right|200px|thumb|RS K6 Keelboat broaching, caused by wind action.
A broach is an abrupt, involuntary change in a vessel's course, towards the wind, resulting from loss of directional control, when the vessel's rudder becomes ineffective. This can be caused by wind or wave action. A wind gust can heel (lean) a sailing vessel, lifting its rudder out of the water. Both power and sailing vessels can broach when wave action reduces the effectiveness of the rudder. This risk occurs when traveling in the same general direction as the waves are moving. The loss of control from either cause usually leaves the vessel beam-on to the sea, and in more severe cases the rolling moment may cause a capsize.
An alternative meaning in the context of submarine operation is an unintended surfacing of a shallow-running submarine in a deep wave trough.
Causes
Wind
Broaching caused by wind action may occur when a vessel is sailing away from the wind and its sails are suddenly overpowered by a gust of wind, causing it to heel excessively. Heeling alters the rudder's orientation, away from vertical, reducing the horizontal force which water can apply as it flows past the rudder. In extreme cases, heeling can raise the rudder out of the water. With loss of directional control, the vessel turns into the wind. In the process, the vessel may heel close to horizontal and may capsize. Such loss of control may be preceded by oscillations of the vessel's mast and course, as the person steering attempts to maintain control. Wave action may contribute to a broach initiated by wind gusts.