A preventer is a line used to keep from the boom from suddenly swinging across the boat in a situation such as an accidental gybe. When the boom swings quickly from one extreme to the other, the forces generated can be huge and result in damage to the boat or injury to anyone in the way of the boom or the mainsheet tackle. Someone can easily be knocked overboard.
When a sailboat sails on a point of sail near downwind, the mainsail is typically positioned far out to the side. If the boat crosses the wind from one side to the other, a gybe occurs and the sail is backwinded and swings across the boat’s centerline to the other side. Ideally it is a controlled gybe and the mainsheet is shortened in advance to prevent the boom from swinging from one extreme to the other. But if a wind shift or gust or a large wave suddenly pushes the bow across the wind, an accidental gybe can occur and cause a serious accident - unless you have a preventer rigged.
A preventer also works like a boom vang to hold the boom down and the sail full when sailing downwind.
A Makeshift Preventer
The concept of a preventer line is simple: a strong line is tied to the boom at a convenient strong point (the outboard end of the boom is better than midboom) and brought up to a secure point forward of the mast. If you do not have a permanently installed preventer line, you can simply tie a dockline or heavy spare line from the boom taut to a toerail fitting once the mainsail is trimmed for the downwind point of sail. Be sure the line can handle the large forces involved, and be sure the attachment points are secure. Do not, for example, tie the line to the small support strut welded onto some stanchion bases – these have been known to snap off under pressure and become a flying missile back at the cockpit!
When changing point of sail, untie the forward end of the line to allow for trimming the mainsail in. If you are making a controlled gybe to continue downwind on the other tack, just retie the preventer on the other side afterwards.
This simple preventer will work when the boat is not equipped with a permanent preventer but is not as easy to use and requires a crew going forward on deck. A better solution is to rig a permanent preventer.
A Simple Permanent Preventer
At minimal cost you can rig a permanent preventer that is easy to use, is fully adjustable, and does not require going forward on deck.
All you need is two blocks (pulley attachments) attached forward on both sides, ideally near the toerail, and enough line to run from the boom attachment up to the blocks on each side and then back to the cockpit. With a larger boat, you may need additional turning blocks between the forward blocks and the cockpit, to run the lines clear. If the boat does not have cleats free for securing each preventer line at the cockpit, you should install them also. Do not plan to use the cleats needed for jibsheets.
With this arrangement, simply secure the preventer on the mainsail’s side of the boat at any point of sail. Haul in the preventer on that side and cleat it. During a controlled gybe, slowly release the preventer as the mainsheet is brought in – so that the boom is controlled at all times – and then pull in the preventer on the other side as the sail is let out.
It is crucial to use a large-diameter line for the preventer (which is also easier on the hands), as well as strong blocks and secure attachments points. Remember, the forces can be enormous. A severe injury can occur if the system fails, as happened in this case.
Starting at about $200, several commercial boom brakes are available that can serve the same function as a preventer but offer more control when gybing. All follow the same general principle. A line rises from a deck fitting on each side to the brake mounted on the boom in a convenient midboom site. When gybing, whether accidental or intentional, the brake slows the movement of the boom through tension on this line.
The boom is not entirely prevented from moving, but when the system is properly installed and adjusted, the boom can move only slowly and thus lowers the risk of damage or injury. Three general types of boom brakes are available, depending on your boat and your needs.
Friction Broom Brakes
Broom brakes like the Wichard Gyb'Easy Boom Brake have no moving parts. The continuous line from port to starboard passed through the device, which produces friction on the line. The more tension in the line, such as during a gybe, the more friction - thus slowing the movement of the boom. This is simple to install and easy to use, with no moving parts to break.
Drum Type Boom Brakes
Drum type boom brakes like the Walder Boombrake function similarly with friction as the port to starboard line wraps one, two, or more times around a drum in the device. This is similar to how friction slows movement in a line wrapped around the winch. The more tension in the line, the more friction – again, slowing the movement of the boom.
Adjustable Sheave Boom Brakes
Boom brakes like the Dutchman Boom Brake use multiple sheaves in which tension on the line can be adjusted with a knob. This is easier than changing the number of wraps around a drum to adjust tension. With more moving parts, this type is usually more expensive.
Advantage of Boom Brakes
While any type of boom preventer or brake can solve the problem of the boom suddenly crashing across the boat wreaking havoc and possible damage or injury, a brake offers one key advantage. Since the boom is not completely locked in place as with a preventer line, if you do accidentally gybe and become backwinded, the boat will be much easier to control if the boom can move to allow the sail to cross over. If the mainsail is held in a backwinded position, the boat can rapidly heel to the opposite side and may broach. On a larger boat, especially, the force of wind is large and makes it difficult to recover from the gybe and get back on course, especially if under-crewed.