How to rig a preventer and boom brake: Our expert guide

A well-designed preventer system is just as important as any other part of the rigging, but it’s often an afterthought, sometimes little more than an old mooring line tied around the boom.

As the average size of a cruising yacht, and a mainsail, increases, it becomes ever more critical. In the preceding pages, we’ve looked at four cases over the last 20 years in which accidental gybes led to serious injury or death of at least one crew member.

Although these fatalities happened for different reasons, many of them shared similar features, such as strong winds and large seas, helmsperson inexperience, fatigue or distraction and the skipper being away from the cockpit.

Two had preventers rigged that failed. In particular, the Platino report demonstrates the huge loads involved, and the importance of how gybe-prevention equipment is fitted and rigged. It makes for sobering reading and has made me sit back and think about how I do things on my boat and when I am aboard other boats. Hopefully, these two articles will serve to make us better, safer and more aware skippers.

Chastened by the reports’ findings, I spent a day with Iain Horlock, chief rigger at Devon-based rigging company, Jimmy Green Marine, to better understand how to design a preventer for a cruising yacht under 50ft. We used Nimrod, my 36ft Cheoy Lee, to create a traditional preventer system and also tried two alternatives to a preventer: the Walder Boom Brake and the Wichard Gyb’Easy.

Rigging a Preventer

What does a proper preventer system look like then? Best practice has always been that it should run from the end of the boom to as far forward as possible, but the Platino report makes this imperative clear. There are several components: a strong point on the boom, the preventer line itself, which might be separated into two parts, and the deck fittings which allow it to route from the bow and back to the cockpit. I asked Iain to help me specify the route and give advice on components.

Strong point on the boom

Fortunately, Nimrod’s boom has an excellent strong point in the form of a fixed stainless bar in a cutout on the underside. This is used for the mainsheet attachment, but there was plenty of space to attach a preventer to. If you don’t already have a dedicated strong point, ask your rigger for advice.

There are a variety of different boom fittings to consider: additional slugs under the boom, a handle on the end of the boom, or attaching saddles to the boom to run a strop around the boom itself.

We checked this with mast and spar manufacturer Seldén, and account manager Richard Le Mare told us: ‘We would always recommend setting up a preventer from the aft end of the boom to prevent the boom buckling. If you have a mainsheet in the centre of the boom we would still recommend the end for a preventer as the spar is designed for downward and sideways force not the direct sideways force you get from a preventer working hard to stop the gybe.’

He noted that some older booms have a cast aluminium boom-end fitting with a large eye or handle in it. This was designed for the purpose of rigging a preventer and should not be used for anything else. Attachments should be made with rope or soft shackles – other metals could damage the fitting.

While a deep reefed main may place the clew of the sail, and so the force of the gybe, further forward along the boom, it is worth keeping the preventer rigged to the strong point at the end of the boom. This will still give it the best mechanical advantage and avoid you having to rerig it.

Preventer pennant

I have always preferred having a short ‘half’ preventer, or preventer pennant, permanently rigged to the boom. It means that when you need a preventer, half the work is already done, and you’re more likely to do the other half of the work. It also removes the need for someone to attach a line to the end of the boom whilst underway, which isn’t particularly easy or safe if the boom has already been partially eased.

I gave Iain the length between the strong point at the end of the boom, and the kicker fitting. He made up the correct length pennant with two spliced eyes. One end was attached with a soft shackle to the strong point so that it would allow articulation (which might need anti-chafe for long passages), and the other end connected to a bungee on the kicker fitting to keep it out of the way. Iain used Dyneema for this so that it could be a smaller diameter and was less bulky for a line which is permanently rigged.

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It’s worth adding that although we set up just one preventer and pennant on the day of the test, it’s probably worth rigging one on each side, so that you do not need to go forward and swap the preventer every time you put in an (intentional) gybe.

Preventer line

For the preventer line itself Iain prepared a 10mm braided polyester line, about 1.5 x the boat length. He put a snap shackle in one end so that it could be connected to the pennant quickly without losing much strength. According to the Platino report, snap shackles are not entirely reliable under dynamic loads, but they are better than using bowlines.

A soft shackle might be better in terms of reliability, but for ease of use we decided to stick to the snap shackle for a boat of Nimrod’s size.

Routing

An in-depth analysis of the strength of the deck fittings isn’t realistic for most cruising yachts, but an experienced rigger should be able to determine whether they’re fit for purpose on a moderate-sized cruising yacht. Take a look beneath your deck fittings to see whether they’re reinforced with backing plates, or just small washers.

For many boats the bow mooring cleats will be the best position, with the advantage that their backing arrangements can be seen inside the anchor locker, and reinforced relatively easily if needs be.

As you’d expect for a boat designed in the late 1960s, Nimrod has substantial deck fittings. The fairlead on the bow is let into the toe rail, which gives it strength at the preventer apex. With open fairleads, there is a risk the line could jump out so a proper turning block may be better long term.

The bow cleats and midships cleats made natural turning points for a clear run back to the cockpit, with the addition of two relatively inexpensive low friction rings to achieve the best lead. Using snatch blocks instead would help everything flow, but this is less important for static loads than for one which is in constant play, like a spinnaker sheet.

Pre-rigging

We prepared the preventer alongside in Plymouth’s Sutton Harbour. If you’re setting off on a downwind passage it’s much safer to rig it in port. Clip the end which connects to the pennant to the guardwires amidships, so that it’s easy to access when needed.

Keep it secure using a thumb knot in the block at the cockpit end and coil the line over the guardwires to keep the cockpit clear.

TEST 1: The preventer

We set off into Plymouth Sound with a light westerly breeze. It was ideal test conditions: strong enough to fill the sail but light enough that we could experiment safely. A preventer should always be connected on a safe point of sail, with an experienced person at the helm.

If possible, don’t ease the mainsheet too much before connecting it, so that if there is a crash gybe at this point it won’t have so much travel. If you’re short-handed, it’s better to ask the novice crew to do the deck work whilst you remain on the helm: taking the preventer off the guardwires and reaching up to the boom to connect it to the pennant is easy if it’s already rigged.

Alternatively you could heave-to, which will bring the preventer to the high side, and then gybe out of the heave-to and ease the mainsheet once set on a broad reach. All of these options depend on the conditions, and your sail plan – the more mainsail there is up, the more difficult it is to come up to wind.

On most boats the mainsheet needs to be well-eased for the preventer to clear the forward stanchion posts. Once attached, the preventer needs to be well-tensioned to reduce any dynamic loading in the event of a crash gybe.

We gybed the boat with the preventer on. The mainsheet came slightly slack – there was some stretch in the preventer system, but the boom held. It felt precarious and it wouldn’t have been comfortable to stay that way
for any length of time, but it gives an opportunity to either ease the preventer and sheet in the main (but beware, this may risk bending stanchions as the angle to the boom changes), or gybe back again onto the original course.

TEST 2: Gybing device – Wichard Gyb’Easy

The Gyb’Easy is a ladder-shaped device with three different ‘rungs’ and a pin at the top. It’s a one-size-fits-all solution for boats with mainsails up to 40m², which covers cruising yachts up to about 40ft. It should sit just aft of the vang bracket, although on Nimrod the only option was to attach it to the vang bracket itself.

A bight of line is threaded through the rungs and over the pin. The stronger the wind the more friction you need and the more rungs of the ladder you incorporate.

The two ends of the line then run symmetrically from the boom to the base of the shrouds, through a block and back to the cockpit winches. On Nimrod this setup wasn’t possible for a test sail, but low-friction rings on the midships cleats worked well instead. Wichard provides a specific line for the job, called the ‘Gyb’flex’, which is designed to help absorb shock loads. We used a substitute polyester braided line instead for the test sail.

We set it up with two ‘rungs’ of the ladder and eased the mainsail out and tensioned the Gyb’Easy. During the first gybe the boom swung across fast, although not quite full pace. Clearly we needed more tension on the line, or more friction. I was glad we were positioned well forward in the cockpit and hadn’t got in the way of the mainsheet.

Iain added an extra ‘rung’ of the ladder and we tightened up the control line. On the second gybe the boom didn’t budge, but by easing the tension on the line we could bring the boom across in a controlled way. After a bit more trial and error we found the sweet spot: the setting with the least resistance but with plenty of tension on the line. The boom swung smoothly through the gybe.

TEST 3: Gybing device – Walder Boom Brake

The Walder Boom Brake is a drum-shaped device around which a line is passed two or three times depending on the conditions and amount of friction required. The device comes in several different sizes depending on the size of the mainsail, the largest of which can be used on a boat larger than 50ft.

We used the Walder 203 (boats up to 12m or 8 tonnes). It’s recommended that it replaces the kicker, or sits aft of a rigid vang.

Feeding the line onto the Boom Brake takes time, but can be done in advance and left in the locker pre-fed, or rigged prior to departure.

In a similar way to the Gyb’Easy, the Boom Brake needs a strong attachment point at the chainplates or toe rail by the shrouds. On one side, the Boom Brake can be made off, running back to a cockpit winch on the other side for tensioning.

The Boom Brake worked well on our first gybe; perhaps we’d learnt how much tension to apply to the line after testing out the Gyb’Easy. However, since we’d rigged it asymmetrically by tying it off to one side, it worked better on one gybe than the other.

When the control line is running to a winch on the windward side, in light conditions there’s a tendency for it to drag the boom to the centreline as you tension it. This could have been avoided if the line was rigged to a winch on both sides, so that you could tension the leeward control line, or if there had been more pressure in the mainsail.

Overall though the Boom Brake performed well, taking the energy out of the gybe in a controlled way.

Preventers and boom brakes verdict

All three systems worked well and I’d be happy sailing downwind with any of them.

The advantages of preventers are that they’re simple, cheaper to install, and a pennant permanently attached to the boom is much less obtrusive to leave rigged than the other two devices. However, a traditional preventer adds an extra layer of complication to gybing because they need removing first, then the mainsheet needs sweating in, and then after the gybe it needs re-rigging on the other side. This means someone needs to leave the cockpit every time you gybe.

If you opt for a preventer, I would rig a pennant and line each side so that you can set the new side without going to the foredeck. The downside is that you then have more lines in the cockpit, and additional slack lines hanging down from the boom during a gybe could be a hazard. If you’ve got ingenious ways of mitigating this, YM would love to hear from you.

Three weeks after our test, I sailed from Plymouth to Roscoff on a broad reach. The preventer stabilised the boom in a 2m sea, and I was able to release it and round up behind a couple of ships when I needed to. The line had chafed in the fairlead after just 20 hours, so it does need some protection. I bought some cheap firehose online, which also works well as chafe protection for mooring lines. It would be hard to get a turning block far forward enough, and I like the solidity of using the fairlead.

The friction systems, when they’re properly set up, make gybing effortless. They’re also more versatile, since they don’t require the mainsail to be fully eased, you could use them on other points of sail to prevent the boom from swinging if there’s a rolly sea, which can be very hazardous even in light airs. They don’t hold the boom out to windward after a gybe as a preventer does, which still leaves you in a sticky situation and could give way suddenly. Instead, boom brakes let the boom across gently, which in many ways is preferable to a complete preventer.

However, to work well, we found they need two cockpit winches. If you’ve poled out a headsail and have a guy and downhaul rigged, you might not have enough winches left for this. They also have strong points by the shrouds, which some boats may not have.

The Walder Boom Brake felt like the more substantial piece of kit, but I liked the simplicity of the Gyb’Easy and it was easier to adjust the friction setting than on the Boom Brake. There’s also a lot of slack in the mainsheet when gybing so, although the gybe should happen slowly, you need to brief the crew to keep well clear. We all agreed that you need to know your boat well and become familiar with any system before trusting it in a blow.

I’m not sure I would ever trust them to gybe the boat safely with the mainsheet loose in more than 20 knots, but they’re there as a safety backup, rather than as a replacement for sailing your boat properly through a gybe.

Choose a preventer to suit you

Ultimately the choice will come down to your boat, your crew and how you sail. ‘A preventer system will always be fairly bespoke,’ said Iain at Jimmy Green, ‘because every deck layout is different.’ For occasional inshore use, or offshore passages on the same gybe, a traditional preventer is the simplest option, especially if the boat is a heavy-displacement cruiser which is more likely to sail deeper angles downwind and put in fewer gybes.

If I was sailing a more modern boat with an asymmetric in the sail wardrobe which favours reaching downwind, rather than running, and putting in more gybes, I’d be more inclined to use the Boom Brake or the Gyb’Easy. For tradewind sailing across the Atlantic on a boat over 40ft, I’d seriously consider the Boom Brake, especially if I had any inexperienced crew.

‘I’d be wary about designing a preventer system which is going to hold under any load,’ said Iain. ‘Something has to give eventually.’

None of these systems is guaranteed to eliminate the destructive consequences of a crash gybe. They do, however, move the balance of probability in your favour.

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