A preventer’s principal role is to keep the boom (and mainsail) from flogging when running in moderate conditions, especially when the main is set by the lee. Under these conditions, the rolling of the boat can make the boom swing back far enough to backwind the main and start off an accidental gybe.

A preventer is great and I use mine (read details of Sunny Spells’ preventer in this post) most of the time when going downwind (“gentlemen don’t sail to windward”, so that’s a lot!), but you really don’t want it to be the only thing restraining the boom when the poo hits the proverbial…

Regardless of how you set it up, during a planned gybe the preventer has to be let off and re-set on the lee side. SO, this is NOT a part of the running rigging that you can (or should) rely on when the going gets rough or you need to do things quickly. This was graphically illustrated to me recently when crewing on a fifty footer in the Sydney – Gold Coast race: with wind gusting to 45 knots we had to gybe, and opted for a “granny gybe” (tacking the bow through the wind) to reduce the strain on the rig. The crew responsible for the preventer was a bit slow throwing it off, resulting in a pad-eye ripped off the bow and a broken preventer line which proceeded to wrap itself around the prop… Thank gawd we didn’t attempt a gybe!

Also, if you really get the boat out of shape and the main is backwinded, you need to release the preventer pretty smartly, and if you can’t, it needs to be fail-safe – i.e the line should break at a lowish force rather than a high one (this is where the boom brake comes in). Imagine the 2 tonnes of force in a 1/2in preventer being released instantaneously when it breaks! A recent post on CruisersForum suggested using a velcro strop to attach the preventer to the boom – this sounds very sensible as it would allow the preventer to release in an emergency without breaking anything.

Boom Brake

Before I left Sydney for Hamilton Island in May, I rigged a boom brake (details in an earlier post here) to control the boom during gybing especially when sailing short-janded. The boom on a sail boat can be lethal:

• Earlier this year a man was killed off the Queensland coast when he was struck in the chest by the boom on S/V De Ja Vu during an accidental gybe. He was sailing with his wife who was brought to shore by rescue helicopter after the accident.
• The boom striking one of the crew members is thought to have contributed to the disappearance of three yachtsmen in the Whitsundays in April 2007.

The boom brake was initially a bit of an obstacle on the side decks, but I got used to it pretty quickly. I initially thought I might unshackle it from the toe-rails when it’s not in use, but never got to the point where I felt it necessary (we were running most of the time though).

I run my jacklines OVER the boombrake lines, which also keeps the tether hooks off the deck, at least over that area. However, having an intermediate “catch point” in your jacklines (by running them uner the boom brake) is not such a bad thing – if you get washed along the deck by a “green one” you’ll get stopped midway rather than dangling over the transom! It’s just the clipping and unclipping (on a dark and stormy night…) that becomes an issue.

Yesterday I rigged a preventer to use when running downwind. It consist of about 30 meters of 8mm polyester braid line(stretchy) that runs all the way to the stem (outside everything), through a snatchblock and back to a jammer on the cabin top.

I can control the preventer from the cockpit and put it on a winch to set and (most importantly) ease off gradually when necessary. Sunny Spells has the main sheet attached to the end of the boom, and the preventer is attached at the same point, using a snap-shackle. This prevents bending moments being applied to the boom by the preventer and main sheet working in opposite direction, resulting in a broken boom… Fortunately this arrangement is also good at preserving the boom intact should it get dragged in the water!

At 25 meters, the preventer is long, almost double the required length. I did this on purpose so that I can just throw off the jammer when changing tack without worrying that the line will pull through the clutch. I just leave it shackled to the boom and re-run it after the gybe. Also, the snap-shackle stops the line from pulling through the snatch-block on the bow. When I need to re-set (or stow) the preventer, I just pull it through in the cockpit, flaking it on the cockipt floor, until the snap-shackle stops in the snatch-block on the bow. Now I go forward, swap the snatchblock to the opposite toe-rail, take the snap-shackle (outside the lifelines) and walk back to the cockpit, pulling it through as I go. I just snap-shackle it to the boom end, close the jammer and pull it tight – too easy.

Before I left Sydney for Hamilton Island in May, I rigged a boom brake to control the boom during gybing. I knew I was going to be short-handed some of the time and a recent fatality on the coast where a man was killed when struck on the head by the boom during an accidental gybe was fresh in my mind.

My boom brake is very simple – a Figure 8 “rescue descender” used for rock climbing, which I got off EBay for $45, and 20ft of 1/2in polyester braid line. The line is shackled to the starboard toe-rail and runs through the Figure 8 to a block shackled to the port toe-rail and then back up to the cabin top jammers via a free sheave in the line organiser. I use a winch to grind it on. Because of the set-up, it effectively has a 2:1 purchase.

When the line through the Figure 8 is slack, there is virtually no friction and thus no resistance. Wind the boom brake line on tight though, and quite a bit of braking effect is generated. The tighter the line, the higher the friction. I had to play around a bit with the attachment points on the toe-rail and boom to even out the friction through the boom’s swinging arc, but as the boom attachment lug can be moved anywhere between the vang attachment and the boom end, this was not an issue. I’ve opted for higher friction at the end-of-arc, with less friction on the centreline. This way I know the boom will be gradually slowed down as it reached the end of its travel.

A boom brake has a number advantages:

• The forces on the boom are controlled by friction, so it won’t over-stress the boom when dragging the boom in the water.
• The boom brake acts as a second vang, pulling down on the boom towards the toe-rail. This is beneficial side-effect is most noticable when running with the main let off. Under these conditions I’ve found the vang to be a bit under-powered; the boom brake significantly reduces the strain on the vang.
• If you managed to break the traveller or otherwise stuff up the main sheet, the boom brake could be used as an emergency main-sheet.