through a reach. In a race that's usually a place or two. Before you start the loop, check the water to windward is clear. Turning up and tacking takes you across the path of the boats behind.
Crew calls
Most gybing injuries happen to someone who didn't know it was coming. Agree the calls before you leave the dock and use them every time, even on a calm day:
- "Stand by to gybe." Crew get ready: preventer off, runner ready, spinnaker crew in position, everyone thinking about where the boom will go.
- "Ready." Each person answers. No answer, no gybe.
- "Gybing" (or the traditional "gybe-ho") as the helm starts the turn.
- "Heads!" just before the boom crosses, from whoever can see it best.
On a keelboat, give the person on the foredeck extra thought. They can't see the boom coming and may be holding a spinnaker pole. And anyone new aboard should be told before the start where not to sit or stand during a gybe.
Gybing also changes your tack, and your rights with it. Once you're on port, you must keep clear of boats on starboard (rule 10). A gybe done in a hurry to dodge one boat can put you in front of another.
If someone is hit by the boom, stop racing and look after them. Head injuries can be worse than they look at first. Rule 1.1 of the Racing Rules of Sailing requires every boat to help anyone in danger, and getting help for your own crew is allowed under rule 41.
Doing it with Sail Tactician
Questions
What causes an accidental gybe?
The boat turns past dead downwind without meaning to, usually because a wave pushes the stern round, the wind shifts, or the helm is sailing very deep. The wind gets behind the mainsail and throws the boom across.
What does sailing by the lee mean?
Sailing with the wind coming over the same side of the boat as the boom, past dead downwind. It's the step before an accidental gybe.
Where should a preventer be attached?
To the end of the boom, led forward to a strong point near the bow and back to the cockpit, so it can be released under load. Never to the middle of the boom.
What is a chicken gybe?
Changing from one gybe to the other by tacking instead: heading up, tacking through the wind, and bearing away again. It's slower, but the boom crosses with no power in it, which is why sailors do it in strong winds.
Sources
Racing Rules of Sailing 2025–2028: rule 1.1 (Helping Those in Danger), rule 10 (On Opposite Tacks) and rule 41 (Outside Help). Gybing and preventer practice is standard seamanship as taught in sailing schools and club racing.
What changed
- First published.
Put a racing instrument on your wrist.
Start line, wind shifts and VMG at a glance, and a coach in your pocket afterwards. Free to download for Apple Watch and iPhone, with the full debrief on your first five races.
Download on the App Store