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Is my preventer rigged incorrectly?

2020 September 1

Dear Boat Doctor,

I have had a few close calls with accidental jibes, and I now typically use a preventer when sailing downwind. I just use a spare line and tie the boom down the to rail at the point where the boom crosses the rail. However, I’ve had a few people tell me this is a bad idea. What am I doing wrong?

Bob Sampson

Portland, Maine

Dear Bob,

I am glad to hear you use a preventer because an accidental jibe can be dangerous to both your boat and its crew. A particularly violent  jibe can dismast the boat, being hit by the boom can be deadly. But the technique you are using is not the best.

With the midpoint of the boom lashed to the rail, you run the risk of bending and breaking your boom. The midpoint lashing places the load on the boom during an accidental jibe, in a place on the boom where it was not designed to be applied.

Better preventer geometry can be achieved by taking a line from the end of the boom, through a turning block up forward on the rail and then back to the cockpit. With this setup, the load is transferred to the end of the boom and puts compression load on the boom, which booms are designed to handle. By rigging a preventer this way there is also the benefit of being able to adjust or release the preventer from the cockpit. 

I like to place the turning block on the rail at the point where the boat starts to taper toward the bow. This is usually about halfway from the mast to the bow. If you have a slotted toerail, you can use a snatch block, if not, you may need to add some hardware to the deck.  You don’t need fancy low-friction snatch blocks for this application. Colligo Marine (www.colligomarine.com) makes some great soft snatch blocks, these are simple, strong and relatively inexpensive blocks.

It’s important to use a relatively elastic line in the preventer because you want the line to absorb as much of the energy as possible. Three-strand nylon is  a good choice because both the material and construction are elastic. For people who don’t like handling three-strand, single-braid line can offer a little nicer handling.

A good preventer will reduce stress and make your boat a safer place.