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How to rig a sailboat boom preventer (and avoid tragedies)

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boom preventer

The boom breakage on the X-4.3 Agecanonix, on the basis of the first “unofficial” reconstructions, could have been caused by an involuntary gybe (“chinese gybe”) with the boom braked by the preventer (frame taken from the video released by the German newspaper Bild).

The tragedy aboard the X4.3 ‘Agecanonix’ at the ARC, and the death of a French crew member, Max Delannoy, due to boom impact, puts the focus back on crew safety.

Boom preventer is an essential and necessary rig for safety when sailing aft and to avoid the uncontrolled passage of the mainsail over the new walls in the event of an unintentional gybe. We asked “guru” Davide Zerbinati (expert, boat builder, sailor and author of monumental books on onboard work) to help us understand how to rig this rigging correctly, how the boom brake works as an alternative, what the risks of incorrect installation are and what must be done to increase crew safety.

Boom preventer, all you need to know

Davide, what was the cause of the accident on the ARC?

It looks like an unfortunate unintentional gybe, but we can’t speculate until the World Cruising Club technicians establish exactly how the accident happened.

What we do know is that the incident occurred in challenging conditions, with a three to four metre crosswave and winds of between 30 and 40 knots. We do not know under what circumstances and in what sequence, we only know that the boom, or a piece of the boom that had already broken off, hit the victim in the head.

What’s the boom preventer and why is it so important for safety?

For safety, in that sea, in a 3000-mile ocean voyage, at downwind speeds, it is essential to rig the boom preventer. The latter is a rope that restrains the boom and prevents it from swinging over the new walls in the event of an involuntary gybe (the “Chinese gybe”), an event which, with waves and oceanic winds, subjects the mast and the boat to stress with often disastrous consequences, including dismasting.

It should also be remembered that the keel holds the boom in place and preserves the batten and rigging from the stresses of the mainsail. We do not know if and how it was rigged on the Agecanonix, but it is known that incorrect installation of the preventer may not be effective or may cause the boom to break.


VIDEO – How the boom preventer is rigged on Davide Zerbinati’s boat

How to rig the boom preventer properly?

A line that starts at or near the end of the boom (varea) and reaches the bow on a bollard. Or better still, with a loop fitted with a block to return to the cockpit, on a winch or on a bollard, with which it can be adjusted.

When the mainsail is open at the stern, the sail must not touch the spreaders or shrouds, but once the preventer has been fixed, the mainsheet can be recovered a little. The best equipped boats already have two preventers.

boom preventer

The advantage of having the rig on a winch is that you can fine-tune it, but it is also easier to leave in an emergency.

Boom preventer rigging

Boom preventer rigging

What lines to use and how to attach them to the boom?

The preventer line must be of a larger diameter than the mainsheet, because the mainsheet passes through reefing blocks, whereas the reefing is a direct operation. To give you an idea, on a 50ft boat a 16mm dyneema line can be used to hold back the mainsheet, on a 45ft boat 12 or 14mm, also in dyneema.

Some boats already have it, Selden has it, Hallberg-Rassy has it as an option. It’s best to use a loop around the boom or a strap rather than the classic carabiner. On some boats there is a hook that can be anchored to the gunwale or to padeyes on deck.

What happens on a gybe with a boom preventer?

If the boat goes into an unintentional gybe, the preventer goes into tension, holding the boom. If you can’t get back to the previous tack in time, you have to let go of the preventer slowly and use the sheet to get the boom over the new tack and resume steering the boat. This is why the preventer should be easily and quickly adjustable. It goes without saying, therefore, that if the leeway is fixed in the bollard it should be tied with a simple knot that can be untied under tension.

What are the mistakes that can break the boom?

If the preventer is incorrectly fixed in the centre of the boom, it will create a deflection in the centre of the boom-beam, which can easily break the boom. The anchor point should not be the same as the sheet block (it will not withstand the stresses), or a metal-to-metal joint with a shackle.

The anchorage of a preventer can be made with a dyneema webbing loop, wrapped around the sail. Recent tests carried out by Practical Sailor magazine have shown that on modern boats with very quartered spreaders, since the mainsail cannot be opened much, the working angle of the leverage arm of the retaining line is lower, so that only 2% elasticity of the retaining line is sufficient to cause the boat to gybe (this is why we recommend dyneema, a static line with low elasticity). Other problems arise on smaller boats, which can easily find themselves with the boom in the water. The boom, if held, could break or damage the mast.

What to do if the boom breaks?

It is conceivable that with a hacksaw and a few rivets, a crew could find a makeshift solution to repair a broken boom, but having the mainsail on board, which can be rigged even without a boom, could allow them to continue sailing and save themselves.

What is a boom brake and when is it used?

On small to medium-sized boats, an alternative to a preventer is a boom brake, for example the one made by Walder. It has a different function, it does not restrain the boom, but slows down the boom swing, converting the kinetic energy of the boom into heat, through the friction of the lines passing through its structure.


boom brake

Boom brakes are metal components in which a line runs, with friction (in the case of Walder, the line is wound spirally along a drum). Tensioning the sheet increases the friction and therefore the braking force of the device. The base of the mainsail is often used as an anchor point, and the brake is attached to the centre of the boom.

Are equipment failures predictable?

Those who only have experience of sailing in the Mediterranean have difficulty understanding the effects of 3, 4 or 5 metre ocean waves and the mechanical stress placed on all the equipment. Every day, or at the change of shift, a tour of inspection called “hands, eyes, binoculars” must be carried out. By observing and touching the equipment, one can detect cracks, loose shackles, play on the structures, premonitory signs of failure.

Prevention and preparation of the boat are essential: for example, those who have no experience in the ocean tend to underestimate the effects of the rigid spring or mechanical vang, which, due to prolonged wave stress, risks fracturing the boom at the very point where the vang is attached. Some boats replace the piston with a classic textile vang. Adjustments are also important; a good vang adjustment, for example, can reduce mechanical stresses on the rig.

How to increase crew safety and avoid tragedies?

When moving towards the bow, pass upwind, obviously tied up, and always crouch down. Where you sit in the cockpit is also important for safety. In dismastings, fortunately, the mast almost always falls to one side, so the cockpit is a safe area, but it often happens that a block breaks under tension and hits those on board.

The further aft you are from the rigging, the safer you are. Modern racing boats with cockpit covers, such as IMOCAs, also protect the skipper from such events. But it is essential for safety that the whole crew knows what to do and how to move, and how to steer the boat safely to continue sailing in the event of an accident. There are incredible stories of deaths of the skipper on board, and of crew members, who did not even know how to use the VHF.

Is it safer for the crew to stay below deck?

When you are in the middle of 3, 4 or 5 metre waves, it is like sitting on the mechanical bull. Those at the helm find it hard to stand. It’s hard to stay below deck for long in those conditions: you have to find a safe position in the cockpit, where you always have your feet up and your hands gripping on. It’s not easy even for the most experienced crews.



Davide Zerbinati

Davide Zerbinati

About Davide Zerbinati

Davide Zerbinati is a yachtsman, engineer and nautical architect, CEO of Valle Scrivia Srl, a company specialising in supplies for long-range navigation and on-board safety. In his activity since 1998 he has surveyed about 5000 boats, more than 1000 models and supervised about 100 refittings. He is skipper of the Stadtship 54 Aluaka, a boat with which he won Leg 1 of the Las Palmas-Mindelo (Cape Verde) leg and the Mindelo-Santa Lucia leg of the ARC+ and is ready for new adventures and training.

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