Preventer Rigging -- Lessons from the Platino Disaster

[Dockhead]

Originally Posted by Cpt Pat

I won't use a preventer on my boat under any circumstances! They are an accident waiting to happen. They are a lubberly kluge. If I'm sailing so close to dead downwind that I might need a preventer - I fly twin jibs instead, and take in my main.

And to keep the boom from flopping, I use a Dutchman boom brake. It won't "prevent" the boom from swinging due to a backwinded main, but it will slow it down enough to prevent damage (especially to your skull) and will stop the boom from flopping in light winds or a slight backwinded-main condition. If the main is fully inflated while taken aback in heavy airs, you want the main to swing over in a controlled fashion. The alternative (if the preventer doesn't fail) is a knockdown and the boat being pushed astern and athwart against the rudder. That's how people fall overboard and how rudders get destroyed. Backwinding the main is how people used to "back boats up" before "sailboats" (wind-assisted motor vessels) had engines.. . .. .

Well, with respect, these are all arguments for getting rid of the main when going downwind, not for not using a preventer. If these arguments show that preventers are "evil", then what are spinnaker poles? The devil?




I use a preventer whenever the end of the boom is out beyond the rail, just like my father taught me and like all good sailors I know do. Preventers serve many purposes besides preventing a gybe; they also prevent the boom from flopping around in other situations, including when the vessel is rolling in a calm. In my opinion, diligent use of a preventer is basic seamanship, at least on a vessel too large for a boom brake, and even then, there are cases where a preventer is essential.

[Yellowtulip]

Originally Posted by Cpt Pat

I won't use a preventer on my boat under any circumstances! They are an accident waiting to happen. They are a lubberly kluge. If I'm sailing so close to dead downwind that I might need a preventer - I fly twin jibs instead, and take in my main.

And to keep the boom from flopping, I use a Dutchman boom brake. It won't "prevent" the boom from swinging due to a backwinded main, but it will slow it down enough to prevent damage (especially to your skull) and will stop the boom from flopping in light winds or a slight backwinded-main condition. If the main is fully inflated while taken aback in heavy airs, you want the main to swing over in a controlled fashion. The alternative (if the preventer doesn't fail) is a knockdown and the boat being pushed astern and athwart against the rudder. That's how people fall overboard and how rudders get destroyed. Backwinding the main is how people used to "back boats up" before "sailboats" (wind-assisted motor vessels) had engines.

With preventers, as crew (never as skipper) I have personally seen a boom bent nearly in half when it "dipped" into the sea, a lower shroud and mainsheet block destroyed by a preventer failure and resulting crash gybe, and half a dozen knock-downs caused by a prevented main when it got heavily backwinded. I've also heard direct accounts of many bent booms/failed deck fittings and read of many injury-accidents involving preventer failures; including this one that could easily have resulted in death were it not for an incredible effort by the California Air National Guard.

Excerpt:

At about 5 AM Michael Kalahar and another crew member were on watch when a sudden swell twisted the vessel sideways back winding the main and causing the sail to travel hard to port. In doing so, the force of the reloaded sail shredded the preventer line allowing the main sail to travel at high speed across the cockpit and swing the mainsheet across the face of Michael Kalahar thus throwing him against the port side winch with the sheet wrapped around his throat and face.

https://web.archive.org/web/20140116...n-Pacific.html

Preventers are pure evil.

But the preventer wasn't the cause of the problem. If a preventer hadn't been fitted, the boom would still have flown across the cockpit, possibly with even more force and more devastating results, perhaps bringing down the mast with even worse consequences for all aboard. The guy was very unlucky that he got caught in the preventer. There's no such thing as a safe accidental gybe.

It would be interesting to compare figures of deaths and injuries due to flailing booms with and without preventers fitted. Impractical, though.

[Cpt Pat]

Originally Posted by Yellowtulip

But the preventer wasn't the cause of the problem. If a preventer hadn't been fitted, the boom would still have flown across the cockpit, possibly with even more force and more devastating results, perhaps bringing down the mast with even worse consequences for all aboard. The guy was very unlucky that he got caught in the preventer. There's no such thing as a safe accidental gybe.

It would be interesting to compare figures of deaths and injuries due to flailing booms with and without preventers fitted. Impractical, though.

> The guy was very unlucky that he got caught in the preventer.
Actually, he got caught in the mainsheet. This incident could be used as a safety argument for end-boom versus mid-boom sheeting.

There are alternative sail configurations that don't entail the risk of having the mainsail taken aback. Mine is to fly twin jibs instead of sailing wing-on-wing.

Here are some references to the twin-jib strategy and rationale for using it:

https://www.uksailmakers.com/news/20...ising?rq=simbo

Downwind voyaging - Ocean Navigator - January/February 2013

And in all other configurations, my Dutchman boom brake allows the boom to swing across gradually, under control.

Restraining the boom with a preventer allows the main to be fully and forcefully backwinded before the preventer fails and the boom swings violently across. Once the mainsail is backwinded, the forces are divergent, causing the instability to escalate. If the boom were unrestrained, it would gybe sooner when less destructive force had built up.

And even when the preventer doesn't fail, there is the risk of knock-down and rudder failure from the boat being shoved astern and athwart.

So far as luck is concerned: luck should never be used as a strategy. In chaotic seastates, Murphy's Law will eventually assert itself.

[Yellowtulip]

Originally Posted by Cpt Pat

> The guy was very unlucky that he got caught in the preventer.
Actually, he got caught in the mainsheet. This incident could be used as a safety argument for end-boom versus mid-boom sheeting.

There are alternative sail configurations that don't entail the risk of having the mainsail taken aback. Mine is to fly twin jibs instead of sailing wing-on-wing.

Here are some references to the twin-jib strategy and rationale for using it:

https://www.uksailmakers.com/news/20...ising?rq=simbo

Downwind voyaging - Ocean Navigator - January/February 2013

And in all other configurations, my Dutchman boom brake allows the boom to swing across gradually, under control.

Restraining the boom with a preventer allows the main to be fully and forcefully backwinded before the preventer fails and the boom swings violently across. Once the mainsail is backwinded, the forces are divergent, causing the instability to escalate. If the boom were unrestrained, it would gybe sooner when less destructive force had built up.

And even when the preventer doesn't fail, there is the risk of knock-down and rudder failure from the boat being shoved astern and athwart.

So far as luck is concerned: luck should never be used as a strategy. In chaotic seastates, Murphy's Law will eventually assert itself.

I certainly wouldn't say that boom brakes are ineffective. Many people here have said that it's not practical to have them on such large yachts as the one in the incident under discussion.
They have to be well-designed and installed, like any other piece of equipment.
How is yours attached to your deck?
Would you say that a brake is effective in preventing the boom from crossing the cockpit in all circumstances?
Preventers can be very effective, they don't always end in disaster.
Perhaps what we need is a comparison between brakes and preventers.

I certainly wasn't advocating that luck should be used as a strategy. When there's a boom flailing around, it's impossible to predict what it may hit, or catch. That's what I meant by bad luck. On another occasion the boom may have missed everything.

[Don C L]

Originally Posted by Yellowtulip

Many people here have said that it's not practical to have them on such large yachts as the one in the incident under discussion.
They have to be well-designed and installed, like any other piece of equipment.
How is yours attached to your deck?

I know fittings I see are often on the deck, and many seem strong and well backed, but I'd humbly suggest that the better way is to mount a separate chain plate bolted on the hull, if you are going to fit it in that vicinity.

[a64pilot]

No matter what safety system you pick, you will find instances of it having killed folks.
There is nothing that is perfect. Even automobile air bags that were not defective have killed, and will continue to do so.
They save far, far more than they kill though so you will likely see them being used until something better comes along.

So far on my smallish boat, I’m hanging my hat on a boom brake, largely because I already had a good one, but also I get nervous around things that are supposed to never break, I prefer whenever possible a controlled failure mode, and or designs that have some give or flex designed in.

I look at my mast, the thing that of course handles the force that the sails dump into it, and see that there are three rather large steel wires that are meant of course to resist that force, that is quite a robust design.
If I were to run a preventer, I’d want to be sure as much as possible to both take the force out near the end of the boom, and spread out that force as much as possible, cause it may be quite a lot of force, and of course subjected to a snatch load. Maybe several times.
Then don’t forget, everything wears or fatigues, the components in a preventer or brake ought to be inspected and replaced just like any other rigging.

[transmitterdan]

I’ve never understood why the recommendation to take the preventer line up to the bow ever got into the books. That’s a terrible vector and the load on all the line and fittings is relatively much higher.

The “best” (lowest stress) for preventer line is to land on the boom at a 90 degree angle or as close to as you can. The “best” place to attach at the boom is rig dependent. End boom attachment maximizes load on gooseneck. Mid boom maximizes load on preventer and fittings. A compromise is somewhere aft of the boom center of effort under backwind. So each rig will have its own ideal boom attachment point. The boat designer or a good rigger can determine optimum points of attachment. But blanket statements don’t seem right to me.

If you are frightened by preventer failure may I suggest you probably have too much sail up. It’s really easy for cruisers to be over canvased going downwind. Racers seem to always be over canvased.

[Yellowtulip]

Originally Posted by Don C L

I know fittings I see are often on the deck, and many seem strong and well backed, but I'd humbly suggest that the better way is to mount a separate chain plate bolted on the hull, if you are going to fit it in that vicinity.

Yes, I agree, which is perhaps a point against boom brakes, (expense and complexity) unless you are intending to sail downwind in very strong winds for long periods of time.
Similarly, some of those who venture into the southern oceans have extra 'chainplates' fitted so as to stream a drogue when necessary.

[Dockhead]

Originally Posted by transmitterdan

I’ve never understood why the recommendation to take the preventer line up to the bow ever got into the books. That’s a terrible vector and the load on all the line and fittings is relatively much higher.

The “best” (lowest stress) for preventer line is to land on the boom at a 90 degree angle or as close to as you can. The “best” place to attach at the boom is rig dependent. End boom attachment maximizes load on gooseneck. Mid boom maximizes load on preventer and fittings. A compromise is somewhere aft of the boom center of effort under backwind. So each rig will have its own ideal boom attachment point. The boat designer or a good rigger can determine optimum points of attachment. But blanket statements don’t seem right to me.. . .

I don't really understand this. I guess different rigs might have different geometry, but I can't imagine a boat where the best force vector taking into account leverage wouldn't be end of boom to as far forward as you can find a strong point. Moving the preventer towards the goose neck will slightly improve the angle but at the cost of a large amount of lever arm -- cannot possibly be worthwhile unless I am missing something (which is highly possible).


On my boat, the math is pretty simple -- J dimension of 6 meters and the bow cleat is next to the forestay, so call it 6 meters. The boom is 6 meters long. The spreaders are swept back about 20 degrees.


Preventer rigged from bow cleat to boom end looks like this:





So the angle multiplies the force on the preventer by 1.8x. There is no lever arm.


If I were to rig the preventer to the middle of the boom, I would get this:





That improves the angle from 35 degrees to 48. But the force is still multipled by about 1.6x, so the difference is negligible, while a 2x lever arm has been added.




Would that look different on different rigs? I don't know, but I can't imagine it.

[sailingharry]

Originally Posted by Cpt Pat

Excerpt:

At about 5 AM Michael Kalahar and another crew member were on watch when a sudden swell twisted the vessel sideways back winding the main and causing the sail to travel hard to port. In doing so, the force of the reloaded sail shredded the preventer line allowing the main sail to travel at high speed across the cockpit and swing the mainsheet across the face of Michael Kalahar thus throwing him against the port side winch with the sheet wrapped around his throat and face.

https://web.archive.org/web/20140116...n-Pacific.html

Preventers are pure evil.

Can you explain how this was caused or aggravated by a preventer? Sounds like the preventer failed, and was a non-issue. Perhaps caused by having too much sail up for the conditions, but the preventer doesn't sound to have caused any damage, injury, or boat handling issues.

[sailingharry]

Dockhead's math is spot on. As I mentioned in an earlier post, I did this math years ago to determine "ideal" mounting points for a Lightning vang. Unfortunately, the math didn't fit the boat -- the ideal location is an infinite distance out the boom regardless of mast fitting location, and an infinite distance down the mast regardless of boom fitting location. I went with the class norm..... LOL


One big reason to go to the boom end, regardless of leverage, is that it eliminates bending forces on the boom. A dipped boom can put huge aft forces on the boom, countered by the preventer. If the preventer is not at or near the boom end, a bent boom can result.


The gooseneck loading issue is an interesting thought. Indeed, the further out the boom the higher the compression loads on the gooseneck (I think... I may do more math here...). However, the further in the boom, the higher the "sideways" loading on the gooseneck. At the boom end, the sideways loading is zero. At the mid point, the sideways loading on the gooseneck is equal to the aft-ward loading at the clew (wind or waves). Not sure the tradeoff is in favor of the gooseneck.



On my boat, a Saga 43 with a small bowsprit, the preventers go to eyes welded on the side of the tip of the sprit, a nice clean solution over the classic bow cleat, while improving the angles somewhat.

[Dockhead]

Originally Posted by sailingharry

. . . One big reason to go to the boom end, regardless of leverage, is that it eliminates bending forces on the boom.

Depending on the construction of the boom, this could be an extremely important issue.

Originally Posted by sailingharry

. . . A dipped boom can put huge aft forces on the boom, countered by the preventer. If the preventer is not at or near the boom end, a bent boom can result.. .

I am speculating, but I have always assumed that a dipped boom at any kind of speed would be catastrophic, and no preventer or boom brake would be of any use. Water is 784x denser than air . . .



This is the main reason why I put the mainsail away when conditions are such that there seems to be any material risk of that.

Originally Posted by sailingharry

. . . The gooseneck loading issue is an interesting thought. Indeed, the further out the boom the higher the compression loads on the gooseneck (I think... I may do more math here...). However, the further in the boom, the higher the "sideways" loading on the gooseneck. At the boom end, the sideways loading is zero. At the mid point, the sideways loading on the gooseneck is equal to the aft-ward loading at the clew (wind or waves). Not sure the tradeoff is in favor of the gooseneck.. . .

I think you're right, and I have never been able to visualize that the gooseneck would be better or worse off with any given location of the preventer. Indeed it is not only compression loads which can break a gooseneck, it seems to me . . .

[transmitterdan]

The boom can bend if the preventer is at the end. In fact, for my boat I feel confident the bending moment is not minimized by preventing at the boom end. By moving the fixing point away from the end the bending moment is reduced. The force on the attachment point goes up but the boom bending moment goes down. May not be intuitive but it’s true nevertheless.

[fxykty]

Wouldn’t you just put the preventer in the same position as the main sheet? The boom is built to take main sheet loads in various directions at that point.

So if end of boom sheeting, then preventer at end of boom. If mid boom sheeting then preventer mid boom. Is this too simple and missing something?

Is possible load on preventer higher than the load of the mainsheet? If so, perhaps we shouldn’t use preventers at all other than as flopper stoppers? Unless of course your boom and mast is designed and built for those additional loads.

[Dockhead]

Originally Posted by transmitterdan

The boom can bend if the preventer is at the end. In fact, for my boat I feel confident the bending moment is not minimized by preventing at the boom end. By moving the fixing point away from the end the bending moment is reduced. The force on the attachment point goes up but the boom bending moment goes down. May not be intuitive but it’s true nevertheless.

I get what you're saying, but it certainly cannot apply to loose footed mains like mine.