Broach (sailing)

A sailboat broaches when its heading suddenly changes towards the wind due to wind/sail interactions for which the rudder cannot compensate. This happens when the aerodynamic force on the rig greatly exceeds the hydrodynamic force on the hull, due to a sudden increase in wind strength or turbulent sea conditions. In small boats and dinghies, broaching can lead to a death roll. In larger boats broaching can lay the mast horizontal, putting both rig and crew at risk. It can be particularly dangerous when racing other boats at close quarters.

More succinctly, a broach is "to slew around on a wave front." Another source says it is "extremely dangerous" and likens it to turning broadside and losing control in following seas, so as to present the ship's side to oncoming large waves. In that event, the ship may "trip" on its keel, roll, capsize and turn turtle.

Historically, it has been defined as:

"Broach-to. To fly up into the wind. It generally happens when there is considerable sea on, and the ship is carrying a press of canvas with a good deal of after-sail set. When a ship sails with the wind aft, or on the quarter, the wind acts in the direction of the ship s course and the pressure on the sails is very much diminished. If from this position the ship suddenly presents her broadside to the wind, the sails, masts, and rudder will be endangered, and in extreme cases the ship may capsize or be forced down stern foremost. Broaching-to is generally occasioned by the difficulty of steering the ship; by the negligence or incapacity of the helmsman; or by an accident happening to the helm which renders it incapable of governing the ship. See BROUGHT BY THE LEE."

This can cause the boat to enter a Death roll, rolling dangerously and if not controlled may lead to a capsize and turning turtle

Broaching most typically happens when a boat is on a broad reach with spinnaker set and there is an overpowering weather helm. This may be due to excessive heeling, swell that causes the rudder to be partially lifted out of the water, or because the stem digs into a wave causing a strong lateral force. The boat will come quickly across the wind with too much sail set. It can also happen on a close reach where it is typically precipitated by sudden waves. With an inattentive helmsman it can be a consequence of gybing, which can be a dangerous manoeuvre even when performed under controlled circumstances. When it happens unexpectedly and the helm isn't corrected, the heading of the boat may veer into the wind causing a broach.

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Wikipedia