Broaching
From The Dock Line, Dec. 1997
For purposes of this report a "broach" is defined as an uncontrolled turn to wind ward in response to a wind gust. Detailed explanation of the mechanics of a broach would be too lengthy for purposes of this article, therefore the following is a very simplified version. While a broach can occur on any point of sailing it is most pronounced when sailing close hauled in strong gusty winds. Being close hauled in a strong wind means that the boat is usually already at a large angle of heel and the rudder deflected to weather to counter the turning effects of heeling. When true wind velocity increases during a gust the apparent wind shifts aft and windward pressure on the sails in- creases markedly (wind pressure varies directly as the square of the velocity, i.e. if wind speed doubles the pressure increases to four times its former value). This pressure increase causes the angle of heel to increase sharply, and increasing aerodynamic and hydrodynamic forces act to cause the boat to rapidly round up to windward The high turn-rate can also generate a centrifugal force which acts upon the mass of the mast and rigging to further increase the angle of heel. A normal reaction is to attempt to counter this sudden turn with full rudder deflection. This usually leads to rudder stall and/or brings the leading edge of the rudder close enough to the surface of the water to allow air to be sucked down along the rudder surfaces. This results in a marked de- crease of rudder effectiveness allowing the high tun-rate to continue, and may even cause the rate to increase. When the boat has turned far enough to windward to start the sails lulling the overturning wind pressure begins to ease allowing the angle of heel and the turn-rate to decrease and the rudder to start regaining some effectiveness. By the time all of his has occurred the boat is usually in irons, and may even have inadvertently tacked.
So what can be done? When sailing close hauled in heavy air and gusty conditions you must anticipate and be prepared for a broach. The key operative word is anticipate. Learn to anticipate the arrival of a gust by watching the water surface for the "dark spot" accompanying a gust. Watching nearby boats (particularly those to windward) will allow you to judge if the gusts are strong enough to initiate broaching. When an approaching gust is detected start preparing for its arrival by easing your sails a bit more, even to a point where you carry a slight luff. When the gust arrives, or preferably slightly before, rapidly ease your sails, completely luffing if necessary, to minimize the increased heeling effect, and use no more than half rudder to compensate for the start of the turning effect. Recovery from a lulling condition is much easier and quicker than recovering from being in-irons. If the boat is allowed to heel too far the end of the main boom can strike the water (known as "tipping the boom"), the boom will be unable to go out and the boat will broach. If your timing is right the eased and slightly luffing sails should become full as the apparent wind shifts aft and boat speed should increase, and the faster a boat goes the more effective its rudder becomes so not as much deflection is necessary. As boat speed increases you can "milk" the sheets in or bear off slightly to maintain speed and extract as much energy from the gust as possible. As the gust passes trim the sheets in and resume your former course.
Sounds simple but you must practice this technique constantly to develop the ability to react instinctively and correctly. After all there is a lot of "stuff' happening in a very short time and you will have no time to stop and think about what to do. Obviously you will not be able to curtail all broaches but with constant and diligent practice you should be able to negate a large percentage of them and minimize the effects of those you missed. Once again there is a key operative word, practice. Broaching on other points of sail or in gusty medium-air occurs in a much less violent manner and more slowly so there should be ample time to react.
For purposes of this report a "broach" is defined as an uncontrolled turn to wind ward in response to a wind gust. Detailed explanation of the mechanics of a broach would be too lengthy for purposes of this article, therefore the following is a very simplified version. While a broach can occur on any point of sailing it is most pronounced when sailing close hauled in strong gusty winds. Being close hauled in a strong wind means that the boat is usually already at a large angle of heel and the rudder deflected to weather to counter the turning effects of heeling. When true wind velocity increases during a gust the apparent wind shifts aft and windward pressure on the sails in- creases markedly (wind pressure varies directly as the square of the velocity, i.e. if wind speed doubles the pressure increases to four times its former value). This pressure increase causes the angle of heel to increase sharply, and increasing aerodynamic and hydrodynamic forces act to cause the boat to rapidly round up to windward The high turn-rate can also generate a centrifugal force which acts upon the mass of the mast and rigging to further increase the angle of heel. A normal reaction is to attempt to counter this sudden turn with full rudder deflection. This usually leads to rudder stall and/or brings the leading edge of the rudder close enough to the surface of the water to allow air to be sucked down along the rudder surfaces. This results in a marked de- crease of rudder effectiveness allowing the high tun-rate to continue, and may even cause the rate to increase. When the boat has turned far enough to windward to start the sails lulling the overturning wind pressure begins to ease allowing the angle of heel and the turn-rate to decrease and the rudder to start regaining some effectiveness. By the time all of his has occurred the boat is usually in irons, and may even have inadvertently tacked.
So what can be done? When sailing close hauled in heavy air and gusty conditions you must anticipate and be prepared for a broach. The key operative word is anticipate. Learn to anticipate the arrival of a gust by watching the water surface for the "dark spot" accompanying a gust. Watching nearby boats (particularly those to windward) will allow you to judge if the gusts are strong enough to initiate broaching. When an approaching gust is detected start preparing for its arrival by easing your sails a bit more, even to a point where you carry a slight luff. When the gust arrives, or preferably slightly before, rapidly ease your sails, completely luffing if necessary, to minimize the increased heeling effect, and use no more than half rudder to compensate for the start of the turning effect. Recovery from a lulling condition is much easier and quicker than recovering from being in-irons. If the boat is allowed to heel too far the end of the main boom can strike the water (known as "tipping the boom"), the boom will be unable to go out and the boat will broach. If your timing is right the eased and slightly luffing sails should become full as the apparent wind shifts aft and boat speed should increase, and the faster a boat goes the more effective its rudder becomes so not as much deflection is necessary. As boat speed increases you can "milk" the sheets in or bear off slightly to maintain speed and extract as much energy from the gust as possible. As the gust passes trim the sheets in and resume your former course.
Sounds simple but you must practice this technique constantly to develop the ability to react instinctively and correctly. After all there is a lot of "stuff' happening in a very short time and you will have no time to stop and think about what to do. Obviously you will not be able to curtail all broaches but with constant and diligent practice you should be able to negate a large percentage of them and minimize the effects of those you missed. Once again there is a key operative word, practice. Broaching on other points of sail or in gusty medium-air occurs in a much less violent manner and more slowly so there should be ample time to react.