Rigging a boom preventer will allow you to sail a true downwind course without a constant worry about crew safety. I also use it to pin the boom in its preferred position in light winds with sloppy seas.

When racing short-handed with a symmetric spinnaker it also allows me to use aggressive windward heel to make extra metres to leeward. Here are a few of my top pointers for getting the most out of this valuable set-up.

Fixing point

The preventer should be attached to the outboard end of the boom to avoid damaging the tube in the event of an accidental gybe. Some boom end castings have a designated hole through which a preventer can be attached.

A large bowline loop (the red striped line) affixes the preventer to the boom. The knot can be reached without sheeting the boom in to the boat

If this is not the case then a large bowline loop passed around the end of the boom between the clew and the end casting will work just as well. The loop should be long enough so it can be undone from the side deck without the need to re-centre the boom.

Alternatively, to avoid hauling the boom in every time the preventer is required, make a strop around two-thirds of the length of the boom with an eye in both ends. One end can be permanently attached to the boom and the other will be attached to the running part of the preventer.

The strop can be accessed easily from within the footprint of the deck while the mainsail is out. When not in use, the strop can be tensioned with an elastic cord from either the kicker fitting or inboard boom casting.

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Think of the preventer as a part of your running rigging. It works in opposition to the mainsheet and so needs to be accessible from the cockpit and easy to trim in as you let the mainsail out. For maximum resistance to an involuntary gybe the preventer line should lead from the boom end as far forward as possible, then back to the cockpit.

I often use a forward mooring cleat in lieu of a turning block, making use of the fairlead to avoid toe-rail chafe. If you have no mooring cleats available then use a snatch block on the toe rail or mid-foredeck.

The preventer line should be trimmed from the cockpit using a winch. Often the best winches for this function are halyard winches as they tend not to be in permanent use. This may mean feeding your line under a sprayhood.

If you have absolutely no winches free, then it is also possible to take the line back along the deck to a stern mooring cleat. If choosing this option, check for chafe as the line passes down the deck and ensure the preventer leads into the cleat with a fair or open angle so it can be eased smoothly under load.

Gybing

As soon as your preventer is rigged, make sure your crew are aware of what to do in the event of a gybe, both planned and accidental. For a planned gybe ease the preventer out as the mainsheet is pulled in. Once the attachment point can be reached safely, a crewmember should detach it, working from the leeward side of the boom in case of an early gybe.

Once gybed, set it up again on the other side. Accidental gybing with a preventer rigged can be alarming, especially in the dark, with the noise made by the sail and the windward heel of the boat. In most cases, if the main loads up from behind, the helmsman should gently steer the boat back onto the original gybe. Be aware that the windward heel caused by the backed mainsail will bear the boat away further, so take action promptly.

In the worst cases, boat speed will slow significantly, and steering the yacht back onto the original gybe becomes impossible. In this case the preventer must be eased under control. Make sure all crew are away from the path of the boom and traveller, then gently ease the preventer with sufficient wraps around the winch to maintain smooth control. Pull in the mainsheet as it becomes slack, then gybe the main as normal.

Boom preventer tips

• Do not tie off the preventer forward. This would require a crew member to go forward for a release in the event of a gybe, whether voluntary or not.
• Resist the temptation to improvise a ‘quick fix’ to avoid pulling in the main by, say, tying vang fittings to the toe rail.
• On longer passages regularly check your preventer for chafe, particularly where it crosses the toe rail, or if you are using a mooring cleat as a turning block.
• A preventer line should be around 1.5 times your boat length and the same diameter as your mainsheet. Double braid polyester is ideal.

First published in the September 2017 edition of Yachting World.