When sailing downwind, a preventer is usually run from the boom forward to the boat to avoid having the boom swing across the boat during an accidental jibe. The theory is sound, but the practicality is often overlooked.
The setup is simple, the boom is held out in place via four sheets: Mainsheet, Preventer, Topping Lift, and Vang.
The Mainsheet keeps the spar from moving forward.
The Preventer keeps the spar from moving aft.
The Topping Lift keeps the spar from moving down.
The Vang keeps the spar from moving up.
While it may seem very simplistic, specific details must not be overlooked to ensure proper function and prevent catastrophic damages.
First, the preventer and the mainsheet should connect to the same point on the boom, and this point should be nearest the end of the boom. If you mainsheet attaches as various points to the boom, place the preventer on the most aft attachment of the mainsheet.
Second, the preventer should be of equal strength to the mainsheet. If the mainsheet is setup with a block and tackle system, the preventer should be setup with an equally sized block and tackle system.
The purpose of the preventer is to avoid the boom from swinging across the yacht during an accidental jibe. Should the wind get ahead of the boom, the preventer will act as the working sheet and will be placed under incredible loads. If these two simple rules are not followed, the boom could break, or the preventer could snap and an accidental jibe would ensue.
If the Preventer is attached to the Vang attachment on the boom, the boom could snap when the Preventer is loaded. The force exerted on the boom is incredible, and the mainsheet attachment area is reinforced to handle these loads. The Vang area is not set up to handle these forces as it's job is only to hold the boom down and resist the opening of the sails leech.
If you attach the preventer to the vang area and subject the yacht to an accidental jibe situation, the force on the clew is going to push the boom aft, and the preventer is going to pull on the spar near the mast forward. This will cause the boom to bend and snap as the clew is pushed aft and the vanged portion of the boom is held forward.
Another common issue is to attach the vang to the toe rail to act as a vang and a preventer. The vang is more efficient when attached to the toe rail, as its pull would be directly downward, but it will not double as a preventer. If an accidental jibe were to occur, the boom could bend and snap at the vang attachment point and cause a catastrophic disaster.
If you decide to run the vang to the toe rail, you need to also have a dedicated preventer run forward that is attached to the mainsheet and appropriately sized for the task at hand.
This yacht owner has setup a permanently and properly installed preventer. The preventer is attached with padeyes between the two mainsheet blocks and the line is appropriately sized for the task at hand. To avoid issues with chafe, the preventers are not led forward of the spreaders, allowing the system to work with minimal chafe on the rigging on any point of sail.
The lines are setup with appropriately sized blocks and have fair leads leading the lines back to the cockpit. This allows the captain to set and release the preventers from the cockpit without the need to go forward. This would let you manage all the sheets from the helm in quick order, allowing you to execute your maneuvers quickly and easily.
Preventers are another part of the running rigging that needs to be setup properly and managed the same way as the rest of your running rigging. If you are going to rely on it to save your rigging from an accidental jibe, you need to make sure that it can withstand the forces and loads that it will be subjected to.