Broach (sailing)
Encyclopedia
A sailboat broaches when its heading
suddenly changes towards the wind due to wind
/sail
interactions for which the rudder
cannot compensate. This causes the boat to roll dangerously and if not controlled may lead to a capsize
. 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.
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 in to 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 a gybe
, 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.
Also when sailing on a dead downwind run an inexperienced or inattentive sailor can easily misjudge the real strength of the wind since the boat speed subtracts directly from the true wind speed
and this makes the apparent wind
less. In addition the sea conditions also falsely seem milder on this point of sail as developing white caps are shielded from view by the back of the waves and are less apparent. When changing course in a brisk wind from a run to a reach
or a beat
, a sailboat that seemed under control can instantly become over-canvassed and in danger of a sudden broach.
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. Spinnaker
s 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 rig
ged 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, and 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.
The key to stopping or rescuing a broach is to depower the sails at the first opportunity. This may be done by releasing the main or jib sheet by a foot or more or by releasing the boom vang
(kicking strap) which will spill wind out of the top of the mainsail. The main means of preventing a broach is to ensure that the amount of sail set is reduced to suit the sailing conditions.
Course (navigation)
In navigation, a vehicle's course is the angle that the intended path of the vehicle makes with a fixed reference object . Typically course is measured in degrees from 0° clockwise to 360° in compass convention . Course is customarily expressed in three digits, using preliminary zeros if needed,...
suddenly changes towards the wind due to wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
/sail
Sail
A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...
interactions for which the rudder
Rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
cannot compensate. This causes the boat to roll dangerously and if not controlled may lead to a capsize
Capsize
Capsizing is an act of tipping over a boat or ship to disable it. The act of reversing a capsized vessel is called righting.If a capsized vessel has sufficient flotation to prevent sinking, it may recover on its own if the stability is such that it is not stable inverted...
. 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
Death roll
In a keel boat, a death roll is the act of broaching to windward, putting the spinnaker pole into the water and causing a crash-gybe of the boom and mainsail, which sweep across the deck and plunge down into the water. The Death Roll often results in destruction of the spinnaker pole and sometimes...
. 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.
Broaching most typically happens when a boat is on a broad reach with spinnaker set and there is an overpowering weather helm
Weather helm
Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward in order to counteract the effect. Weather helm is the opposite of Lee 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 in to 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 a gybe
Gybe
Gybe may refer to:*Gybe, an alternative spelling of jibe, a sailing maneuver**Chinese gybe, a type of jibe*To deride or tease with taunting words, also spelt "gibe" or "jibe" and done with a Sneer...
, 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.
Also when sailing on a dead downwind run an inexperienced or inattentive sailor can easily misjudge the real strength of the wind since the boat speed subtracts directly from the true wind speed
Wind speed
Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
and this makes the apparent wind
Apparent wind
Apparent wind is the wind experienced by a moving object.-Definition of apparent wind:The Apparent wind is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer....
less. In addition the sea conditions also falsely seem milder on this point of sail as developing white caps are shielded from view by the back of the waves and are less apparent. When changing course in a brisk wind from a run to a reach
Reach
- Companies and organizations :* Reach Global Services Ltd, a company operating a large cable network in Asia Pacific* Reach for the Top, a Canadian high-school trivia competition* Reach Canada, a NGO in Canada, that help people with disabilities...
or a beat
Beat
-Film:*Beat , the smallest unit of dramatic action in a play*Beat , a film about writer William Seward Burroughs*Beat , a 1998 Japanese film*Directorial beat, an exchange of behavior between characters in a screenplay...
, a sailboat that seemed under control can instantly become over-canvassed and in danger of a sudden broach.
One of the less obvious causes of broaching is an oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
developed when running
Points of sail
Points of sail describes a sailing boat's course in relation to the wind direction.There is a distinction between the port tack and the starboard tack. If the wind is coming from anywhere on the port side, the boat is on port tack. Likewise if the wind is coming from the starboard side, the boat...
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
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
. Once this oscillation reaches a certain point, the extreme change in heel angle will cause the boat to broach. Spinnaker
Spinnaker
A spinnaker is a special type of sail that is designed specifically for sailing off the wind from a reaching course to a downwind, i.e. with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of...
s 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 rig
Square rig
Square rig is a generic type of sail and rigging arrangement in which the primary driving sails are carried on horizontal spars which are perpendicular, or square, to the keel of the vessel and to the masts. These spars are called yards and their tips, beyond the last stay, are called the yardarms...
ged 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, and 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
Sea anchor
A sea anchor, is a device external to the boat, attached to the bow used to stabilize a boat in heavy weather. It anchors not to the sea floor but to the water itself, as a kind of brake. Sea anchors are known by a number of names, such as drift anchor, drift sock, para-anchor, and boat brakes...
may be used at the bow in extreme weather to keep the bow pointed into the wind and waves.
The key to stopping or rescuing a broach is to depower the sails at the first opportunity. This may be done by releasing the main or jib sheet by a foot or more or by releasing the boom vang
Boom vang
A boom vang or kicking strap is a line or piston system on a sailboat used to exert downward force on the boom and thus control the shape of the sail. An older term is "martingale"....
(kicking strap) which will spill wind out of the top of the mainsail. The main means of preventing a broach is to ensure that the amount of sail set is reduced to suit the sailing conditions.