Broach (nautical)

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.^[1]

^ Saunders, Harold E. (1965). "10: Definitions and nomenclature for Seakeeping". In Taggart, Robert (ed.). Hydrodynamics in ship design. Vol. 3. New York, NY: Society of Naval Architects and Marine Engineers. p. 160.

[Saunders_V3_1965-1] Saunders, Harold E. (1965). "10: Definitions and nomenclature for Seakeeping". In Taggart, Robert (ed.). Hydrodynamics in ship design. Vol. 3. New York, NY: Society of Naval Architects and Marine Engineers. p. 160.

[1]