How should I rig a preventer?

Dear Boat Doctor,

I experienced my first accidental jibe a few weeks back. I was flying downwind on a beautiful afternoon, and my autopilot lost control. The boat turned hard to leeward, the boom came flying across the boat. I know I should have rigged a preventer, but I’m unsure as to the best way to do that. My plan was to run a line from the boom down to the rail, but now I’m hearing that’s a bad idea. Can you help?

Mark Matthews

Raleigh, North Carolina

Dear Mark,

I am glad it turned out OK, because accidental jibes can be very dangerous.  As you discovered, keeping the boom under control is important when the wind is aft of the beam.

It seems pretty natural to lash the boom down to the rail, right at the point where the boom crosses, but there are a few problems with this technique. The biggest issue is that you prevent the boom from rising should the boat heel enough for the boom to hit the water. Lashing the boom down is a recipe for a bent or broken boom.  

The other problem with this approach is that you are loading the boom in an area that the designer did not intend to load. In mechanical engineering terms, you are applying a bending moment on the boom, which increases the likelihood that the boom will end up bent. Boom brakes and similar devices that are attached at mid-boom work because the system merely slows the motion of the boom rather than locking it down. This can prevent a catastrophic, out of control boom, but doesn’t fully prevent an accidental jibe.

A better way to rig a preventer is from the outboard end of the boom to a turning block forward on the rail. With this arrangement, the boom is held in place but can rise should the boom hit the water. By attaching it to the outboard end, there is no bending moment applied to the boom; instead it is loaded at the same point where the mainsail clew is applying load.

Use a low-stretch line to rig a preventer and apply a bit of tension to it, but not so much as to require a winch. Using a low-stretch tensioned line will ensure the boom stays in place if you jibe. Line with more stretch will allow the boom to swing slightly inboard, causing the preventer which has been rigged far forward to change angle to the point where it will be up against the cap shroud, causing loading the shroud is not designed for. 

Lastly, when you rig a preventer, make sure that it can be easily eased or taken off a winch in an emergency.