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RS K6 Keelboat broaching

A sailboat broaches when its heading suddenly changes towards the wind due to wind/sail interactions for which hull hydrodynamic reactions cannot compensate. This happens when the aerodynamic force on the rig greatly exceeds the hydrodynamic force on the hull, usually due to a sudden increase in wind strength. In small boats and dinghies, broaching often leads to capsize (see 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.

Sailing in variable, gusty winds is a common cause of broaching when sailing on a close reach and in strong gusty winds on a broad reach. When sailing on a broad reach, the helmsman must be careful to make sure that the wind is not allowed to cross the stern. If this happens, then the sails may fill from the leeward side, resulting in a sudden reversal of the direction of thrust as the sail moves from one side of the boat to the other. This is called a gybe, and it can be a dangerous maneuver even when performed under controlled circumstances. When it happens unexpectedly, the boom may live up to its name, as it crosses the boat under full load and may sweep unwary crew members overboard, unless a preventer is rigged on the boom.

One of the less obvious causes of broaching is an oscillation developed when running downwind. When running dead downwind, the sails are set with an angle of attack of nearly 90 degrees to the wind. The high angle of attack causes turbulent bubbles to form on the leeward side of the sail, and when there is insufficient attached flow to keep these bubbles attached to the sail, they separate. The sudden separation of a turbulent bubble drops the lift of the sail to nearly nothing, which causes the boat to rock slightly. At angles of attack near 90 degrees, lift can generate on the leech, or trailing edge of the sail. As the boat rocks due to separation of turbulent bubbles, a constructive interference is formed, as bubbles begin to shed from alternating sides of the sail. Once started, the oscillation builds quickly, especially in modern ultra-light displacement centerboard boats, which lack the damping effects of a large keel. Once this oscillation reaches a certain point, the extreme change in heel angle will cause the boat to broach. Spinnakers generate lift from the top, rather than the side, and thus they are less prone to this form of broaching than other modern sail types. Square rigged vessels also generate lift from the tops of their sails, which in fact, combined with the large keels they traditionally use, makes them immune to this type of broaching.

Another cause of broaching is encountered in heavy seas and high winds. If the bow of the yacht is not kept pointed into the waves, then the waves will push the bow aside, turning the boat side-on to the waves. Once side-on to the waves, the waves will roll the yacht side to side violently, causing severe discomfort to the crew, the yacht may broach and may even capsize. Since a sailing ship cannot steer unless the sails are providing forward motion, and since heavy seas are most often from the same direction as the wind, a sea anchor may be used at the bow in extreme weather to keep the bow pointed into the wind and waves.

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